Question about backfire on throttle release

So I've rebuilt my carb, installed new rotor, distrib cap, ignition coil, plugs, installed new header gaskets and collector gaskets. Things were running fine, but a little weak on acceleration and a rough first to second gear shift. After a lot of reading I decided to change out the vacuum modulator to see what it would do for me and it helped the rough shift a lot, and also gave me a decent amount more acceleration. I had to readjust my carb to run better with the change in vacuum and it's the peppiest the motor has ever been. Run's absolutely great. The only problem is the backfire on throttle release. When i'm under load and driving at mid to high rpm and release the throttle to decelerate (like a highway exit) it cackles and backfires out the exhaust a LOT. Before I adjusted my mixture it did the same just not nearly as bad and was manageable. I would love to keep the carb set up just how it is but I gotta get rid of all the popping. Any suggestions where to start?

It's more than likely an exhaust leak somewhere.

What's your timing set at?

Maybe the float level’s a little too high or the idle mixture is way too rich. It sounds like that’d be the direction you adjusted it, and then there’s another issue going on that richening up helped to mitigate. I’d keep this in mind, and my next concern is definitely timing. It wouldn’t surprise me if you were running at like 1*+ ATDC, and you’d need to advance the base timing a good little bit to burn that raw fuel going into your exhaust.

Used to be GM installed a mechanism on the carb linkage to slow the closing of the throttle plates to prevent backfire.

I'd agree on the timing being just a little off. but cackling on deceleration isn't necessarily a bad thing, it means that your tune is pretty close to good. popping like that through the exhaust is the ignition of unburned gasses in the exhaust system which would generally mean you have a good scavenging effect. as mentioned above, I'd try advancing the distributor a few degrees to see if that solves the problem. And make sure the vacuum advance is hooked up and working as it should.

Here in a little while when I get the chance I'll hook up the light and mess with my timing a bit and see if I can get anything figured out. How far advanced are we talking here?

If that doesn't work, I'll be back!

I’d start with 6* BTDC base, and if it’s better, see what 10-12* BTDC does for you.

1987 GMC Jimmy said:

I’d start with 6* BTDC base, and if it’s better, see what 10-12* BTDC does for you.

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what he said, and if you haven't also make sure you have the vacuum advance unhooked and plugged when you are setting your initial timing. And once you get it dialed in, hook it back up.

So I timed it about 8* BTDC with advance off, and voila no more popping. But I have lost the improved acceleration that I had before. Even playing with the carb it it slower, maybe a little more “boggy” on the get go. But other than that it’s running good. I wonder where I’m losing that low end?


Edit: Side note question. I don't really understand how to actually measure my timing once I get my advance hooked back up. I've seen in the forums what it should add to it approximately, but the pointer obviously only goes up to 14(?) or so. How are people getting accurate readings way higher than the stock pointer? I understand that if my overall is higher than 36 I need to adjust the curve but I don't know how to see how high I actually am.

Iamthewalrus said:

So I timed it about 8* BTDC with advance off, and voila no more popping. But I have lost the improved acceleration that I had before. Even playing with the carb it it slower, maybe a little more “boggy” on the get go. But other than that it’s running good. I wonder where I’m losing that low end?


Edit: Side note question. I don't really understand how to actually measure my timing once I get my advance hooked back up. I've seen in the forums what it should add to it approximately, but the pointer obviously only goes up to 14(?) or so. How are people getting accurate readings way higher than the stock pointer? I understand that if my overall is higher than 36 I need to adjust the curve but I don't know how to see how high I actually am.

Click to expand...

With keeping the OEM pointer you could get some timing tape and put it around your balancer starting at the zero mark. this will give you a larger range. Have to make sure the tape is placed 100% correct then whatever number on the tape lines up with zero on your pointer is what you'll be set at. Some people including myself just get a balancer which has the timing marks in it, then get an aftermarket pointer that has just one point. works well but is more involved.

Or depending on your timing light ... you can have a offset... rev it up to 2500 and ad just the offset till your at zero ... then look at the offset and it'll tell you your total timing ... I run mine at 35 ... cam heads and headers

Did it ping with more timing than that? If not, it take it all the way to 12* base and see if you can recover some of what it lost.

@1987 GMC Jimmy I don't know exactly what you mean by pinging, but I don't hear anything else other than the lifter that I already needed to adjust.

I’m talking about detonation or pre-ignition. It’s hard to describe, almost like a tinny, crunchy sound. If it’s not doing it at idle or under load/acceleration, you’re in good shape, and I’d press for the highest number possible. 350s seem to be universally safe at 12*. Some can handle more, a few can’t handle that much.