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Discussion Starter #1
Hello,

I can’t seem to find the reason as to why my car back fires seem like a lean backfire. I checked my air fuel at higher rpms and its goes to about 12.5-12.7. When im at idle and reving it wont backfire but when im driving and i slam the throttle after about 3,000 RPM it starts backfiring like crazy and when i let go off the throttle i hear lots of pops from the exaust.

My timing is at 14 initial and 35 total.

I have a dual feed carb.

What do you think it may be?
 

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Hello,

I can’t seem to find the reason as to why my car back fires seem like a lean backfire. I checked my air fuel at higher rpms and its goes to about 12.5-12.7. When im at idle and reving it wont backfire but when im driving and i slam the throttle after about 3,000 RPM it starts backfiring like crazy and when i let go off the throttle i hear lots of pops from the exaust.

My timing is at 14 initial and 35 total.

I have a dual feed carb.

What do you think it may be?
Are you logging AFRs when you slam the throttle ? My guess is you are running rich and then the unburnt fuel is exploding in the exhaust....

I am using a wideband to log AFRs right now and it has been interesting....
 

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Discussion Starter #3
Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.
 

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Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.
Well I am using the innovate wideband as well (LM2). Under throttle I am more like 12.5 - 13 range at the leanest. If you are seeing an AFR of 15 when WOT then you are tuned too lean IMHO. Optimal AFR is 14.7 (as I am sure you know), but there are a number of other factors to consider and with a carb I doubt you will ever be able to tune the car as precisely as a newer computer controlled fuel injected motor. I generally am happy if the car is at 12 - 13 AFR under WOT.

Obviously when you let off the throttle, your AFR will be much greater than 14.7. I believe I see greater than 20 IIRC while coasting.

Your plugs don't look "bad" to me, but you are definitely not running rich by the looks of them (syncs to your wideband readings). Please confirm when you are hearing the backfire.

Generally a backfire indicates you are running rich (as there is unburnt fuel in the exhaust that ignites) or you have a timing issue. (That said, it can happen if you are too lean as well.)

What is your vacuum at idle? Have you verified you don't have a vacuum leak? What is your AFR at idle?
 

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Discussion Starter #5
Hi ,

I was using an air/fuel reader by innovate. On my tesrlt run i was getting readings between 13-15 but as the rpms pass 3000rpm im at about 12.6-13.5, on my second test run , the clamp said goodbye lol, but for that run thats the reading i got. Im going to show you my spark plug , looks a bit lean.
Well I am using the innovate wideband as well (LM2). Under throttle I am more like 12.5 - 13 range at the leanest. If you are seeing an AFR of 15 when WOT then you are tuned too lean IMHO. Optimal AFR is 14.7 (as I am sure you know), but there are a number of other factors to consider and with a carb I doubt you will ever be able to tune the car as precisely as a newer computer controlled fuel injected motor. I generally am happy if the car is at 12 - 13 AFR under WOT.

Obviously when you let off the throttle, your AFR will be much greater than 14.7. I believe I see greater than 20 IIRC while coasting.

Your plugs don't look "bad" to me, but you are definitely not running rich by the looks of them (syncs to your wideband readings). Please confirm when you are hearing the backfire.

Generally a backfire indicates you are running rich (as there is unburnt fuel in the exhaust that ignites) or you have a timing issue. (That said, it can happen if you are too lean as well.)

What is your vacuum at idle? Have you verified you don't have a vacuum leak? What is your AFR at idle?
Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.

I double check my timing and it still the same as what i set it at. My initial is 14 and my total @ 35.

I shouldnt have any vacuum leaks i check a while ago but i used to have the same backfiring issue as well. The device im using isnt accurate at idle shows me 25+ but as soon as i raise the rpm to about 1500 starts reading better
 

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Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.
Well I think you are running a bit lean and may not be getting a great combustion. You could try retarding the timing 2* and see if that helps.

25" of vacuum at idle is great (congrats) - that would tell me you don't have a vacuum leak.
 

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Discussion Starter #9
Hello,

As i said , my AFR at Wot goes between 12.5 to 13.7 (+\-) and when i let go of the throttle like you said it goes high but that’s completely normal (Coasting). I start hearing the backfiring at around 3000RPM + when im on WOT.
Well I think you are running a bit lean and may not be getting a great combustion. You could try retarding the timing 2* and see if that helps.

25" of vacuum at idle is great (congrats) - that would tell me you don't have a vacuum leak.
I tried changing the timing but still got the same backfiring.

Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?
 

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I tried changing the timing but still got the same backfiring.

Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?
If you retarded timing and still had the backfiring then I'm guessing it is just too lean a mixture and you will need to richen it up a bit.

25" of vacuum would be nice, but I believe is atypical for Pontiacs....

When does your full timing come into play (RPM?) ?
 

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Discussion Starter #11
I tried changing the timing but still got the same backfiring.

Im going to check the vacuum and let you know what happens, but isnt 25” of vacuum a bit too high?
If you retarded timing and still had the backfiring then I'm guessing it is just too lean a mixture and you will need to richen it up a bit.

25" of vacuum would be nice, but I believe is atypical for Pontiacs....

When does your full timing come into play (RPM?) ?
Im going to retard my timing again and see just to double check, around 4 degrees and start from there.

I have a vaccum guage, im going to see what i get on idle, whats a good number to be above?

I usually set my total @4000 rpm.

Thank you
 

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What type/brand/size of carb do you have?
Many things to check as to what it could be. More info might help us ID the problem better or narrow it down, so here is a list and a few more things to consider.


Does it stumble/bog at any point during acceleration?

What spark plugs/gap are they?

Point gap and dwell setting checked/confirmed?

Checked PCV valve/hose/grommet for operation and/or vacuum leaks?

Headers or cast iron exhaust manifolds?

Any leaks or bad gaskets at the exhaust ports allowing air to be sucked in?

1.5 or 1.65 rocker arm ratio?

Rocker arms/hydraulic lifters adjusted too tight keeping valves slightly open at RPM.

Weak valve spring?

Bad cam lobe?

With the high vacuum number of 25", maybe it is pulling too much vacuum for the vacuum can on your distributor? Maybe try an adjustable vacuum can which can be used to set the speed at which the vacuum advance pulls in when you let off the gas.

Just for fun, have you tested the vacuum with the air cleaner off? Is is possible the air cleaner is not flowing as it should and has become a restriction?
 

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Im going to retard my timing again and see just to double check, around 4 degrees and start from there.

I have a vaccum guage, im going to see what i get on idle, whats a good number to be above?

I usually set my total @4000 rpm.

Thank you
Full disclosure I am an OLD CAR NEWB and have learned a lot recently about mechanical and vacuum advance and Carb design. That said, there are many related aspects to EFI / computer tuning and older style setups.

Lars sent me a very good write up on timing and if I remember correctly my full timing comes in much earlier (more like 3K rpm).

Vacuum at idle - the higher the better but I doubt you will see more than 25" with a decent gauge. From what I have read many GTOs are below 18" and if they have a more aggressive cam will be ~10 - 12".
 

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Discussion Starter #14
Im going to retard my timing again and see just to double check, around 4 degrees and start from there.

I have a vaccum guage, im going to see what i get on idle, whats a good number to be above?

I usually set my total @4000 rpm.

Thank you
Full disclosure I am an OLD CAR NEWB and have learned a lot recently about mechanical and vacuum advance and Carb design. That said, there are many related aspects to EFI / computer tuning and older style setups.

Lars sent me a very good write up on timing and if I remember correctly my full timing comes in much earlier (more like 3K rpm).

Vacuum at idle - the higher the better but I doubt you will see more than 25" with a decent gauge. From what I have read many GTOs are below 18" and if they have a more aggressive cam will be ~10 - 12".
Hello,

I set it my total @ 4000 because its said that it could be more accurate and set my intial with the distributer advance port disconnected as well.

Ill be really happy with 14-16” but i actually have an aggressive cam!

Im going to test it tomorrow and let you know of the results hopefully.

Where is the highest manifold pressure on the intake?
 

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I am also going to throw this one at you just for consideration - your previous posting:

"455 troubles

I know this is a old post , but i thought id let you guys what went on.

Did a compression test all were within 10% of each other, except for one, when i removed the spark plug coolant bursted out of the spark plug hole!!!

So i removed everything, changed the head gasket and basically did a overhaul since i was at it.

And now no more coolant loss and it runs a little better!

Im glad it was only a head gasket."

Did you have the head checked to make sure a valve was not bent? If you had coolant bursting out the spark plug hole when you removed the plug, it is possible that the water in the cylinder could have caused what is sometimes referred to as "hydrolock." Could have bent a valve, pushrod, or ? Just a thought on the popping exhaust problem.

My brother operates a railroad and one of his engines experienced this. He knew he had one cylinder which had a water leak and it is possible on a train engine to drain it off - but this was done in an attempt to keep the train running until it could be put out of service and repaired. His luck ran out and the engine locked up at idle - hydrolocked. When they pulled down the engine to check the cylinder, the connecting rod he pulled out was pretty bent up. He told me that connecting rods in a diesel engine are pretty big, but water does not compress too well so something has got to give. A new liner, piston, and rod and it's all fixed up and running again.
 

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Discussion Starter #16
What type/brand/size of carb do you have?
Many things to check as to what it could be. More info might help us ID the problem better or narrow it down, so here is a list and a few more things to consider.


Does it stumble/bog at any point during acceleration?

What spark plugs/gap are they?

Point gap and dwell setting checked/confirmed?

Checked PCV valve/hose/grommet for operation and/or vacuum leaks?

Headers or cast iron exhaust manifolds?

Any leaks or bad gaskets at the exhaust ports allowing air to be sucked in?

1.5 or 1.65 rocker arm ratio?

Rocker arms/hydraulic lifters adjusted too tight keeping valves slightly open at RPM.

Weak valve spring?

Bad cam lobe?

With the high vacuum number of 25", maybe it is pulling too much vacuum for the vacuum can on your distributor? Maybe try an adjustable vacuum can which can be used to set the speed at which the vacuum advance pulls in when you let off the gas.

Just for fun, have you tested the vacuum with the air cleaner off? Is is possible the air cleaner is not flowing as it should and has become a restriction?
Hello Pontiac Jim,

I have a thunder series AVS 800 cfm edelbrock Carb.

Doesnt stumble nor bog at acceleration.

Spark plugs are delco R45T’s

Gap checked and confirmed i think at 0.40 if I remember correctly.

PCV valve no leaks! But i still need to do a full vaccum check.

Dougs long tube headers!

The exaust gasket may have a small leak but i dont think so but even before this leak i had the same problem so im 100% sure its not it.

1.5 rocker arms ratio, i tightened them myself.

Cam is fine i dont think its a a bad lobe but how do i check?

For the vacuum as i mentioned above i need to do a vacuum test to be sure.

I have double valve springs that i installed when i installed this cam.

And ill check that once i do the vacuum test!
 

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Discussion Starter #17
I am also going to throw this one at you just for consideration - your previous posting:

"455 troubles

I know this is a old post , but i thought id let you guys what went on.

Did a compression test all were within 10% of each other, except for one, when i removed the spark plug coolant bursted out of the spark plug hole!!!

So i removed everything, changed the head gasket and basically did a overhaul since i was at it.

And now no more coolant loss and it runs a little better!

Im glad it was only a head gasket."

Did you have the head checked to make sure a valve was not bent? If you had coolant bursting out the spark plug hole when you removed the plug, it is possible that the water in the cylinder could have caused what is sometimes referred to as "hydrolock." Could have bent a valve, pushrod, or ? Just a thought on the popping exhaust problem.

My brother operates a railroad and one of his engines experienced this. He knew he had one cylinder which had a water leak and it is possible on a train engine to drain it off - but this was done in an attempt to keep the train running until it could be put out of service and repaired. His luck ran out and the engine locked up at idle - hydrolocked. When they pulled down the engine to check the cylinder, the connecting rod he pulled out was pretty bent up. He told me that connecting rods in a diesel engine are pretty big, but water does not compress too well so something has got to give. A new liner, piston, and rod and it's all fixed up and running again.
Hello,

When i replaced the head gasket i checked the springs, rockers , pushrods, valves and everything and polished and resurfaced the head since i was at it just to be sure.

How do i check if the cam lobes are gone?
 

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OK on your heads as you had things checked out - was just a consideration.

Two things. 1.5 rockers and you tightened yourself. How? Factory torque down or are you using poly locks & "zero lashing" them?

Next, since you have been playing around a lot with the carb, throw away the air/fuel gauge for now and adjust it old school. I found this on another site which might help? -- "Idle mixture screws too far in and the butterflys too far open at idle will do it. Drawing on fuel/air from the main circuit and not from the idle circuit, so when you cut the throttle there's excess fuel being drawn in through the main circuit."

Fatten up your idle mixture screws and back out of the idle screw. Don't use a vacuum gauge either. Adjust it by ear. I like to begin by backing out the idle mixture screws 2 1/2 turns each, but in your case back them out about 4 turns to fatten them up. Then back your idle screw out to bring your RPM's down to around 650 RPM's (or whatever the lowest idle RPM your cam like to idle at). Then, one at a time, turn each in until the engine RPM's drop, stop adjusting, and turn the idle mixture screw back out to get the engine back up to 650 RPM's. See if that setting changes anything. I might even try it with 4 turns out and the idle at 650 RPM's and take it for a spin to see if it gets worse or better.

You are going to have to experiment, the old way. If richening it up improves it, then that's the way to go. If it gets worse, then leaning it out may be the way to go and then test that with a drive. The carb also has the step-up springs rated for specific vacuum settings. 25Hg of vacuum at idle seems way too high for a performance cam, but I am no expert on the subject.

There are other adjustments on the carb as well which the online owners manual points out. Check the trouble shooting list near the end. Do you have the correct 4-hole spacer if you have it on a factory Q-jet, or spreadbore type manifold?: http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

Might be worth a call to Edelbrock as they could possibly steer you in the right direction as well. :thumbsup:

Another long shot. Maybe your headers are over scavenging? This may also be cam related based on cam overlap where the intake/exhaust valves are open at the same time. Unburned fuel mixture may be getting sucked out for an instant when you decelerate rather than "force" the exhaust gases out the headers under load when you open up the throttle. I have also read that headers require a step up in carb jetting to richen up the carb when they are used. Just another thing to confuse you. :yesnod:
 

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Discussion Starter #19
OK on your heads as you had things checked out - was just a consideration.

Two things. 1.5 rockers and you tightened yourself. How? Factory torque down or are you using poly locks & "zero lashing" them?

Next, since you have been playing around a lot with the carb, throw away the air/fuel gauge for now and adjust it old school. I found this on another site which might help? -- "Idle mixture screws too far in and the butterflys too far open at idle will do it. Drawing on fuel/air from the main circuit and not from the idle circuit, so when you cut the throttle there's excess fuel being drawn in through the main circuit."

Fatten up your idle mixture screws and back out of the idle screw. Don't use a vacuum gauge either. Adjust it by ear. I like to begin by backing out the idle mixture screws 2 1/2 turns each, but in your case back them out about 4 turns to fatten them up. Then back your idle screw out to bring your RPM's down to around 650 RPM's (or whatever the lowest idle RPM your cam like to idle at). Then, one at a time, turn each in until the engine RPM's drop, stop adjusting, and turn the idle mixture screw back out to get the engine back up to 650 RPM's. See if that setting changes anything. I might even try it with 4 turns out and the idle at 650 RPM's and take it for a spin to see if it gets worse or better.

You are going to have to experiment, the old way. If richening it up improves it, then that's the way to go. If it gets worse, then leaning it out may be the way to go and then test that with a drive. The carb also has the step-up springs rated for specific vacuum settings. 25Hg of vacuum at idle seems way too high for a performance cam, but I am no expert on the subject.

There are other adjustments on the carb as well which the online owners manual points out. Check the trouble shooting list near the end. Do you have the correct 4-hole spacer if you have it on a factory Q-jet, or spreadbore type manifold?: http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

Might be worth a call to Edelbrock as they could possibly steer you in the right direction as well.


Another long shot. Maybe your headers are over scavenging? This may also be cam related based on cam overlap where the intake/exhaust valves are open at the same time. Unburned fuel mixture may be getting sucked out for an instant when you decelerate rather than "force" the exhaust gases out the headers under load when you open up the throttle. I have also read that headers require a step up in carb jetting to richen up the carb when they are used. Just another thing to confuse you.
Sorry for the later reply!!! I dont know how to turn on notifications on the forum.

As im still having the same problem,

1 - for the rockers, factory torque down.

2- thats how i usually adjust the carb and it seems to be running great and now i keep the idle @700 rpm

3- my manifold is a an edelbrock performer so i dont have to run an adapter.

As for the headers im not sure? How would i know if they were ‘over scavenging’

I did notice something yesterday tho im running a stock style valley pan with a pcv provision but i dont have a grommet installed with the pcv , could this be an issue? So the pcv is plugged directly into the valley pan without a grommet.

This has taken so long and i cant seem to figure it out.

I have now installed an MSD distributor and ignition box, still the same.

Im going to change out the spark plugs as well just to make sure i eliminate the ignition issues.
 

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Sorry for the later reply!!! I dont know how to turn on notifications on the forum.

As im still having the same problem,

1 - for the rockers, factory torque down.

2- thats how i usually adjust the carb and it seems to be running great and now i keep the idle @700 rpm

3- my manifold is a an edelbrock performer so i dont have to run an adapter.

As for the headers im not sure? How would i know if they were ‘over scavenging’

I did notice something yesterday tho im running a stock style valley pan with a pcv provision but i dont have a grommet installed with the pcv , could this be an issue? So the pcv is plugged directly into the valley pan without a grommet.

This has taken so long and i cant seem to figure it out.

I have now installed an MSD distributor and ignition box, still the same.

Im going to change out the spark plugs as well just to make sure i eliminate the ignition issues.

You say you have a radical cam. In one of your posts, you stated you had 25" of vacuum at idle. You won't have 25" of vacuum with a radical cam. Now you state you will be happy with 14"-16" of vacuum. I'm confused. :thumbsup:

You could have a bad/worn cam lobe(s).

What is your real vacuum reading at idle?

Run plug gap at .035".

PCV valve needs the rubber grommet in the valley pan to which it plugs into.

What happened to all your AF ratio's and the wideband monitor?

So begin the whole process again and post your numbers.

MSD distributor. You eliminated the factory resistor wire and have it wired for 12volts? Most aftermarket distributors require 12volts, not 8 volts like points. Your instructions for the MSD should say what you need.

Cam Specs ?

New balancer?

Confirmed No.1 piston at TDC and timing mark on balancer at 0?

Did you degree the cam to manufacturer's specs and confirm them?

Initial Timing - with vacuum advance disconnected and plugged at the carb?

At what RPM does your advance begin to move? Should not move until you get to about 1000-1200 RPM's.

What is your total timing in degrees when the distributor advance stops ( vacuum disconnected) ? You stated 4,000 RPM's. Double check. You should see 34-36 degrees total at what RPM?

Some additional reading on the wideband tuning. You may still need to step up on your jetting if you have a radical cam and used the carb right out of the box.

https://www.enginebuildermag.com/2008/07/carburetor-tuning-the-airfuel-equation/

Mixture, Air/Fuel Equation
Henry P. Olsen,author

Now that the ignition spark timing advance curves are optimized for the blend of reformulated and/or oxygenated gasoline your customers are using we will now show you how we use tools such as a 5-gas exhaust analyzer and wideband Lambda air/fuel (A/F) meter to tune the mixture.

The Air/Fuel Mixture

A lean fuel mixture can cause an engine to have a surge or miss at idle and part throttle stumble on acceleration, leading to engine overheating and lack of power. A rich fuel mixture can cause an engine to “load up” at idle, foul the spark plugs, and also lack power or run sluggish.

If the A/F mixture that is delivered to the engine is excessively rich for too long the engine could leave leftover fuel from the combustion process, washing the oil off the cylinder walls. Without the oil to act as an anti-wear agent, the pistons and rings will make metal-to-metal contact with the cylinder walls. Also, if enough fuel gets past the rings and into the crankcase the oil can become diluted and lose much of its lubricating properties and accelerate engine wear.

Theoretically, the ideal stoichiometric A/F mixture (the chemically ideal mixture of air and fuel that is required to provide a complete burn) for a properly tuned engine running on pure gasoline is 14.7:1; that is, 14.7 lbs. of air to 1 lb. of fuel. However, because of operating losses in the induction system due to intake runner and cylinder wall wetting, plus the fact that fuel may not fully vaporize in the combustion chamber, a 14.7:1 A/F mixture is often too lean for actual operating needs. A more realistic light-load, cruise A/F mixture for a stock carbureted engine running on reformulated unleaded gasoline is in the 14.1:1 range.

The A/F mixture always varies from cylinder to cylinder, therefore we tend to tune the average A/F mixture slightly on the rich side to avoid engine misfire in the leanest cylinder. It is possible to target an A/F mixture leaner than 14.7:1 for maximum fuel economy but this can lead to driveabilty problems if any one cylinder is leaner than the others. The power mixture we target for maximum horsepower is in the 12.2:1 – 13.5:1 A/F range, depending on the engine package and its combustion chamber design.

The original equipment carburetor(s) that came on a muscle or classic vehicle’s engine was tuned for the leaded gasoline of the day, so in most cases the engine will tend to run lean with the reformulated and/or oxygenated unleaded gasoline of today. The gasoline of today also has lower volatility than the leaded gasoline of days past, which will cause most carbureted engines to need a slightly richer A/F mixture at idle and light load part throttle driving conditions to have the same drivability as it had with the leaded gasoline of the ’60s and ’70s.

Back in the 1950s and early 1960s, the car manufactures tended to calibrate their carburetors on the rich side of the ideal A/F mixture needs of the engine with the leaded gasoline of the day. Then starting in the late 1960s, the carburetors were calibrated more toward the lean side of the ideal A/F mixture needs of the engine so the vehicle could pass the exhaust emission standards that were just coming into existence.
The modern reformulated conventional and oxygenated gasoline of today will cause the A/F mixture to shift leaner when compared to the leaded gasoline of the 1960s and 1970s. This means if the A/F mixture was lean with leaded gasoline it will be even leaner with today’s gasoline blends.

The high performance and replacement carburetors sold today are sold with an A/F mixture curve designed for a generic engine; therefore they must be tuned for both the specific engine and the blend of gasoline they will be used with. These aftermarket carburetors should be designed with an A/F mixture that is rich enough for a wide variety of engine packages with different exhaust systems, but this is not always true. Some of the aftermarket carburetors we see need a lot of tuning work to get the A/F mixture correct for the engine’s demands with the reformulated unleaded gasoline of today.

Air/Fuel Mixture Tuning Guidelines

Back in the days of leaded gasoline an experienced tuner would adjust the A/F mixture the engine was getting from its carburetor by reading the color the fuel left on the insulator of the spark plug in the exhaust port and in the first 6 inches of the exhaust header. The reformulated unleaded gasoline we have today has made reading spark plugs almost impossible because it leaves little or no color on the spark plug insulator.
However, modern technology has made available at an affordable price both portable 5-gas exhaust gas analyzers and wideband Lambda (“oxygen”) sensor based digital A/F meters that can be used to accurately “read” the A/F mixture in an engine by analyzing the content of the engine’s exhaust gases. These modern tools can allow you to observe what A/F mixture the engine is getting from the fuel system while driving the car in real world conditions at any rpm and load condition.

The ideal A/F ratio for maximum power or fuel economy may be best calculated at the factory with the engine on a dynamometer, but the readings that are available from a 5-gas exhaust gas analyzer allow you to tune the A/F mixture for what your engine needs in real world driving conditions. The readings from an infrared exhaust gas analyzer will indicate A/F ratio, engine misfire, engine combustion efficiency and excessive combustion chamber heat (detonation) by looking at the following exhaust gases:

CO (Carbon Monoxide): The reading from an infrared gas analyzer that we use to determine the air to fuel ratio when the A/F mixture is on the rich side of stoichiometric. (Note: CO is partially burned fuel.)
The other readings that exhaust analyzers provide are:
HC (Hydrocarbons): The amount of unburned fuel in the exhaust (a indicator of an engine misfire).
CO2 (Carbon Dioxide): A gas that is the product of complete combustion (the best A/F mixture gives you the highest CO2 reading). The ideal ignition-timing advance will also create the highest CO2 reading
O2 (Oxygen): A high O2 reading indicates a lean mixture; an exhaust leak or the engine has a “hot” cam. Note: if O2 content is above 2 to 3 percent, air dilution of the exhaust gases being measured is indicated and the accuracy of the all of the gas readings may be negatively affected.
NOx (Oxides of Nitrogen): A gas created by excessive combustion chamber heat. This gas can be used as a precursor to detonation.

The readings you can get from a 5-gas exhaust gas analyzer can help an experienced tuner calculate what A/F mixture and how much ignition spark timing advance the engine needs to perform at its best.

A wideband sensor lambda sensor based A/F meter calculates the A/F mixture by “reading” the unburned combustible content of the exhaust gases (note: a lot of people call the lambda sensor an oxygen sensor but Bosch calls it a lambda sensor). The wideband lambda sensor measures the amount of oxygen that must be added to or subtracted from the exhaust gas to form a stoichiometric gas mix in its reference chamber, the A/F meter then calculates the A/F mixture of the exhaust gas from that value.

The readings you get from a wideband lambda sensor based A/F meter can be quite accurate, but false readings can be created by an exhaust leak, engine misfire, or an engine with a high performance camshaft at lower engine speeds. These false readings are caused by the Lambda sensor misreading the unused oxygen and/or unburned combustibles that are in the exhaust gas mixture

Tuning with a 5-Gas Analyzer and Wideband Lambda Meter

The use of a portable 5-gas exhaust gas analyzer and/or a wideband sensor based A/F meter can allow a tuner to observe the A/F mixture the engine is getting from its fuel system at any engine operating condition.

A starting point for A/F mixtures for most mild performance engines is:
• Idle: 1.0% to 3.0 % CO or a 14.1-13.4:1;
• Cruise rpm: 1.0% CO or a 14.1:1 with a mild performance engine; or 1.0% – 3.0% CO or a 14.1 – 13.4:1 with high performance cam; and
• Power mixture and acceleration: 6.0% CO or a 12.5:1 for a “normal” engine or high performance engine with improved combustion chamber design such as a Pro Stock or a NASCAR engine; in some cases you may be able to use a slightly leaner power mixture of 4% CO or a 13.0:1.

When we are tuning fuel systems, we use both infrared exhaust gas analyzer and the wideband Lambda sensor methods. This way we can take advantage of the strengths of both tuning methods. The infrared exhaust gas analyzer, while slower in reaction time than a wideband sensor based A/F meter can actually best determine A/F mixture needs. The misfire rate can be observed with the HC (hydrocarbon) reading.

Efficiency can be observed by the CO2 reading (carbon dioxide) reading, and the NOx reading (oxides of nitrogen) can also be used as a precursor to detonation. A wideband Lambda sensor-based A/F meter systems available from companies such as Innovate Motorsports or FAST have almost no delay, while a 5-gas exhaust gas analyzer has a 6 to 10 second delay.

If the engine you are tuning has an air-gap style intake manifold and/or high performance camshaft you may need to tune the idle and cruise mixtures richer than a stock engine with the same gasoline. The added performance from an air-gap intake manifold and the increased valve overlap from a high performance camshaft can often come at the price of lower fuel vaporization at lower rpm operating conditions.

The richer A/F mixture can help cover up the driveability problems when the fuel is not completely vaporized. The heat the intake manifold gets from the exhaust gas crossover in a conventional intake manifold helps the engine vaporize the fuel as it travels from the carburetor into the cylinders combustion chamber.

A/F Mixture Delivery Circuits

A carburetor has an accelerator pump, idle, main jets, and in most cases a power system that is designed to supply the correct A/F mixture for the demands. The accelerator pump system adds fuel as the throttle valves are opened. Tuning the accelerator pump squirter volume and duration is mainly done by trial and error to obtain the best throttle response, but a 12.5:1 A/F mixture is a good place to start.

An idle system will have an idle jet/restriction that must be changed to supply the desired fuel mixture for idle and off idle engine demands. If the engine you are working on is equipped with a power valve (no metering rods), the main jet size is what determines the A/F mixture that will be delivered to the engine at light-load/cruise speeds.

The power valve restriction (under the power valve) determines what A/F mixture the carburetor will supply when the power valve is open; under high power demands a 6.5? power valve will be open, supplying richer A/F mixture any time the vacuum is below its 6.5? opening point.

Power valves have a reputation for being a weak link in certain designs, but the carburetor can be retrofitted with backfire protection, which will improve reliability. A carburetor that uses metering rods in the primary jets will use the metering rods to change the A/F ratio for both the power and cruise mixture demands of the engine; the larger the metering rod diameter the leaner the A/F mixture.

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, as well as checking to be sure there are no vacuum leaks, the next step is to determine what the A/F mixture is at idle through 3,000 rpm. If the cruise mixture is off, first change the jets to get the A/F mixture correct at 2,500-3,000 rpm cruise range. Then check and set the idle mixture. If the A/F mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle jet.

If the mixture is still lean at 1,000 through 1,800 rpm after enlarging the idle jet, the idle channel restriction (if used) may have to be enlarged slightly to allow more fuel to be delivered at part throttle. It is important to note that any changes other than basic adjustments and jet changes should be done by a “carburetor expert” to avoid damaging a vintage carburetor. If the carburetor is damaged a replacement numbers matching carburetor could be quite expensive.

A modular design carburetor, such as a Holley, with a metering block does not use an idle channel restrictor. When we want to richen the part throttle we often must slightly enlarge the idle well in the metering block. When the A/F mixture is too lean at part throttle the engine may miss or stumble on light acceleration and at 5 – 25 mph light throttle cruise conditions.This lean off idle problem has become more prominent as the ethanol content in today’s gasoline is increased and as the gasoline formulation is changed.

If the A/F mixture is too rich at idle and/or part throttle, the idle jet or part throttle idle restriction may be too big. You may need to be replace it with a smaller one. Once you have the idle, part throttle and cruise A/F mixture curves correct, the next step is a road test.

A road test using a portable infrared exhaust gas analyzer and/or a wideband oxygen sensor will allow you to check the cruise speed A/F mixture, followed by a check of the power A/F mixture under load. This type of test allows you to see what the A/F mixture is under real world driving conditions. During this road test you will be able to read and then correct the A/F mixture.

If you see an A/F mixture reading that goes too lean at high engine loads, the first thing to do before you change jet size is to check the fuel pressure. The fuel pressure must stay above 5 psi at wide-open throttle; if not, the carburetor will starve for fuel.
The most common accelerator pump-related complaint we hear is a hesitation on quick acceleration. This hesitation is most often caused by the changes in the gasoline’s volatility and changes in carburetor manufacturing. The accelerator pump duration spring used on most replacement carburetors is not as strong as the spring that was on these same carburetor designs used in the 1960s.

We use an accelerator pump upgrade kit on most Holley modular style carburetors that consists of a stronger duration spring, a 0.031? squirter and a “pink cam” (Ole’s p/n 1330), this makes the accelerator squirt more active.

When we are working on a engine with an Edelbrock Performer or Thunder series carburetor we use an improved accelerator pump (Ole’s p/n 1010). This accelerator pump has a stronger duration spring that allows the pump to be more active and thus help cure most of the accelerator pump related hesitation we see with these carburetors.

Selecting the Correct Carburetor

The big four suppliers of 4 barrel carburetors today are Edelbrock, Holley, Quick Fuel and Barry Grant, each of these carburetor designs have strong and weak spots. The carburetor that we would recommend is based on how the customer will drive their car and the engine package that is in the car.

The Carter-designed Edelbrock Thunder and Performer are reliable low maintenance carburetors with great electric chokes but if the driver likes to drive fast around corners they may not bethe best carburetor to select for that application. The off-idle system design in these Carter-designed carburetors can lead to a lean off-idle stumble problem when the engine has a “hot cam” or an air-gap style intake manifold. Enlarging the idle channel restrictor on the 500 thru 650 cfm units will often cure this lean off-idle stumble problem but we have not had the same success solving this lean off-idle stumble problem on the 750 and 800 cfm carburetors of this design.

Modular carburetors manufactured and sold by companies such as Holley, Barry Grant and Quick Fuel are very good carburetors to select when the driver likes to drive fast around corners or when you are tuning for maximum power. Quick Fuel also sells billet metering blocks with changeable idle jets, power channel restrictors and emulsion well restrictions for the Holley style modular carburetors, which allow you to custom tune the fuel curve.
When the customer wants a high performance modular carburetor with an electric choke we often recommend a Holley brand carburetor because their chokes have a choke pull-off built in. When we are tuning a high performance engine with a “hot cam” (over 240 degrees of duration @ .050?) or any engine with an air-gap style intake manifold, we often recommend a race-designed modular carburetor with a four corner idle system.
 
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