Backfire Through Carbs and Exhaust

I have an old KZ900 engine that has been sitting for some time(atleast 4 years). I am currently trying to use it in my bike while I build up my new motor. When I tried to start it it fired up quickly. However it intermittenly backfired through the carbs and exhaust at and above idle, however it doesnt seem to really do it in the higher rpms. I set the points and they are now set correctly. However the problem continues. Last weekend I dissassembled the carbs and cleaned them good. I know that the carbs are out of sync would this cause this problem?(I plan on tackling that this weekend). After cleaning the carbs the bike ran for almost 2 minutes or so at idle without backfiring but then started to do it every 5-10 seconds or so. Any ideas?

did you check the manifold rubbers for cracks or maybe a missing or crackes vaccuum plug?

Have you tuned the bike; that is, set ignition timing or adjusted the valve clearances? Have you checked the amount of voltage going to your coils?

I well, my boss, recently bought a little carberator
for a project dodge, in the pamplet it said
that a carberator could never cause a backfire. If
you do not believe this go to your local auto parts store and have them open up any carb, they all have them, a little cartoon guy telling the do's and don'ts
of the new carb. I do not know why they could say this if it werte not so.

I found the cause of the problem. I have a bent exhaust valve on the number 3 cylinder. My guess is that it was stuck in the guide in the open position after sitting for so long. And when I fired it up it must have nicked the piston. The piston looks good though so I think I will just have to replace the valve and do a quick valve job on it and it will be good to go. Thank goodness I can do all of this myself at our shop.

lemo32 wrote:

I well, my boss, recently bought a little carberator
for a project dodge, in the pamplet it said
that a carberator could never cause a backfire. If
you do not believe this go to your local auto parts store and have them open up any carb, they all have them, a little cartoon guy telling the do's and don'ts
of the new carb. I do not know why they could say this if it werte not so.

Since all a carb does is meter fuel and control airflow, I guess that would be technically correct.
But a rich mixture can cause backfiring out the exhaust, while a lean mixture can cause a popping out the intake, so I guess the backfiring could be caused by to much or to little fuel, while the carb is just metering the (incorrect) fuel it was setup to do.

But jets do wear, as do throttleshaft bushings, so it could be the carbs fault after all

Are you really gonna believe something a funny little cartoon charactor says?? :laugh: :laugh: :laugh:

Oh, i don"t know, there seems to be quite a few
cartoons happening right here at Kzr, but you got a
point, i wouldn"t believe em" all