'79 650 stalls while on highway

I’m having an issue with my bike stalling during highway cruising. First, I should say that it’s a ’79 KZ650 B3. I recently received it back from the mechanic’s where it had the carburetor cleaned, rejetted, adjusted, and so on. It’s had a slight issue starting since I got it back. However, I replaced the lines going from the petcock to the carburetor today and it’s much more responsive. I’m a new rider, but this stalling issue doesn’t seem like it’s an issue with me since I’m cruising along when it happens.

Anyway, when this situation arises I’ll be plugging along on the highway. The bike however, either will not accelerate or go higher than around 50-55MPH. If it’s going faster, the speed just flat out cannot be maintained. In both instances I end up with a wide open throttle because I’m either trying to accelerate or just keep the bike going. In the end I end up coasting along the median and stopping the bike. After sitting for a few minutes I can kick start it and be on my way. At this point it will accelerate and the issue doesn’t reoccur.

Any ideas?

Thanks in advance for any suggestions.

Check and see if the vent in you gas cap is clogged. If it is, as you use fuel the tank will develop a vacuum and fuel won't flow after a point. Stopping, or even just opening up the cap while it's happening, will break the vacuum and let fuel flow, until the vacuum develops again.

If you're driving and it starts to happen, pop open the fule cap. If it takes off again, that's the problem.

If it turns out not the be the gas cap vent, make sure the fuel filter in the tank isn't clogged. Did it do this at all before the mechanic worked on it? Ed

Z1Boise and 650ed have offered some good advice. The easiest fix is to look at the gas cap venting.

I had a similar problem to this a couple of years back. I removed the gas cap and carefully disassembled the underside. You will see a venting route and a couple of air holes. I simply enlarged the vent holes with a little larger drill bit and the stalling problem went away.

Give this a try if the problem persists and let us know the results.

You didn't mention if the bike will pull beyond the problem rpm range when the bike is in neutral or if the problem is only when the engine is under load. This information might be helpful in helping to diagnos the problem.

Cheers-Colin Firth-Ontario Canada

Thanks, for all of the suggestions.

The bike doesn't recover from the problem once it starts happening. It just requires throttle to be opened more and more in order to keep running. Once opened fully or laid off, it'll die.

I'm nearly 99% sure that it's the gas cap after looking at it and taking it apart. There was all sorts junk in the vent path. Plus, more evidence from the past week to the lack of venting was suction release sound that would be given by opening the cap after running it for a bit. I just never thought about it since I'm a newb without much experience. I gave the cap a good cleaning so hopefully that will fix the issue. It's too late to go for a test drive tonight, but I'll check it out tomorrow. I'll keep you guys updated on the situation.

Thanks again.

Post edited by: jamesjp, at: 2007/08/25 00:12

Man I hate to admit this on the forum :blush:... but I had the same symptoms.

Turns out that all 4 of my spark plugs were lose! :pinch: I guess that I forgot to tighten them after inspecting them.

Best of success!

I went riding this morning and cleaning out the cap seems to have fixed the problem. It never once stalled during city and highway driving. Overall, it seems to working quite well now. Thanks again for the help.