79 KZ650 Loss of Power, Engine Quits

I'm new to the forum - I guess this is really a troubleshooting question because I don't know fit's a carburetor problem but it feels like it.

This problem came on very suddenly at the tail end of a 100 mile ride during which it ran fine.

The starts and idles fine but on the road it has very little power, acting like there is no gas. After i or 2 minutes it becomes very hard to keep it running. I have to rev it very high to get any power at all, and soon it is very hard to start. I limp the mile or so home and shut off the bike. After a few minutes it starts and idles fine. Back out on the road, same symptoms.

I have replaced a cracked fuel line from the petcock to the filter and the filter to the carbs. I have replaced the fuel filter. The plugs have about 1,000 miles on them. I have tried a short test ride with the air filter removed - same symptoms.

Does anyone have any suggestions for what to try next? Thanks.

-Paul

Sounds to me like something is getting hot and failing. Coils come to mind have a look at them and ohm test them with a heat source (hair dryer) directed at them. Good luck.

Thanks Slayer. I'll look for information on impedance testing of the coils.

-Paul

Take a look at your gas cap vent.

Hi Peter. I wondered about that. I'm not sure how to check that but I'll look into it. I did think though that a 2-3 minute ride should not use all the air in the tank - do you think it could? The other symptoms seem to fit.

My tank vent was plugged up tighter than a mouse's ear...to rule it out...pop the gas cap, & take her for a
ride, (no leaning!) see if she does it.

Make sure there is no kink in the fuel line; this can cause the exact symptoms you describe (guess how I know :laugh: ). Also, make sure the filter permits full flow of fuel. Ed

Grumpy - that crossed my mind but it made my a little nervous; I'll give it a try though - it would not have to be a very long ride.

Ed - the lines from the the tank to the carb, and the filter is brans new. I'll take a closer look at vent & vacuum lines though.

-Paul

Perhaps the petcock screen is clogged or the petcock is mal-functioning for whatever reason.

Could test run with the petcock set in PRIme position, which by-passes the vacuum dependent feature.

Could also test for free fuel flow through the petcock in PRIme position, with the engine NOT running, by disconnecting the fuel line at the in-line filter and allowing fuel to discharge from the fuel line into a catch pan.

If there's fuel in the tank with petcock set to PRIme, fuel is supposed to flow from the petcock's fuel outlet, regardless of whether the engine's running or not running, as the petcock in PRIme position doesn't need vacuum in order to allow flow.

Under these circumstances, if fuel fails to flow, then the absence of fuel flow is likely to be an issue with the petcock (perhaps the petcock screen that protrudes up inside the fuel tank is obstructed with crud, or maybe there's crud blockage inside the petcock fuel passages).

Dis-assembly and cleaning the petcock might restore proper fuel flow, but a new diaphragm or petcock repair kit is sometimes required.

Good Fortune!

Thank you Patton. The previous owner replaced the factory petcock - there is no prime position, I think it is just a simple three position valve. Nevertheless your suggestions applies. I'll put them on the list.

Just a quick update. I cleaned the tank using vinegar and rinsed it with acetone to dry out water, then let it sit to dry for a couple of days. A lot of gunk came out with the vinegar so I did it again.

I sent the petcock/fuel valve to Pingel who re-built it for $15.00, and I mounted it on the tank.

I replaced the fuel line from the petcock to the filter, replaced the filter, and replaced the fuel line from the filter to the carbs.

Despite my great hopes, the problem persists - initially idles fine, runs okay for a couple miles, then loses power to the extent that I can barely get it home.

I have not tested the coils as suggested in that thread. I will do that the next time the tank is off but it seems like a long shot to me.

BTW, this problem came on suddenly at the tail end of 1 100+ mile ride.

If you have other suggestions (acceleration pump? clogged needle jet?) I would be glad to hear them; mostly I wanted to update the thread for those who have contributed.

This REALLY sounds like your coils. Have you tested them yet? I had those exact same symptoms but also noticed that the loss of power seemed to be due to misfiring or lack of firing.

Your coils sit beneath your gas tank right above your engine. They get VERY hot and will eventually fail over the years. What happens when they are cold is that everything on the inside is shrunk down and the connection is fine so while the bike is cold or cool it runs great as it is able to get fire. Once the bike warms those coils up they expand and break the conductive paths causing loss of fire.

Luckily when mine started acting up I had a brand new set of high performace Accel coils sitting on the shelf for another project. I just threw them in and wired them up and took her for a ride and my problem was 100% solved.

If you can't figure out how to test them, just buy a new set. Could always have them on hand for the future if that wasn't the issue. (Really think it is though). Also make sure when you get a new set to replace the sparkplug cables too if you haven't already. They can go bad also. Best to just replace at the same time as the coils.