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Discussion Starter #1
So, I'm not a mechanic but I like to fix stuff and figure stuff out. I'm in a little over my head here, but I'd like to figure this out and learn along the way.

Here's the deal, I purchased a 68 gto a week ago. Car was restored looks pretty, and sounds great. Car looks like a rocket ship, but on my test drive felt like a slug. The good thing was the way it drove got me a good deal on the car. On Friday I did a basic tune up. Replaced the plugs, wires, cap, and rotor. All the plugs where all equally fouled. They where all actually a little wet. After the tune up the car drove like it looks. Spin the tires at will. After a short drive the car started lacking the ability to burn rubber, and it seems like it's progressively getting back to where it was. Not quite as bad, but getting there. I'm hopeful that this is a carb adjustment issue, and assuming the plugs are fouled again. The car is set up with two four barrel eddelbrock carbs. I was told they are 500 cfm, but have not had a chance to confirm this (only had it a week and have too many other irons on the fire). The car was supposedly bored "30 over" too. The timing read at 8, but I did not check it again after the tune up. Only other thing I've done mechanically was clean the carbs with gumout and lubed the linkages. I did notice yesterday that one of the electrical connections to the choke was disconnected. I'm guessing that this was done in an attempt to lean out the mixture.

So, I ordered a couple vacuum gauges and will be ready to have at it again when I get a moment. Any suggestions or advice would be greatly appreciated.

Thanks
Tooth
 

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First, you did not say if they were hooked up together or if you have progressive linkage. If hooked together, that's the beginning of your problem - way too much fuel for a street car. If you do not have progressive linkage, get it. The second carb should snap open when the first carb is typically about 3/4 open.

The secondary carb does not need a choke, but it should be wide open, nothing closed or set at an angle. This will only cause the carb to run rich.

Next, if both carbs are identical, you are running the idle circuits on both carbs - too rich. The factory dual 4 set-up used a secondary carb with no idle screws. You only adjust the carb you run off of.

You should have vacuum secondaries on each carb, so they should open as needed and provide a smooth transition without bog or stumble. Your jetting may need to be changed as they are most likely set-up to be run as a single use and not for dual 4's.

Other suggestions would be an an MSD unit and hot coil. The MSD does what it is designed to do - multispark at lower RPM's which will help fire the plugs better. You might also try a step or two hotter plug for better burning.

Points or electronic? Either way, I like to run the spark plug gap at the factory .035, but it is recommended with electronic distributors to run .040" or more. I just feel it takes too much energy to jump the wider gap, especially if you are already running rich. I also run an 8mm wire but I went with an 8.8mm on my present build - I like the Taylor brand.

I would also play around with the timing. Don't know what cam you have or compression, but 8 degrees at the crank may actually not be enough. It may be retarded to help with pre-ignition, but with all the excess gas right now, I don't think that should be a problem. I might bump it up to 9 degrees and upwards into the 12 degree range, moving a few degrees at a time and taking out on the road for a wide open blast to see how it goes and listen for detonation (rattling). This is where you want to check your distributor advance curve and set it up for the best performance. Do a forum search as this has been covered in depth, and very well.

I once ran a 409CI that I built up not knowing what I know now. Had a big solid cam that shook the windows. I installed a pair of 460 Ford 750 CFM AFB carbs after doing a rebuild. I screwed in the idle screws on the secondary carb. Even with progressive linkage, the plugs ran rich and would foul. I installed the MSD and hot coil- it ran smoother, but still had fouling plugs. Engine had the factory Mallory dual point with no vacuum advance set-up to be all in a 2,500 RPM's. So I bought a set of those aircraft style 5/6 electrodes that JC Whitney used to sell. Never fouled again like the regular plugs. Car ran like a raped ape and because it was really over carb'd, would keep on pulling past 6,500 RPM's and never quit pulling. Gas mileage was 8-10 MPG's.

Hope that helps. :thumbsup:
 

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Discussion Starter #4 (Edited)
Looks like a progressive. This is my set up. I know it's a crappy picture, but I'm at work. Not sure if this and the picture of the spark plug add to the story at all. I just want to go home and have at it. Lots to learn along the way.

Thanks again
Tooth
 

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Looks like a progressive. This is my set up. I know it's a crappy picture, but I'm at work. Not sure if this and the picture of the spark plug add to the story at all. I just want to go home and have at it. Lots to learn along the way.

Thanks again
Tooth
Certainly looks like the Edelbrock progressive linkage as well as Edelbrock's fuel line to the carbs. If you don't have the tuning manual for these carbs, here is the link. You can download it, print it out, whatever you need. It'll give you lots of info on tuning for the correct mixture.

Best of luck!!

http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf
 

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Discussion Starter #6

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The manual has a lot of good info. Pay attention to page 3 on the "Step-Up"rods and springs. Manifold vacuum plays the big role with these. Don't know what cam you are using, so you want to get an idea of the vacuum the engine has at idle. You can also get different diameter rods.

You can also change out your jets as well. It might be worth while to purchase an Edelbrock Calibration Kit for your carb. It has an assortment of step rods, springs & jets. https://www.summitracing.com/search/product-line/edelbrock-performer-series-carburetor-calibration-kits

What I have always heard is that once you get one carb set up, the second carb will be the same. So I would disconnect the secondary carb and try dialing in your main carb and get it set up so the plugs aren't wet with gas. This kind of thing is trial and error until you get it dialed in.

Here is a good write up which you want to read that covers what I have said. Mind you, this is for a Chevy engine, but the same thoughts still hold true. Ask Away! with Jeff Smith: Tuning Tips for Edelbrock 2x4 Carb Setup on a Small-Block Chevy - OnAllCylinders
 

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Discussion Starter #8
Poked around at the car a little before work this morning. The carb in the front seems to be the primary, and the one in the back the secondary. The electric choke, though disconnected is on the secondary. The adj screws on the secondary are open all the way. I adjusted the primary screws by ear, just because I needed to mess with things and tried to keep it on the lean side. The out of the hole power is still not there like it was after I changed the plugs wires cap and rotor, but the car seemed much happier at cruising speed on the way to work. I think my next step is going to see where the vacuum is at on both to have a baseline, then start with the timing changes. Feel like I'm all over the map and have allot to learn.
 

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Discussion Starter #9
Ok, so I've got the car a little closer to being on the right track. I played with the timing first. It was at 8, and after some trial and error left it at a hair under 12. I dialed the idle back from 740's to 620's, but it stalled out in drive at that idle speed. I bumped it back up to 700 for now.

The car does indeed have the edlebrock progressive linkage, but the whole thing is set up wrong. The choke though disconnected is attached to the secondary, there was no spring to the front throttle because it was not set up as progressive yet the carbs where set up so. What I meant by that was that the primary carb was set with the idle screws, but the secondary was set wide open. I changed it so that the linkage is now progressive (need to dial this in better), but left the secondary idle screws as is. I think my next step will be to dial both carbs in with a little better with a vacuum gauge, play with the timing again and try to get the car to idle a little lower.

So, am I on the right track? The car is definitely running much better, but still lacks the out of the hole power I had after doing the initial basic tune up. I did take the plugs out and clean them all again, but it didn't seem to do much. I think one of the many projects I need to do is put in a better ignition system. I'm guessing that will certainly help, but I've got allot to learn to really get this thing right.

Thanks in advance.
Tooth
 

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First, idle speed is set in Drive, not park or neutral. Set it so when you pull it into Drive, it will run about 650-700 RPM's as your starting point. Don't necessarily use the factory idle speeds. If you read through them, you will note that different years have different idle speeds with/without the electric idle solenoid stop. You will also notice that the manual/high HP engines have higher idle speeds. The different idle speeds take into account the cam grind. The more aggressive it was, the higher the idle speed needed.

As you learned, when you drop it into drive it stalled. So idle it up (650-700RPM in Drive,) to where the car runs smooth when you drop it into gear and you don't have to use a lot of pressure on the brake pedal to keep the car from rolling forward. Light pressure should be OK, but you don't want to have to use heavy brake pressure to overcome the higher idle setting. I suspect you don't have a higher after market 2,500 stall converter (more slippage) which allows for a higher idle setting without the car wanting to move forward in gear. So play around with 650-750RPM's in the Drive range with lower RPM's being best as long as the engine runs smooth and it does not stall out.

Timing sounds better at 12 degrees. You can work on the advance curve later. The advance curve can may a big difference in the seat of the pants acceleration.

Your carbs should be the same, right? So why not simply swap them and make the carb with the choke your primary? Then you want to screw in the idle mixture screws in all the way on the secondary carb. Again, turn them in easily until they stop, no need to crank them down real tight as you will damage the carb. Then make sure the idle screw on the side of the carb is backed off so the primary throttle plates are closed. You don't want them open. Typically as you turn your idle up, the throttle plates open to let more air/fuel in which increases your engines idle speed. You don't want any extra air/fuel going through the secondary carb until you pop it wide open via the progressive linkage.

As suggested earlier, you want to get the engine running good on one carb by getting the primary carb adjusted first. So disconnect the secondary carb for now and make sure it is "shut down." - it is only adding to the problem. Run the car on just one carb. Get the engine running right on the one carb, then hook up the secondary carb and dial it in. You may be like a cat chasing its tail trying to figure out all the combinations of adjustments with one affecting the other and even each being thrown off when adjusting one carb or the other.

Better/hotter spark can help. Do you have points or an electronic distributor? Points are fine if you do. I would get a hotter coil for starters if you don't already have one.

My experience when the plugs run rich is that the engine will run a little sluggish because it "loads up" with carbon (the black sooty stuff you see on the plugs) - at least that's what we used to say. You have to run it hard to blow/burn this off to get the engine to run crisper. My 409 would get sluggish when I took it easy on it trying to keep my foot out of the gas in an attempt to get better gas mileage. I would start to ring it out and run it hard to clear out the carbon and sure enough it would wake the performance back up and run great again. So it was always better to flog it than to take it easy on the engine. So you may be experiencing this with the carb running rich even though you cleaned the plugs.
 

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Discussion Starter #11
Jim

Thanks for all the info. There is allot to digest there. You have pointed out a couple things I'm doing wrong. First off, the idle screws on the secondary are all the way open! I'll also need to dial back the idle adj screw on the secondary. And, yes I'm currently fighting the brake when stopped at idle, so that needs to be adjusted. I've actually been putting it in neutral, so I'm not fighting the brake pedal. Also, it is a points system not electronic. I put in the spark plugs cap rotor and wires from a tune up kit I got from rock auto. Other than the iridium plugs I'm guessing everything is close to stock. What would you suggest for a hotter coil and plugs? The plugs in the kit where XR5IX 7355 NGK.

In the long run am I better off scraping the dual quads and going with an appropriate single carb? I'm looking for performance and reliability over esthetics.


Thanks again
Tooth
 

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OK, here are my thoughts on all this. IF you are not bent on having to have the dual quads, then your best bet is to go back with a stock Q-Jet intake and rebuilt Q-Jet. You could sell the dual quad set-up and use that money to finance a manifold and carb and get the car back to a stock configuration.

If you do a search using the "Search" feature of the forum and search for carburetor suggestions, Q-jet, or intake, you should get a number of hits that will give you some ideas. You don't have to buy any special intake as the factory cast iron is one of the best and you should be able to pick one up for $50-$100. Now you can get an aluminum version if weight savings is your thing, but its not needed and you will have to pay higher for it.

You don't have to go numbers matching on the Q-jet unless you insist. You do want a "Pontiac" Q-jet due to the inlet location of the gas line. I suspect you might have to buy a new gas line from the pump to the carb if yours has been cut or modified. A reproduction line would simply make it easier for you to hook up with no grief. You may also have to buy a throttle cable or bracket if yours has been modified or swapped out, but yours may be still factory so it could be OK for you.

Option #2 would be to keep the dual quads, but run off of one carb only. I did this on one of my old cars back in the day to get the look. The engine was stock and would have never run with both working. So I fabricated a thin steel plate and put a gasket under it to seal off the carb, and then bolted the carb on. Ran a "slip" linkage that made it look like it was hooked up and the dual air cleaners. Fooled the unknowing, but not the real motorheads. Still, I got the look I wanted and it ran well with the one carb working.

I am not a fan of Holley carbs and never ran one, so I can't steer you in that direction. I always ran the Q-jets on the Pontiacs I had as a kid and rebuilt them to run pretty good.

You can continue to play around with the duals, but you have to do it step at a time, tweak at a time in order to know what works and what does not. It can be a timely process.

As far as "hotter" coil, there are several, but it boils down to opinions. I have used the auto store 40,000 volt coils with success and have also used the Pertronix coil for a stock look. I use an 8mm wire. Prefer a distributor cap with brass electrodes as well as the rotor. Points are fine and can last a long time. Just make sure the gap is correct and you set the dwell.

Plugs? Found this article which states the older copper plugs are better for older cars and that newer type plugs may not run as hot as the old type copper plugs - which might be something to consider: "Many people ask "Can I use a platinum spark plug in my old car?" The quick answer is "yes", but you may not want to. The copper plugs designed for older ignition systems were of a higher heat range than their equivalent platinum or iridium brothers. A hotter plug means it retains more heat in the core of the plug. Colder plugs shed heat more rapidly, keeping the plug "cooler." Using a platinum or iridium spark plug in an older engine may lead to the fouling of the plug rather rapidly because the plug is too cold to burn off any excess fuel and oil that get into the combustion chambers of the engine. Burning off extra fuel and oil is what the hotter copper core spark plugs did very well. Today’s engines run cleaner and ignition systems are more powerful. The need for copper plugs doesn’t exist. Platinum plugs are the spark plug of choice for most vehicle manufacturers."
 

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Discussion Starter #13
Thanks again Jim. I think I'm going to continue to dick around with the dual quad set up over the summer and possibly into the fall. If nothing else, it will be a good learning experience. Over the winter I'll need to make a decision as to whether or not I'm sticking with the dual quads. I already ordered a "hotter plug" that I should have in the mail tomorrow (gotta love amazon), they are iridium as I was just trying to go one plug hotter. Since the copper plugs are cheaper I may wind up just ordering a set of those as well while I'm screwing around and learning.

My plan this weekend is to pop in the hotter plugs, correct the things you pointed out on the secondary carb ie. idle screw and air mixture screws, and try to dial in the linkage so that the secondary isn't coming on as soon.


Thanks again
Tooth
 

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Sounds like a plan. You can't learn if you don't play around with your car. I prefer dual quads for looks. My project 455 will get dual quads EXCEPT I gotta have a tunnel ram. I don't like the aftermarket tunnel ram that is available for Pontiac engines as it looks more like an old Pro-Stock intake and does not have the individual runner look I want. So, I am going to have to try my hand at fabricating and welding one up. Got an older big block tunnel ram top which positions the carbs at a slight sideways angle for that "cool" effect and I will have to use some adapters to mount a pair of Q-Jets and then fabricate all the rest. Never did anything like this before, so it may or may not turn out OK - but I like a good challange. If it proves to be a bust, then I'll go with an available aftermarket dual quad intake like yours and call it a day.

If you are willing to experiment, I would pull out the springs under the step-up pistons (check out the Edelbrock carb manual). Simply unscrew the small screw that holds the cover/plate over the top of it. Lift up/out the step-up piston and metering rod. Lift up/out the step-up spring, then set the step-up piston and metering rod right back into place.

My thinking is that this will keep the metering rod down at its lowest/leanest position as opposed to being lifted up to richen the already rich fuel condition. By doing this you should be creating a leaner condition on the primary side of the carb at wide open throttle. You won't be changing anything at idle or lower engine speeds.

The piston/metering rods are held down by engine vacuum and as vacuum drops (carb opening up) the springs don't have enough vacuum to hold the piston/metering rod down in place and it rises to allow the additional fuel needed to feed the wide open engine. The metering rods are tapered, so they are thicker near their tops which when the power valve is bottomed out under vacuum, restricts the amount of fuel going through the jets at lower vacuum. The Taper narrows towards the bottom and as the spring overcomes the vacuum of the engine, it raises the power piston up and extra fuel is able to flow due to the narrow taper allowing more gas to go through the jets.

You are not doing anything that can't be reversed as you can put the springs right back in again. These springs have different colors which are rated based on the engine's vacuum. They are part of the Edelbrock tune-up kit. If it runs better and you find the plugs are not running so rich, then you know you are going in the right direction with the carb - which is to lean it out. This would tell me that your jets need to be dropped down a couple sizes as they are too big for your application.

If the carb becomes too lean, it will typically "pop" or stumble when you floor the pedal as the engine is sucking in too much air and not enough gas - stop right there and reverse whatever you have just done as you will be going in the wrong direction in your changes. Any testing out on the road you do, make sure you always have the air cleaners in place and I always suggest a good fire extinguisher in the trunk. They are very cheap and can be bought locally at your auto parts store. :thumbsup:
 
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