81 kz1100a1 backfiring

Hello its my first time posting I have a 81 kz1100 a1 with a 4-1 header bs34 carbs with 135 mains 140 pilots and pods. i just got the motor back together and have serious backfiring issues at idle. it sounds like a gun going off. I did a valve clearance check they are all in range. im thinking exaust valves not seating. Any ideas would be great. Thanks

It could be the emissions system if your bike has it. They easy way to test is to connect a hose between the two big ports on the top of the valve cover, and seal off any vacuum ports that have hoses going to the air valve assembly under the tank.

When that unit fails, it can cause cherry bomb explosions in the exhaust.

Thanks. Come to find out I have the dreaded twisted crankshaft.

Turns out I have a twisted crankshaft.

Aw, that sucks

if in need I have Brand New " J " Model crank in the box available

Rossb16a wrote: Turns out I have a twisted crankshaft.

What does that mean, and how do you know that your crankshaft is twisted?

:ohmy:

When I did the upper gaskets I noticed that when number one piston was Tdc number four was a quarter inch lower. After tear down it’s quite noticeable that it is twisted. And I had major backfiring when running. Also puffing back through number one carb. I tried to upload pictures.

Rossb16a wrote: When I did the upper gaskets I noticed that when number one piston was Tdc number four was a quarter inch lower. After tear down it’s quite noticeable that it is twisted. And I had major backfiring when running. Also puffing back through number one carb.

:ohmy: Yikes

if you noticed it when you did the upper gaskets. why did you continue. you must have know something was wrong then because you noticed it.

In order to reduce rotational resistance, cranks sometimes use roller bearings. In order to use roller bearings, the crank must be made from several pieces and is usually pressed together. Unfortunately this means the pressed area can sometimes slip under stress and the crank ends up "twisted". The solution on race motors is to weld them after assembly. This is why you sometimes see "welded crank" as a selling point on used KZ's of large displacement. Most/all of the smaller Kz's use a one-piece crank with split plain bearings.

maybe its your 140 pilot jet? With it being bigger than you main it will cause reverse vacuum and draw fuel into the engine at the wrong time causing a big boom