KZ750H2 backfiring breakthrough!! PLease read!!

Hey Guys, I have been fighting bad backfiring through the exhaust on this 1981 KZ750 H2 and I might have found the problem! The bike has the clean air system on it but it is not being used due to an aftermarket exhaust and pods! I discovered that the large hose that comes from the vacuum valve under the tank was really sucking a ton of vacuum so as soon as I plugged it with my index finger the backfiring was gone!!! I mean it disappeared period! What is the proper way to disable this system? Can I just cap off the two large ports coming from the valve cover? Any other suggestions? Thanks

APE sells some beautiful block off plates, but the systems does no harm if all hooked up and working correctly.

So I can block off the ports and it won't hurt anything? It gets rid of the backfiring out the exhaust when I plug that big hose so something in the system is not working right correct?

dosent hurt anything at all to block them off, they were put on to help the bikes meet emmissions when new, but most of the time they stopped working.

Thanks Irburke, my '82 kz750 Ltd has a lot of mild popping once it warms up. I'm going to try blocking off the big tube and see if it goes away.

can you get a picture of this area posted? it would greatly help.

I have done exactly the same procedure on my bike and used the black hose that ran from the pressure release valve back to the air box that you have removed, placed one end on one of the openings and the other end on the other side opening and used the clamps that were there to hold them. Works perfectly.

As noted above, also okay to just connect the reed valve caps to each other with a hose.

Or may plug each cap individually.



Good Fortune!

The backfiring you are having is excess fuel burning off in the hot pipes.This is common with a jet kit and pods.The air that is pulled in is not going into the combustion chamber,but comes in after/past the exhaust valves.Yes,the system was meant to reduce emmissions at the tail pipe.If you limit/remove the freah air into the system,the backfiring will stop,but you may get some black smoke out of the tail pipe.IDK if you watch NASCAR,but it's basically the same thing as when a car goes into the corner,and the driver lifts off the gas and you see flames shoot out the pipes.Lots of unburnt fuel getting into the exhaust and burning there.You may be running too rich,and may have to lean it out a little.Check the "Reed Valves" under the covers to make sure they are there.if you're REALLY pulling in that much air,they may be worn out or missing.You DO want them in there to help seal the covers,and to keep hot exhaust from coming back up and out into the system.You can check the check valve with a hand vacuum pump to see if it's working right also.

Usually the system fails when the diaphragm/switch mechanism above the motor will no longer shut off the air flow when you close the throttle. The fresh air is only supposed to get sucked into the exhaust when the throttle is being opened (or being held open, but I haven't checked). When the throttle is being closed, the fresh air has to stop or it will back fire.

Other leaks in the system can also cause backfiring in the exhaust. You can just cap off everything connected to that valve/contraption and remove it. That also means capping the two vacuum tubes coming from the intake manifolds.

Thanks for all the good advice guys! I think I am finally getting a handle on this ole Kaw! Larry

My Clymer manual for the 77-83 KZ650 has a troubleshooting procedure for the Air Injection System. The index lists pages 247-248 BUT that's a typo. The info is actually on pages 267-268. Just in case others can't seem to find the info.