Topic: popping from exhaust

[the_genrl]

when im riding at 3k ish, and cut back on the gas (engine brakin') the engine whines down with an erratic popping.  No stall or anything bad, just annoying.  It's not very rapid, like fast popping (or breaking up) on some motors, but maybe a pop every second or so until im at an idle.

I've heard this phenomenon from 1000s of motors on everything from Ore loaders to Z50s, but i dont know why.  Anyone know why?  Or...maybe a way to make it go away? 

[TX300BILL77k7]

Well i am pretty sure that the popping noise you hear is caused by extra fuel being pulled through the carbs by the vacum that forms inside the carbs during engine brakeing. The fuel does not burn becuse there is not enough air in the mixture for a proper burn, so on the exaust stroke the raw fuel gets dumped in to the air inside your hot exhuast pipes where it ignites, causing the popping and crakeling you hear on decel.

[TwoTired]

Well, I don't know what bike or setup you have,  but the decel popping can be reduced by leaning the idle settings or adding some back pressure to the exhaust system.

Closing the carb slides while the engine is spinning at 3K causes very high vacuum on the slow jet system.  This causes more fuel to flow from the slow jet due to the increased pressure differential.  If you have an Iidle Mixture Screw, turning that in should improve your annoyance.  If you have an Air Bleed Screw, turn that out a bit. If the slow jet emulsion tube holes are blocked, the fuel/air premix can also be over rich and increase/cause the popping phenomenon.  This is interactive with the IMS or ABS.  Adding back pressure in the exhaust, retains a bit more of the CO2 in the pipes and spoils the conditions for ignition in the pipe header.

The popping can also be caused by leaks in the exhaust letting air into the header.  The header to head gasket, for example.

[Lumbee]

...I'll concur with twowired on the exhaust leak...my bike started popping after I snapped the ring off the end of one of my exhaust pipes (the ring that presses agains the head).  I jbwelded it back, but when I put my hand in front of the head I can still feel some exhaust leak, along with popping when I change gears.

[dpen]

Check the fit of the muffler(s) to the headers. A lot of 4 into ones are a sloppy fit & any air gap will cause backfiring on the over run. (sounds good too).

[Kamerer]

Don't know what bike and mods you have, but most of these bikes have an air cut-off valve that corrects this problem.  There is one on each carb and it serves to bypass the butterfly to allow a proper air/fuel mix on deceleration and avoid this lean condition.  If you have a functional/intact exhaust, this is often the cause of this problem.

You can get at only the cut-off for #1 cylinder with the carbs on the bike - it is the round "manhole" looking thing with two screws on the side of the carb.  There is a spring, a diaphram, and an o-ring to seal the tiny air passage when the cover is screwed on. 

If you have recently worked on your carbs, one of these may have been mis-installed or an o-ring omitted (BTDT).

If you do a google search for "air cut off valve" you will find some hits that explain how this works, diagrams, etc.

Bryan
Seattle