1969 GMC backfires

[Razr]

I have a 1969 gmc c25/pick up. I put two cherrybomb glasspacks about 2 weeks ago and the truck backfires when i decelerate from high or medium rpm. It never did before. wats wrong. All it is is the glasspacks and no muffler/exahusts.

[lts70]

Nothing is wrong. Should sound pretty good. My 2 year old son claps and giggles when my truck back fires on deceleration.

[Mayo]

my truck backfires when I shut it off

[Stock72c10]

Mine does that too, pretty cool, huh! My neighbor calls it my Jake Brakes.

[Wrenchbender Ret]

Your idle jets are lean. Try turning them out about 1/2 turn. I my young days we used to turn the idle jets way in to get it to do this. It sounded cool. I don't have to be cool anymore.

[lks dcvn]

Sounds like an exhaust leak near the manifold - that can cause a backfire when letting off the gas quickly after accelerating...

Just had my new exhaust installed - it would backfire as loud as can be (scare the crap out of me) when I took my foot off the gas on the highway.

[Stock72c10]

Originally Posted by Wrenchbender Ret

I don't have to be cool anymore.

In that case, just buy a f@rd

[69halfton]

yip, mine does it when driving with open headers, it winds down real nice like 6000 rpm to 2500 or so, then crackles down to an idle. i think it sounds cool

[Longhorn321]

When you close the throttle and the engine is still trying to suck throught the carb, it gets a richer mixture. This rich mixture is not needed since you are not under power so a lot of it is not burned on combustion in the cylinders and finds its way into the exhaust where it finds lots of oxygen in a free flowing exhaust. Heat from exhaust + rich mixture + Oxygen =Result....Ka Booom!

[highperf4x4]

Yeah it's unburned fuel igniting inside your new glasspacks. What's your mpg like?

If you have a manual trans and a carb you can turn your engine off on the highway wait a few seconds and then turn the switch back on and listen to that explosion!

[Razr]

well how can i stop the backfiring i need help to stop it.plz.

[highperf4x4]

There are alot of things to check. Your spark plugs and wires, distributor cap and rotor. Your carburetor. (what type of carb are you running?)

Do a compression test on each cylinder. If you have one that's low, it's probably not igniting the fuel like it should.

[Razr]

its a 4 barrel edelbrock carb

[highperf4x4]

What model? How old is it? Missing any linkages? Does the choke work like it should? Do you know how to adjust it? Is it leaking gas anywhere?

[highperf4x4]

Originally Posted by Razr

I have a 1969 gmc c25/pick up. I put two cherrybomb glasspacks about 2 weeks ago and the truck backfires when i decelerate from high or medium rpm. It never did before. wats wrong. All it is is the glasspacks and no muffler/exahusts.

First off, a glasspack is a muffler. And when you say no exhausts do you mean no pipes behind the glasspacks? What type of exhaust manifold/headers are you running? How far are the glasspacks from them. What size are your glasspacks/headers/pipes? Provide as much info as you can. What type of heads/intake. How old is the motor? Do you know how to do a compression test?

[Razr]

i mean there is no pipes out of the glasspack its just the glasspacks and notin else..

[highperf4x4]

I'm sorry razr I don't have enough info to help you much. I will say this, if you put a pipe after those glasspacks they will probably stop the vacum pop.

[WorkinLonghorn]

Just fix the leak.Tighten (to recommended torque)or replace doughnut gaskets as needed.

[mr48chev]

Before you play with the pipes, check the timing and vaccum advance. They will backfire when slowing down or pop through the pipes if the timing is late.

But I will agree that it may be going a bit lean when the throttle plate closes.

My truck was doing pretty much the same thing and according to my vacuum gage the timing was late but when I swapped carbs I went up several sizes on the jets so I don't actually know which change did the most good but it doesn't pop when I slow down now.

[Longhorn321]

Originally Posted by mr48chev

But I will agree that it may be going a bit lean when the throttle plate closes.

.

Throttle plate closing will cause a rich condition...not lean. You are shuttling off the oxygen supply so it's rich...unless you have a fuel injected engine in which case the injectors are sometimes shut off which would result in a lean condition. He has a carb tho so throttle closed = rich. Also, lean would prolly not cause a backfire or "burble".

[WorkinLonghorn]

I put two cherrybomb glasspacks about 2 weeks ago and the truck backfires when i decelerate from high or medium rpm. It never did before.

So putting on glass-packs will change the timing and carb settings?? interesting

But I would still just find and fix the leak in the exhaust.

[highperf4x4]

How do you know he has an exhaust leak?

[WorkinLonghorn]

How do you know he has an exhaust leak?

Well,He changed his mufflers and all of a sudden started getting backfires. It happens to me too and I always find a leak,that's all. I could be wrong.

[Razr]

how do i adjust the carb and how do i fix the exhuast leak ...wat do i have to do

[WorkinLonghorn]

If it wasn't backfiring with the other mufflers,forget the carb for now. Make sure the exhaust manifold is torqued to specs.(inside bolts 30,outside 20) -assuming it's a small block with stock manifolds.Then check the dougnut gasket clamps just past the manifold. I can't remember the torque for these,get a manual or just make sure they are very snug.If you see signs of exhaust blowing out of the joint and the nuts are tight,you may have a bad gasket.