Exaust backfiring - Page 6 - Pontiac GTO Forum
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post #51 of 65 (permalink) Old 03-31-2019, 08:05 AM
 
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So think of it this wat CJ....total advance and when it comes in are “unrelated” for the moment of the discussion.

Total advance we know is centrifugal +Base and is the most important number......so if it keeps advancing to 4 thousand RPM etc, where it stops is the most important number,...let’s say it stops at 40.....

Now subtract your base, say you had 10, you now know that have 30 degrees of centrifugal advance in the distributor,....your base should then be set at 6......30+6=36......springs have no say in this......

Now if your vac can is pulling 25, I see that all the time by the way, ditch it and get The right vac set up, depending if you are running HEI or not.

I can give you those specifics if you don’t already have them.

You can change the vac can in the car. It is two screws and a little finesse, but easier to do it on the bench.

If your total is way too high, then you have to add a bushing or change the weights and center piece or a combination.

Springs are last, once all this is right then you can experiment with different springs, and they cannot go over your total advance, they just adjust WHEN that total comes in.
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post #52 of 65 (permalink) Old 03-31-2019, 12:18 PM
 
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Lemans guy, that makes sense. But I read elsewhere you wanted a base timing of 10-14 with vacuum of 10, so 24* at idle. The problem is I’d be more like 6-8 * base to limit total to 36*.

Is there a way to correct for this?

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #53 of 65 (permalink) Old 03-31-2019, 03:34 PM
 
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Cj the most important number is the 36.....so let’s say you have 28 centrifugal and add 8 base for 36....

Now to that 8 base you are going to add 10 degrees of vacumn advance.....so idle will be 18 degrees....it will idle great and run great.

Then just do the springs so it comes in reasonably early, really 2800 to 3600 is good.

You don’t have to have 24 degrees at idle although cars with very radical cams can handle more idle timing. If you have to get then you have to use a bushing on the centrifugal weights or use different weights and center piece.

But if your numbers are like I said above,..I would run it like that if it does not ping or knock, then try turning the base up 2 degrees to 10 base....for total of 38.....andd then your idle would be 20...........10 base and 10 vacumn

Lot’s of Pontiac engines can take that much some more, but compression set up intakes all play a roll.
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post #54 of 65 (permalink) Old 04-02-2019, 05:55 AM Thread Starter
 
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Lemans guy, that makes sense. But I read elsewhere you wanted a base timing of 10-14 with vacuum of 10, so 24* at idle. The problem is I’d be more like 6-8 * base to limit total to 36*.

Is there a way to correct for this?
My problem was fixed after eliminating everything.

But the problem was a distributor problem, i got an msd distributer with an msd ignition box and set TDC (top dead center) once again.

Then i was playing with the timing.

The stock msd has 18* of mechanical advance (stock spring - black)

So to get 34* of total timing (works with me) i set the initial timing @ 16*.

Got new spark plugs as well.

This is what cured my backfiring issue and the engine started running strong and healthy!
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post #55 of 65 (permalink) Old 04-02-2019, 12:50 PM
 
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What is your vacuum advance?

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #56 of 65 (permalink) Old 04-03-2019, 04:29 AM Thread Starter
 
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What is your vacuum advance?
I run an MSD pro billet distributer without vacuum advance.
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post #57 of 65 (permalink) Old 04-13-2019, 12:22 AM
 
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So more confusion to add to the timing advice / debate....

I have total set to 36*. My distributor has a mechanical advance of 30*, so this means my base is 6*. The vacuum can that came with the distributor adds 25*, so at idle I am at 31*.

If I were to add a vacuum can with just 10*, then my idle timing would be 16* which is way to low.

I have been told by someone far more experienced than me that my current setup has a base timing too low and that I should be more like 16* + 10* vacuum canister which would yield 26* at idle with vacuum and that I should get a distributor with 20* mechanical advance. Frankly I don't understand how the two setups would be materially different as my setup will have a more aggressive mechanical ramp that should offset the lower base timing.

What I am wondering is if it would be worth it to make all of these changes to the distributor ? Maybe I am running too much timing (e.g. on deceleration / engine braking) thus causing the backfire? But under WOT, the car feels great.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #58 of 65 (permalink) Old 04-13-2019, 08:20 AM
 
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cij911 - "I have been told by someone far more experienced than me that my current setup has a base timing too low and that I should be more like 16* + 10* vacuum canister which would yield 26* at idle with vacuum and that I should get a distributor with 20* mechanical advance."

PJ - You might want to let this guy have your car and let him set it up for you.

You have been given the correct info by Lemans guy as well as TheGoatFather1965 and don't seem to want to take their advice and I am not sure why not? If you talk to 100 people you are going to get 100 suggestions - only adding to your confusion.

Maybe you want to start from scratch. Pull the distributor and do a rebuild. Check the gear for wear and endplay. Then go from there. With all this difficulty, I'm thinking you have a bad/worn distributor.

Last edited by PontiacJim; 04-13-2019 at 08:35 AM.
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post #59 of 65 (permalink) Old 04-13-2019, 11:07 AM
 
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cij911, it would be best if you began a new discussion and title it "Backfiring II." Then supply all the specs on the engine - year, cubes, heads, compression, intake, carb, exhaust, ignition type, and any other modifications that may have been done. Rebuilt or not. What octane gas do you run. 4-speed or automatic. Rear gearing. Are you at sea level or at high altitude like Denver (this does make a difference).

All of these things may not seem important, but they are in knowing what your combo is and then matching your timing and advance curve to the engine/car.

Start from scratch with the info and then help can go forward from there. Then do what is suggested and provide results so your situation can be dialed in and resolved.
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post #60 of 65 (permalink) Old 04-13-2019, 04:01 PM
 
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cij911 - "I have been told by someone far more experienced than me that my current setup has a base timing too low and that I should be more like 16* + 10* vacuum canister which would yield 26* at idle with vacuum and that I should get a distributor with 20* mechanical advance."

PJ - You might want to let this guy have your car and let him set it up for you.

You have been given the correct info by Lemans guy as well as TheGoatFather1965 and don't seem to want to take their advice and I am not sure why not? If you talk to 100 people you are going to get 100 suggestions - only adding to your confusion.

Maybe you want to start from scratch. Pull the distributor and do a rebuild. Check the gear for wear and endplay. Then go from there. With all this difficulty, I'm thinking you have a bad/worn distributor.
PJ - He is not local, otherwise I would. He is super knowledgeable and very helpful. That said, I am trying to learn from those of you with more experience.

The distributor is NEW and there are NO problems with it, just that the vacuum can it came with has too much advance 25* vs recommended 10*. Additionally it appears the distributor has more mechanical advance than ideal - 30*, which has my base timing at 6* to get a total advance of 36* coming in before 3500. The car as it sits runs well and pulls hard.

What I am trying to understand is why changing the distributor would help. Basically the recommended setup has base at 16* (vs my 6*) and a 10* vacuum can so at idle it would be 26* (vs. my setup at 31*). My understanding is if a car runs well at 31* (vs 26*) it is even better as it will run cooler at idle. At WOT, I am at 36* (just as the recommended), so really no difference there.

So basically it comes down to partial throttle and transient response (which is a major part of normal driving). My question is am I likely running too much timing? While the recommended distributor has base timing 10* more than mine, I feel that is really a meaningless (as described above). So maybe the concern is I am running too much timing mid rpm range with modest load ? That I could understand, but I am unsure if that is true as I haven't seen any signs. Basically I guess the car could see as much as 51* (full mechanical + full vacuum) vs. the recommended setup as much as 30* (full mechanical of 20* + full vacuum of 10*). So is that the issue ? And if so, what would I see / hear / or feel that would indicate it was too much? (I haven't heard pinging, car feels responsive, and stays under 190* under the hardest driving.)

Thanks for the help.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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