jpc647

I took my car out for a spin tonight and when I let off the gas at 50mph or greater the car pops a few times(3 or4) out of the exhaust. 72 cutlass 350 motor th350 trans dyna flow dual exhaust. Is this the car running lean? Should I turn the carb screws a 1/4 turn for more fuel?

coppercutlass

there might be a hole in the exhast or a loose clamp somewhere. When i drag race and run an open exhaust it pops about 5 to 4 times . I have been told it's whatever unburnt fuel getting re ignited as it is exposed to oxygen while exiting the exhaust usually if the leak or hole is closer to the manifold.

jpc647

Hmm. I don't really hear an exhaust leak Or a tick, but I'll listen more carefully. Thanks. Anyone else have a similar problem is causes of this? It used to happen when I ran wot up to 100mph is so and let off. I always thought that was probably normal though.

jpc647

bump for more thoughts?

Whats the best way to trace a potential exhaust leak? Besides on a cold night, cold start with a super bright flashlight.

TripDeuces

Sounds more like a lean condition. Did you add headers to the car? Idle screws aren't going to effect the popping, you'll have to richen the jets a step if it is lean

jpc647

No, I didn't add anything, except did a tune-up. I don't remember it doing this when I bought it. If it's running lean, maybe there is gunk in the carb. I'll throw a can of that cleaner in the tank, and see what happens.

coppercutlass

I know my car popped bad when i had a header leak at the flange. i snugged it up and it went away. If it was lean wouldn't it show on the plugs trip duces? i know it's only on deceleration., but if it was lean on deceleration wouldnt it run hot in idle since when you decelerate you are on the idle circuits. Not being a smart A$$ here either I just thought jetting wouldnt apply to idle circuits.

rustyroger

British Minis were notorious for this, normally we fixed it by slowing the idle to the correct speed, is your idle a bit high?.

Roger.

jpc647

Why yes, I think it might be. When I shift from park or nuetral into gear, it does jerk a tad bit as I increased the idle a bit for a smoother idle. We might be onto something here.

coppercutlass

Have you tried to turn up the idle speed

455man

My car used to do this. not sure what i did to make it stop. I was thinking maybe timing?

coppercutlass

my car always pops after i get on it take it to about 6k rpm. If i let off quickly it will pop if i slowly take it to the closing position it wont do this . if the idle was high it would lunge in drive it could also be jerking due to not a high enough idle play with it a bit mark your intial settings. Another intresting point i got into the habit of slapping my car into neutral after my runs in the 1/4 mile when i did this it never popped. when i started leaving it in gear it always popped sometimes less sometimes more. I just think it's the fact the engine is decelerating with a load or in gear and it took you a certain amount of gas to get there now you are going from 4k to 600 rpm with what ever gas it idles with . That just my opinion i might be wrong .

rustyroger

I think coppercutlass has put his finger on the cause, essentially the cylinders are still partially filling with air/fuel mixture, in effect making the engine act like it has a very low cr. The mixture is still burning when the exhaust valve opens, with the butterfly fully closed only a tiny amount of mixture goes in, not enough to cause popping in the exhaust.

Any engineers here able to tell me if I'm wrong?, it's only my theory, not backed up by any research.

Roger.