What is making my exhaust backfire as it is throttling down. It has sidepipes and when I start slowing down when driving it will backfire. I always thought this was due to timing but changing it doesn't have any effect. Thanks Larry

 

You could have an exhaust leak and when let off the gas air is sucked into the exhaust and mixes with the hot unburned fuel and causes it to explode.

 

Experiment by turning the idle mixture screws out a quarter turn at a time.
You can eliminate 90% of that noise.

 

I like that sound. The C6R race cars pop very loudly on deceleration in gear. My DeLorean does it, and because of the short exhaust on a rear engines car, sometimes flames are visible. My 79 Vette also does it.

 

Hi tl,
I believe a backfire through the exhaust while decelerating can sometimes be caused by how the engine's timing is set.
Perhaps you might want to check that?
Regards,
Alan

 

That's what I always was led to believe but it doesn't seem to change the problem by changing the timing. Which would be more apt to cause the problem? Too advanced or too retarded?

 

What are you running for a distributer? Could be a problem with the advance

 

I was just reading last night you maybe running a little lean. Im trying to diagnose a carb issue right now as well

 

Probably too lean if it's a mixture issue.

 

Back Fire as in a load BANG or a bunch of burble?

 

Had same symptoms, found solution in post by Lars, i have it still. Is it considered ok to post a picture of his post? Not sure of forum etiquitte or spelling of etiquitte either

 

Timing too far advanced will backfire like this:



Retarded timing will give you after fire (out the exhaust pipes).

Leave you timing as is was and adjust the idle mixture screws to richen up the mixture and the after fire will go away.

 

The backfire is just small pops that are just annoying. The dist. is a stock GM HEI with initial timing around 10 degrees with total timimg around 36 degrees. Didn't notice the problem until I put the sidepipes on. I'll try opening up the adjustment screws a little.Thanks everyone for all the advice. Larry

 

Pretty sure you can share. I've seen others do it. If in doubt PM it to me. Thanks Larry

 

It's usually a combination of things that cause after fire popping in the exhaust. Lean idle mixture, retarded ignition timing and exhaust leaks.

Make sure your vacuum advance is working properly. If it is not working, it will contribute to the after fire popping.

Set your idle mixture screws with a vacuum gauge. Start by turning them all the way in and lightly seating them. Turn them out 1 1/2 turns to start out with. Now start the car and fully warm it up. Hook the vacuum gauge to manifold vacuum. Start turning each in a little at a time. If the reading on the gauge goes up, keep turning them in until the reading starts to drop, then turn them back out until you find the highest reading on the gauge. If the reading goes down when you first start to turn the idle mixture screws in, just go the other way and start turning them out until you find the highest reading. Make sure you turn both mixture screws the same amount on both sides.

Now check very closely for exhaust leaks. Just a small leak will contribute to popping. Fix any leak you find.

If you do all of the above, it should eliminate or at least minimize the after fire popping.

Mike

 

I know I'm probably alone in this camp but if the above advice doesn't work for you. ...try this.... Set baseline mixture as stated above with mixture adjustment backed off from seated 1 1/2 turns. Street test with engine at normal temp....if it still pops on deceleration turn each mixture adjustment clockwise(leaner) 1/4 turn and street test again. Repeat as needed but you still need decent idle when finished. If you have to crank mixture in a lot you may have internal fuel leaks in the carb. My take on this is on deceleration with throttle plates closed, vacuum goes high drawing in fuel from the idle ports, raw fuel looking for ignition....red glowing carbon deposits in exhaust system. I've had luck with this a few times. Good luck.

 

Go ahead an re-post anything I've posted.

Bottom line is this: Afterfire (popping in the exhaust) is almost always caused by a lean condition. Not by timing or a rich condition. Richen it up.

Lars

 

I usually agree with all you say on here, but I have to disagree about the timing not affecting decel pop. If you have ever ridden an old Harley with the timing control on the left grip on the handlebars, you would find that when you have the distributor advanced and decel, there is no pop. Retard the timing and decel, it will pop like crazy. I learned this decades ago. In tuning fuel injection I have cured decel pop many times by yes richening the decel area, but sometimes with fairly open exhaust they still pop. Advance the timing in the decel area and they stop.

Leaks can cause decel pop also.

Mike