I have just had my Tiger cub engine rebuilt and its popping back when I close the trottle, I have a keihen carburetor is it just a question off jetting main jet as tick over etc seems to be OK?

The carb works well on the Cub but I had exactly the same problem. I believe the fault is the length of the inlet tract. I did not extend mine as I changed to the Del Orto which is perfect in all areas with no spit back,but it does have a longer inlet.

Tony

wont be main jet,its either the pilot jet a bit small or air leak on carb joint or exhaust.

edited due to me getting my R's and elbow mixed up..

banging in the exhaust may be caused by too weak a pilot mixture when the throttle is closed - or too rich a pilot mixture and an air leak in the exhaust. the reason in either case is is that the mixture has not fired in the cylinder and fired in the hot exhaust. violent explosions in the silencer during descents may be because the throttle is not closing fully. if the bike bangs when the throttle is open then the problem is ignition.

Edited January 19, 200917 yr by totalshell

Forgive my ignorance but if the pilot jet is too small, how does excess fuel get into the exhaust?

thanks for your edit 4stroke i hadn't put it very well but hopefully my edited version is a little more comprehensible.

Gday, all engines go over-rich on deaccelleration, this is because some fuel sticks to the inlet tract walls during normal running. When you shut the throttle you increase manifold vacuum greatly and so this fuel gets drawn into the engine (over rich!) If there is a hole in the exhaust (leaking gasket etc.) this adds a bit of air and given enough heat, combustion occurs - banging in the pipe! This is a common cause of "backfires". Also as written above the other issues can cause the same thing. If the bike idles and starts OK your jetting is correct so dont change it! Check the other stuff first.

HTH,

Cheers,

Stork.