1981 KZ750 LTD - 4 cylinder sounds like a diesel

Hey Guys,
Having a bit of a problem with my 750LTD 4cylinder. Basically its making a pinging noise out of the top end, kinda sounds like a diesel, in a way. But I noticed when you have it idling while its pinging it runs slightly rough, and if you give it throttle, like rev it up to 1500-2000rpms and hold it there, the motor backfires every once in a while, and not out of the exhaust, more so back towards the carbs...I noticed all this, this week, after I had to ride it home in pouring rain earlier last week. When I got home the day it rained, I pulled it into the garage to park it and check over everything. I looked at the airfilter because on my bike, the airfilter hole is on top of the airbox, so it can easily get wet(kind dumb, but I love this bike so whatever) and sure enough the inside of my uni filter was damp. ever since then its been pinging. I just got done reinstalling the topend(checkd the cam timing) and the pistons looked good, so I dont think water got sucked in, the only other thing I can think of is detonation???
If anybody can understand what I'm trying to say and help me out... That would be great
Thanks,

Could be some crap in the Air bleeds for the idle circuit in your carbs. But a compression test might not hurt to verify No Prob's.
The air bleeds are in the rear air horn of the carb. ! feeds the main circuit the other the low/ Idle circuit. Could even got something in a fuel jet. Besides the removal process the carbs are pretty easy to clean when they are on a running bike.

Could be electrical too.Take off the side covers and clean them out with a good electrical parts cleaner spray.You could also be arcing a spark from one coil to another,causing the plug to fire on a cylinder on the intake stroke (crossfiring).If your wires and caps are OE,it may be time to change them.

I would check the ignition timing and the cam timing. those bikes had the lousy automatic cam chain tensioner and if it went bad, your timing chain could have slipped a tooth or two.

Sounds like you have weak or no spark in one cylinder. Check the plug wires for resistance.

I had two coils that ran great for years and years without a hiccup. Then I got caught in a rainstorm. After riding through it, both coils went belly up the next day, within 50 miles of each other.

Over time the coils get tiny cracks in them, but they can run ok like that as long as they stay dry. After a ride in a heavy rain, the water gets into the cracks and that's the end of those coils.

See if you can figure out which cyclinder(s) is not firing, then swap the coils and see if it's a different cylinder not firing. If it is, then you know it's time for new coils.

What kind of gas you using?
Don't want that high octane stuff.

Thanks Guys,
I try out the ideas and see what happens.


Oh I put 92 or higher octane in all the time, Fill it up mainly at a Shell gas station.

Thanks

Try 87 octane for a while and see if it goes away.

As far as the running noise you're getting, I agree with otakar about a cam chain tensioner. You can find manual tensioners for about $50 online. As far as the backfiring goes, I'd check the reed valves up front for cracks. If you do, resist any temptation to scrub them clean.

Im agreeing with timing/valve issue, sounds advanced to me or releasing pressure when it shouldn't

I have a feeling he either fixed it, blew it up, or sold it. He posted his problem almost 2 years ago. :laugh: