Exhaust popping while slowing down

[sven]

Whenever I use the engine to slow down (like when going down a long hill) the exhaust pops inside the muffler. Im afraid that one day it will backfire and blowup my new muffler. What are the causes of this? Truck is a 73 SIII petrol runs very well. I recently installed a new 45D distributor.

[badvibes]

Sven-

Popping could be excess unburned fuel in the exhaust going off, when decelerating, or an exhaust leak. Make sure there are no leaks in the exhaust to rule out that cause. Engine vacum affects the situation also when decelerating, a lean fuel situation occurs, some unburned fuel makes it into the exhaust and may cause popping. You may be able to decrease the popping by playing with the air/fuel mix or timing. I have a 1 barrel Rochester and the lowest flow electric pump NAPA sells. I get a little popping, I'd call it a soft burble, on deceleration. In fact I sometimes feel the engine idle drop significantly after a long deceleration and I finally push the clutch pedal in to come to a stop. I don't seem to be able to totally tune this out of my truck. It doesn't seem to be a problem at this point and I haven't had any "backfiring" since I removed the bad piping and muffler and replaced it with a generic cheapo muffler and pieces from a local parts house. I've timed my truck where it's running good I then have leaned the air/fuel mix for the best idle and throttle response. At this point I live with the burble and call it good.

Jeff

[ducttape]

Interesting, I was just going to post a question about backfiring and saw this post.

No gurgle, but an intermittent backfire upon deceleration. Sometimes it is so loud I thought I may have dropped the whole rear end off the truck.

I guess by your post above I should adjust the carb?

Also, other than aging me and totally freaking me out, what is the downside of backfiring?

Thx

[badvibes]

ducttape-

I'll skip the obvious joke which I'm sure you've heard way too many times. I'd start by being positive I had no exhaust leaks anywhere in the system. A little leak can allow for backfires even if everything else is good. Eliminating that problem then if the backfire continues I'd set my timing and adjust the carb for best performance. Then if the problem continues I'd rich the fuel mix a bit. When the motor is under deceleration, you let up on the gas pedal, the vacum goes up drawing more air into the carb causing a lean situation with the reduced gas flow to the carb. By "riching" the mix you may get more complete combustion and less gas into the exhaust decreasing the chance for backfire. I've read this a couple of times and I think it says what I'm thinking. You may need to tweak your timing also. I think you retard the timing a bit to try to help with this problem.

Jeff

[LH Drive]

Yep, had the same problem and went through all the common checks like badvibes mentioned. I could'nt find any leaks and had come to think it was my old exhaust/muffler system that was the problem. So I decided to disconnect the vacuum line to the brake booster and plug it. It did'nt change much at idle but when I drove my Series truck the back firing went away. It was the brake booster that leaked when I applied the brakes. Because of this my air/fuel mix was off and after the test drive I adjusted the carb again to where it should have been.