Cylinder(s) cutting out - fueling issue?

Hi guys, I'm trying to diagnose an intermittent but annoying issue with one or more cylinders cutting out on my 1980 KZ750. Here's a brain dump of notes in case something helps:

When I was looking at the bike when I bought it, cylinder 1 wasn't firing and didn't have spark, so the seller replaced the spark plug and it started working.

Since owning it, I found the carb overflow tubes get a couple drops of gas in them, but much more from carb #1.

A couple years later when using the bike to commute, I found sometimes after riding for a few minutes and at low speed or a stop, the bike would start making a ratting noise and lose a lot of power. I later determined this was from one or more cylinders cutting out, and that it will usually go away when I blip the throttle.

I have since cleaned the carbs, and set the float heights, and the #1 carb had its float a bit low, if anything, which seemed odd since that was the one with the most gas in the overflow tube. This tube still gets a few drops of gas in it after being adjusted.

I've also replaced the coils and wires a while ago.

I found the spark plugs looked like when they're too cold and since the cylinders would cut out at low speeds, I switched to hotter plugs. The new ones look pretty good, except for #1, which still looks pretty rich.

The cylinder(s) cutting out issue remains, so I'm thinking something is making cylinder #1 run rich at low throttle, it fouls the plug, and then it cuts out.

I tried turning in the idle mixture screw on that cylinder (clockwise should lean it when the screw is on the engine side, correct?) and it had no effect.

If that's not working, I'm thinking something with the needle jets could be the issue since it's at low speeds, but the needles don't have clips for adjustment.

My other thinking is I could lower the float level on cylinder 1 despite that it measures in the right range, since the symptoms still seem to be those of a float level too high.

Enough of my babbling - let me know any suggestions you guys come up with.

Valve adjustment and carb sync?? Amazing how a carb will spit fuel back when these two things are wrong.

Thanks for the suggestions. I sync'd the carbs not long ago and that didn't seem to do it. A valve adjustment's currently on my to-do list; I might just have to move it up.

Just realized I never updated this thread! In case it helps anybody out, the cause turned out to be the pick-up coils dying, especially when they heated up, hence why it was when the bike was up to temperature and moving slowly or at a stop.

It *finally* happened in my driveway where I could determine which cylinders it was affecting (2 and 3), and then I could test the relevant parts with a multimeter. I recently got a chance to do a valve clearance check, and the intake on cylinder #1 is the farthest out of spec, so I suspect this is what was causing the fouling of that spark plug.

I hope that solves your issue, but if your carbs are still flooding, or are over a period of times and miles you can damage your cylinders as the unburned gas washes the oil off the bores leading to low compression and sometimes ring damage. I'd be suspicious of the coil that supplies a spark for cylinders 2 & 3 if they are the ones playing up. Some times coils can be intermittent in operation and some work fine until they get warm.

Yup, I since rode the bike in several situations where it would have formerly acted up and it ran fine. The coils are new and seem pretty solid.

My suspicion with the few drops of fuel in the carb overflow tubes is that the float needles are juuuuuust short of a perfect seal so the fuel in the line at the end of the ride eventually works its way down, and mostly into carb 1 since it's the lowest when the bike's on the sidestand. Couldn't be too bad though since I don't need to put the petcock on prime to start it even if it's sat for more than two weeks. I could be way off though.

Smell the oil cap, if you smell fuel, dump the oil.
Steve

Has the clear tube test been performed on #1 carb to assure that the fuel level is within spec?

Good Fortune!

Nope, no gas smell in the oil. I set the float heights a while back using a clear tube, and they were pretty much bang-on already. That was actually my first suspicion about it.

While on the subject of the new pick-up coils, how tight does the seal around the pick-up coil cover have to be? I ask because the replacement pick-up coils didn't have a wire for the oil pressure sensor, and the only way I can feed one through the rubber grommet is outside the heatshrink, which might leave a pinhole in the grommet. I can always remove the wire if it causes any issues.

If you're concerned a little silicon will seal it up.
Steve

A little pinhole in the grommet should be ok. There should be a small hole towards the bottom of that cover anyway (to allow condensation out ?) but as Steve says you could always use little a dab of silicon if concerned.

Thanks for letting us know how you fixed the original problem as it is helpfull to others. Many dont bother