Kz750 twin backfiring

Hey all, i just got my Carburetors cleaned and synced by a local motorcycle shop . The bike backfires every so many rotations while idling. When you crack the throttle the bike sounds great until u let off the throttle and it will do a single backfire every so often . I already set the ignition timing and I'm going to measure the valve clearance in a day or so . I dont expect the valves to be out of specs , but who knows . Is it possible that the bike is backfiring because of high idling? I dont really want to mess with the carbs considerig the mechanic told me it wasnt carburetor related. The muffler has some bad rust inside too if that makes a difference . Anyone had this problem?

Yes. Mine was a problem with the timing advancer.

You should also check for vacuum leaks.

Schmeck wrote: When you crack the throttle the bike sounds great until u let off the throttle and it will do a single backfire every so often .

That sounds like a lean pilot system, either vacuum leak and/or the mixture screws are set too lean.

Another thing: the 750 twins with BS-38 come with either 40 or 45 pilot jets which are too lean. They run much better with 50 pilots. The 50s stopped the "pop pop" when I let off the gas.

Is there a more specific way to check the timing advancer ? I turned the crank to the tightest spot and or widest opening and set that so that is set good according to what I read in the manual. As far as the advancer I dont know what is good and whats bad . It is rusty but not too bad . I sprayed it with wd40 when I had it off . If you dont have an airbox lid could it cause backfire? I wouldnt think so but if the bike is that touchy without a cap then I'll need to look for one ( bike had no air box lid just air filter )

Yes, you need the airbox lid. Without the airbox lid the air filter is doing nothing because the air will just go around the filter and into the carbs rather than through the filter. Ed

Okay will get that lid. Is there a better way to check the timing advancer. How does it operate and how do I know if mine is not doing its job?

Do you have access to a timing light and a manual? You can use the timing light to very easily check the operation of the timing advancer. Ed

FSM on the advancer.

Do the weights move freely and smoothly?

Do the springs keep them in or do they wobble and flop around loose?

Another thing to cause backfiring: make sure the fuel levels are correct as verified by clear tube.

Thanks for the replies guys. No Ed I dont have a timing light but I do have a manual that I will look at when I get home. When I had the advancer off before to look at it one spring was pretty tight and the other was tight but not as much. Both moved but were hard to open. Maybe the springs are too rusty . How loose are they suppose to be ? On another note I just had the carbs cleaned rebuilt and synced so I would asume the carbs are okay . If I have to I will play with the idle screw alittle but I personally dont want to mess with carb tuning.

If the bike is lean which I think it idles alittle on the high side, how would I richen it?

At this point, don't bother trying to richen or adjusting the fuel/ air mixture. If you do, you will end up having to do everything over again when you get the lid to the airbox, and you need that lid. It not only will improve the lean condition caused by unrestricted air going into the carbs, it will keep bugs, grit, and who knows what else from being sucked into the carbs. Keep in mind that air will follow the path of least resistance, and that is around the filter - not through it. Running without the lid is no different that running with the carbs wide open to the atmosphere. Not a good thing to do. Ed