Popping in exhuast on deceleration

[Anonymous]

Ok, I have a 1978 Suzuki GS750E, electronic ignition, Kerker 4 into 1 header, accel super coils,Mikuni 29mm smoothbores, the factory air box has been cut open and a K&N element installed. The bike seems to run great, there is not another gs750E around to compare it to. The bike was running really rich, the pilot jet was a 25 and it would start cold without the choke and if you turned it on even a little the bike would die. It still seemed to run good through the RPM range with no flat spots. I decreased the pilot jets to a 20.5 and it now seems to run even better. The front end actual starts to lift a little the only thing I noticed is the exhuast pops on deceleration. I noticed it today on a 85 degree day for the first time. I had accelerated to 9000 rpm through 1,2nd and 3rd. Then I let off the throttle completely coming up to a turn. What causes this? Is it from running lean or could it have a small exhuast leak and sucking cold air into the exhuast pipe. I can't remember. I am thinking maybe it had an air leak before but was not noticeable do to the fact it was running really rich. I think I still need to back the pilot jet down another size. I can turn the choke barely on when it is cold and not kill the bike but it does not seem to help either. I am getting 37 MPG now up from 25.

 

[saaz]

Probably an air leak in the exhaust. If it is only minor popping it is part of the fun!! Be Wes Cooley for a day....

 

[Anonymous]

My 81 does the same thing, always did on heavy decelleration, I always figured it was due to the sudden lack of fuel. Doesn't seem to hurt anything, it still runs :wink:

 

[skip]

lack of back pressure will do it also. what baffle are you running? with the street baffle in my 1150's 4into 1 there is no backfiring but as I install the competition baffle it starts to pop on de-acceleration and if i remove the fiberglass packing it sounds like a popcorn machine during de- acceleration. ...................skip

 

[Jeff Tate]

I had the same thing happen for years with my 1100. Generally this is prone to happen with a less restrictive exhaust system. I got a little annoyed by it after many years and even switched exhaust systems , although not for that reason alone, and it still popped and banged upon deceleration from high revs. I ended up going up one size on the pilot jets which helped a great deal... but it still does it a little now and then. I think just some extra fine tuneing would probably do away with it all together. The more open and louder the exhaust system is, just amplifies any little quirks in the jetting.

 

[Anonymous]

Sounds like you are running a bit lean.

 

[focus frenzy]

it is not from leaning out, it is from the mixture getting rich when you close off the throttle.
this is a common problem for not just bikes but cars also, when the throttle is closed at higher than idle speeds, very high vacuum builds up in the intake, the high vacuum level sucks allot more gas through the pilot jets but the level of air getting past the closed throttle is little more than it normally passes during idle.
this richer than normal mixture sometimes, depending on conditions, wont ignite and will escape into the exhaust system where superheated gasses from a cylinder that did fire will ignite it and you get the pop you hear.

many late Japanese cars that had carburetors used a air valve that would open under high vacuum loads when the throttle was closed at higher rpm's, they would allow a large amount of air to bypass the carb leaning out the mixture and preventing the popping in the exhaust and also lowering emissions.
most modern fuel injection systems shut off the injectors when the throttle is closed at mid, to high rpm's

you may be able to go a little leaner on your pilot jet size (probably still a bit rich) and reduce or possibly eliminate it.

one thing you could do is take the bike to a emissions testing facility and see if they will test the stoichiometric (SP?)ratio of your exhaust (air to gas mixture ratio) this will let you know for sure whether you are lean or rich.