Popping sound on deceleration with CIS

[dberman]

I have an 1983 911SC that had been running fine. This spring I removed the cat and put in a bypass. Afterwards the car continued to run fine for a month including running at the track in New Hampshire. Last week while running at Lime Rock, I started hearing popping noises from the exhaust upon deceleration.

The problem happens both when the car is hot and cold.

I've read a number of threads here and I've seen some comments indicating that when taking out the Cat, I changed the backpressure and that could cause this type of issue. I also see alot of people talking about vacuum leaks. I did check that I don't have any exhaust leaks and I did check the vacuum hoses using the carb cleaner method.

Any other thoughts on what may be causing this problem? I'm considering putting the cat back on just to see if the problem goes away.

Thanks for your help.

[Joe Bob]

Decel popping is an exhaust leak. Wet your hand and run it close to the connections while someone blips the throttle. You should be able feel a leak.

Sometimes, you can see backfires in the dark when standing behind the engine bay and revving the motor.

[Staylo]

When was the last time you checked the head studs?

[Joe Bob]

In my experience head studs sound like playing cards in the biicycle spokes on accel....not decel. But I've been wrong before.

[Staylo]

Movement in the heads can also contribute to the popping.
Just one of the boxes to check off IMHO.....

[dberman]

How would I check the head studs?

[Staylo]

You would need to drain the oil, and remove the lower valve covers. A Bentley manual (or a search) would be helpful here.

I would only do this after confirming abscence of exhuast leaks as JB mentioned, as well as validating correct CIS mixture.

[dberman]

Thanks. I'm going to check for exhaust leaks again and then do the normal fuel pressure test and see what happens.

[Hotwatermusic]

Originally Posted by dberman

I have an 1983 911SC that had been running fine. This spring I removed the cat and put in a bypass. Afterwards the car continued to run fine for a month including running at the track in New Hampshire. Last week while running at Lime Rock, I started hearing popping noises from the exhaust upon deceleration.

Any other thoughts on what may be causing this problem? I'm considering putting the cat back on just to see if the problem goes away.

Thanks for your help.

Problem? The same thing happened in my 83SC when I changed out the stock muffler to a Flowmaster. Immediately after that it started popping and cracking on sudden decel--and I love it. Listen to nearly any high-end sports car, they do the exact same thing. No one has given me any reason to believe it's a problem. There's actually several threads about it on here, and no one has give a discernible reason for concern.

[tyro]

^ exactly. Mine does the same as well with it's euro premuffler. I like it!

[pete3799]

I run a euro pre muffler and 1 in 1 out stock muffler and never experience popping on deceleration.
If not an exhaust leak.... check your timing.

[dberman]

I still can't find any exhaust leak. Timing is right on.

The thing about this is that when I first changed out the cat, I didn't have this problem. It started about a month later. I don't understand if it is "normal" why it didn't start happening right away. I'm tempted to put the cat back on to see if the problem goes away.

[Joe Bob]

A free flow, no back pressure exhaust can cause a "burbble" on decel...popping and worse can be an exhaust leak.....the burbble is mostly with carbs.

[john walker's workshop]

you removed some of the restriction by getting rid of the cat, so more noise out the tailpipe. i hear it from customers all the time after installing a performance muffler. all cars burble and pop on decel.

[Joe Bob]

Yeah....what he said.....one man's pop is another man's backfire.....anyways....if it bugs you look for a leak, if none found, backpressure will cure it.

[redridge]

+1... burble, popping is more "pronounced" with a free flow exhaust. Put the old cat back on, most likely it will go away.... because of greater back pressure.

[T77911S]

Originally Posted by pete3799

I run a euro pre muffler and 1 in 1 out stock muffler and never experience popping on deceleration.
If not an exhaust leak.... check your timing.

retarded timing will do it too, but since you modded the exhaust, that is likely the problem.
see if you cant richen the mixture a little and not have surging.

this is the problem wit CIS, you cant change the mixture in the upper RPMS without changing it across the entire RPM range, so mods like this that help it breathe better may not be a bennefit if it runs lean.