After-firing on deceleration

[JRoper]

My 1982 R100S after-fires throught the exaust on deceleration. I get a slight popping and gurgling sound. No real big bangs. I have asked about this problem before and someone suggested I spray carb cleaner around the carbs while it is running and if the idle goes up, there is a air leak. I tried this test and no leak. I have adjusted the valves, sync'd the carbs after a rebuild, checked the timing, replaced the fuel lines and filters, and replaced the throttle cables. It has plenty of power and idles well after warm up. I had someone suggest it could be an exhaust leak. The cross over pipe doesn't have any clamps on either side. Could that be the culprit? Another person said it could be the valves are getting old and may need trimming. If that doesn't help, a full valve and seat replacement is in order. I can live with the after-firing, but if I can fix it easily I will. Why mess with the heads if it is running OK.

[bmwchuck]

Have you tried the NGK BPR7EIX IX iridium spark plug? My 81 R100s ran a lot better with this plug. Advanced Auto had to order them in and I got them the next day. Cost was about 6 dollars per plug. It might be a cheap fix for your problem.
Chuck

[James.A]

My R75/5 started doing the same thing, very slightly, after a valve/seat replacemnt operation. Personally, I think it is running a little rich. Un-burnt fuel/air in the exhaust.

[Na Cl K9]

My 1982 R100S after-fires throught the exaust on deceleration. I get a slight popping and gurgling sound. No real big bangs. I can live with the after-firing, but if I can fix it easily I will. Why mess with the heads if it is running OK.Actually it sounds like your carbs are set a little lean. The "80-on' bikes were factory tuned to the lean side to meet EPA requirements. That usually results in a pop or two when you back off the throttle. Try re-setting the mixture screw by turning it IN 1/8th of a turn on both sides and see what happens. Turning it in richens the mixture a bit and should clear up the pop. If the mixture screw has to be turned in more than say, 3/8ths of a turn you may have to raise the needles. The key point you made is that the bike runs fine after it warms up...so it IS running lean but not too lean. Does your bike still have all the EPA plumbing intact and functioning?

[20774]

Try re-setting the mixture screw by turning it IN 1/8th of a turn on both sides and see what happens. Turning it in richens the mixture a bit and should clear up the pop.

It was my understanding that on the Bing CV carbs, the mixture screw is in a gas circuit. So, if the screw is turned in, you'd be closing off gas flow and making it leaner still.

[lkchris]

You fail to mention whether the factory air "ingestion" system is still installed.

A bit of burbling on overrun is normal when it is. Excess fuel on overrun is common and it's just the function of this system to ensure it's burned in the headers by introduction of fresh oxygen. When I had it I could actually induce burbling with certain throttle openings just for fun.

Most likely cause of backfires, however, is air leaks between carb and head. This is either or both leaking rubber tubes or the intake stubs loose in the head. It's pretty common for the stubs to unscrew and it's a bit difficult to stop it.

To think brand of sparkplugs makes a difference in this regard is sheer fantasy.

[Na Cl K9]

It was my understanding that on the Bing CV carbs, the mixture screw is in a gas circuit. So, if the screw is turned in, you'd be closing off gas flow and making it leaner still.Yes, I know it seems counter-intuitive and I thought about it for awhile because I have NOT thought about it in awhile. Trouble is, I've been tuning these carbs since 72 and turning the mixture screw IN gets rid of the backfire on deceleration...a problem which is brought about by a lean condition. Normally the way I adjust the idle mixture is to turn the screw in till the engine begins to falter. Then, I back it out while counting the turns until it begins to falter again. I split the difference and then turn the screw in 1/8th of a turn. That setting reduces or eliminates the deceleration pop. Truth is I could be just backwards in theory while the process works.

Another point is that by doing this, a sign the engine is running at a richer carb setting is there is more soot in the exhaust exit point on the mufflers. First take a minute and clean the exit point of each muffler about three inches inside the tube. After a 10 mile ride, pull over, get off and check the density of soot in the muffler by putting your left finger in the left muffler against the inner wall and draw out a quick sample. Do the same thing with your right hand/muffler and compare the two samples. You can tell quickly which carb is lean or rich if the samples differ in density. Faint is lean and strong is rich. This is an old racers trick and simpler than pulling the plugs. Re-adjust the mixture based on this test and ride another 10 to see what difference it makes.

Excess fuel on over-run is a product of switching circuits in the carb by abruptly closing the throttle. If you close the throttle slowely, it should not pop.

[JRoper]

You fail to mention whether the factory air "ingestion" system is still installed.


I have removed the EPA system and plugged the holes as recommended by my local BMW shop. They provided me the required plugs. When I adjusted the carbs after rebuild, I set them for maximum idle like outlined in various carb sync instructions. I'll try giving them an 1/8th of a turn in and see what happens. Sticking my finger up the exaust pipe is getting rather personal, I guess the bike and I have known each other long enough.

[Na Cl K9]

Sticking my finger up the exaust pipe is getting rather personal, I guess the bike and I have known each other long enough.That's way too funny ;-) When you get better acquainted then...don't get burned.

When you removed the air injection plumbing and rebuilt the carbs did you make any jet changes or needle position changes according to what your shop might have recommended? Just curious.

[JRoper]

When you removed the air injection plumbing and rebuilt the carbs did you make any jet changes or needle position changes according to what your shop might have recommended? Just curious.

No, I didn't make any jet or needle position changes. I just cleaned the gunk out of the carbs and installed new o-rings, diaphragms and gaskets.

[RandyB]

Most likely cause of backfires, however, is air leaks between carb and head. This is either or both leaking rubber tubes or the intake stubs loose in the head. It's pretty common for the stubs to unscrew and it's a bit difficult to stop it.

Grab a strap wrench and pull both stubs. Clean with acetone and a steel toothbrush and reinstall with a lot of hi temp thread locker. Unless the rubber tubes are new, replace them "while you're in there."

I fought this for too long before I did this. Then check your idle mixture. That fixed it on both my Airheads.

[JRoper]

Grab a strap wrench and pull both stubs. Clean with acetone and a steel toothbrush and reinstall with a lot of hi temp thread locker. Unless the rubber tubes are new, replace them "while you're in there."

I fought this for too long before I did this. Then check your idle mixture. That fixed it on both my Airheads.

I have already replaced both rubber tubes between the carbs and cylinders. Wouldn't a leak at the stubs show up with carb cleaner being sprayed in the vicinity while the engine is running? I was told that the idle would rise indicating a leak.

[lkchris]

When you removed the air injection plumbing and rebuilt the carbs did you make any jet changes or needle position changes according to what your shop might have recommended? Just curious.

There are no changes required as result of removing the air injection system.

[RandyB]

So I hear, but I have no luck with it. Last time I sprayed it, nothing happened and I could back the stubs out with my fingers. Glue them in and it's one less variable.

[nealart]

Grab a strap wrench and pull both stubs. Clean with acetone and a steel toothbrush and reinstall with a lot of hi temp thread locker. Unless the rubber tubes are new, replace them "while you're in there."

I fought this for too long before I did this. Then check your idle mixture. That fixed it on both my Airheads.


I have this popping problem. I checked the stubs a week ago and they seemed tight.

Today I thought I'd better check them again and the right one turned right out by hand.

Maybe the first time the engine was warm.

I just picked up some high temp thread lock and I cannot wait to listen to the non-popping engine.

[yaktackler]

I have the same issue - what are these stubs you speak of?

[nealart]

The metal intake that is between the carb and the head.

It screws into the head and can come loose.

It is inside of the rubber connection between the carb and head.