Backfiring thru the carbs

Wow, hope someone can help me with this one, been dealing with it for 3 years and have an idea, but not sure. Here goes.

Bought a 1980 KZ1000 LTD with Mikuni carbs (accelerator pump), completely stock, 11000 miles. Bought a Vance/Hines 4 into 1 and associated rejetting kit from Dennis Kirk, along with individual K and Ns. Think I have the jetting nailed since it runs great when I hammer on it, but since I put on the header, the engine backfires thru the carbs (sounds like the carburetor(s) or engine is/are sneezing). Usually it is at lower RPM, but sometimes you can feel the engine hesitate at hiway speed. The carbs have been off at least a half dozen times and completely gone thru. I thought it might be dirt hitting the pilot jet, but have inline filters. Pilot jet from the kit seems to be the same as the stock. Seems almost too small, as I can barely fit a single strand of copper wire thru it. This motorcycle has the suction valves on the front of the valve cover, I tied them together with a single piece of rubber hose as I was told to do. The carburetors have been synched using a mercury vacuum gauge and fuel levels have been adjusted for consistency. My theory is the pilots may be too small and any size piece of dust may be causing the problem? Air screws are set to about 1 and 1/8 turn. Rebuilt the top end on some bad advice so know compression/valve adjustment is not the cause. Bike runs good,just kind of embarassing when you've got such a good looking bike and the engine makes the sound it does. Thanks in advance. Kevin

You forgot to mention that your bike has an accelerator pump. Does it work? This affects jetting. The bike was jetted very lean from the factory and the "jet kit" you installed doesn't have the necessary pilot jets. Check to see if the accelerator pump is working by removing the #4 pod and twisting the throttle. If you see a spray of gas out of the pump nozzle in the venturi, it is likely working. If this is the case, use #17.5 pilot jets, type VM28/486 and if not, use #20 pilot jets of the same type. Adjust each carburetor at idle using the air screw for max idle speed. Don't be surprised if one or more of the cylinders don't change when adjusted. Turn the screws out to 1.5 turns out from lightly seated and then turn the screw about 1/2 turn in either direction for max idle. It will less than 100 rpm in most cases so just do it by ear.

Thanks for the help, sounds like we were thinking the same thing, but have never dealt with 4 carbs at the same time. One final question. When you talk about adjusting to "max idle", does it matter what the setting of the main idle knob is or should it be around a certain amount?

Thanks! Kevin

Your tach isn't accurate at idle. Set the idle by ear. It will generally be 1000-1500 on the tach. Then adjust each carburetor for max idle speed... this will vary from nothing to 100-150 rpm so it is best done by ear. If the idle does go up 150 rpm, adjust the idle speed back down to the rpm at the starting point.

When's the last time you compression tested?