Stutter/stumble at constant RPM

Sometimes my '79 KZ750 stutters or stumbles when I'm cruising at a constant rpm. It seems to happen between 3500-4000 rpm. If I give it a little gas it levels out.

It sort of feels like it's mis-firing or something.

I could use some advice about where to start to look to track this down. I'm thinking of starting with the timing advancer and making sure it's clean and not sticking.

Does that sound like it could cause this?

Thanks!

-Topper

Bump. Maybe this should go in electrical?

Has it always doen this or did it just start happening? Did you recently do some work on the engine and it started right after?
If not then I would start with a complete tune-up and go from there.

I've owned the bike for a little over a year and it's done it the whole time I've had it. I've cleaned and balanced the carbs, cleaned and adjusted the points, set the timing with a light, replaced the coil all in the time I've had it.

It's always done this, but it's one of those things that is difficult to reproduce on demand.

I had a bad plug wire make it act like that.

I got new plug wires with the new coil. But it did this even before the new coil.

Am I right to suspect the timing advancer or is that off base?

I wouldn't think that it would be out of the question. It cost $0 to rule it out so go for it.

Topper wrote: Sometimes my '79 KZ750 stutters or stumbles when I'm cruising at a constant rpm. It seems to happen between 3500-4000 rpm. If I give it a little gas it levels out.

Sounds more like a carb problem. Check for vacuum leaks at the intake manifolds. Check for slits in the carb diaphragms. Check fuel levels in the bowls.

What jets are you running?

If you have the stock BS-38 carbs, increase the pilot jets to #50 and they will run better. If you have the 40's that came in the California models, they will NEVER run right.

Are you running pods and/or low restriction exhaust?

The timing advancer can cause problems, always good to check that.

Run your wires down between your pickups and igniter, check connections also. I've had connections break down when they get warm. Worth a look if you haven't allready.

Les

bountyhunter wrote: If you have the stock BS-38 carbs, increase the pilot jets to #50 and they will run better. If you have the 40's that came in the California models, they will NEVER run right.

Are you running pods and/or low restriction exhaust?

The timing advancer can cause problems, always good to check that.

Believe it or not it had 35s in the carbs when I bought it. Popped like hell but ran. I put 40s in when I cleaned the carbs. Still pops a little on one side especially on decel. I've been thinking about upping to 45s ever since.

FWIW I'm running stock airbox, aftermarket exhaust with mufflers.

I think I'll have some time this weekend to check out the timing advancer.

Topper wrote:

bountyhunter wrote: If you have the stock BS-38 carbs, increase the pilot jets to #50 and they will run better. If you have the 40's that came in the California models, they will NEVER run right.

Are you running pods and/or low restriction exhaust?

The timing advancer can cause problems, always good to check that.

Believe it or not it had 35s in the carbs when I bought it. Popped like hell but ran. I put 40s in when I cleaned the carbs. Still pops a little on one side especially on decel. I've been thinking about upping to 45s ever since.

FWIW I'm running stock airbox, aftermarket exhaust with mufflers.

I think I'll have some time this weekend to check out the timing advancer.

45 pilots run "OK" but you can't get the decel popping to stop. It runs better with 50's. No popping, better throttle response.

Make sure your pilot circuits aren't obstructed. I just went through this on my bike and finally figured out that the air jet for the pilot system was plugged, causing one cylinder to run way too rich at idle and cruising speeds and cause it to stumble at a constant rpm. I don't know anything about your carbs, but if it has a pilot screw to adjust fuel mixture at idle, you could try closing one off to see if there is only one cylinder having the problem.