Bike stalls when hot

My '88 KZ305 is stalling out on me. It runs pretty good when it is cold, but after about a half hour of riding it starts to chug, chug, chug, then looses power and just dies.

If I let it sit for a minute it starts back up and I'm on my way. Sometimes for a mile or two, sometimes longer, then it dies again.

Once it gets warm, like after a day of running short errands it starts to stall out a lot. I can't go but a couple miles then it dies. Have to sit and wait and then I can go again.

My first guess was some gunk in the tank or something with the carbs, but now that it is clearly related to how warmed up the bike is, I'm not so sure.

Someone suggested the ignition coils. Does that seem likely? If so, how do I test them?

I've got a multimeter, but no shop manual for the bike.

Is your gas tank vent clogged by chance, try loosening the gas cap and see if she does better. If that doesn't work I would look at the carbs and a dirty tank.

could be many things,check coils for cracks.could also be vapor locking.....................

Ignition shutdown due to insulation loss (heat) is plausible. Carry a spare spark plug with you, pop the lead off the engine's spark plug, plug in your spare to the lead and ground the plug to the engine block. Try cranking and see if you have spark when the issue occurs...

Would check valve clearances.

Good Fortune!

Kawickrice wrote:

Is your gas tank vent clogged by chance, try loosening the gas cap and see if she does better. If that doesn't work I would look at the carbs and a dirty tank.

Where exactly is the vent located? The tank has been repainted, so it is entirely possible that they vent got painted over.

Its in the cap, try riding with the cap open, but not a full tank as it will tend to slosh. 1/2 tank or less maybe.

If condition is due to fuel starvation resulting from vent blockage or flow obstruction, it's likely while riding that the float bowls are emptying faster than they are filling.

The bowl empties and the bike stops due to lack of fuel.

While stopped, the bowls s-l-o-w-l-y fill again, and the bike runs until the bowl empties.
Ditto, ditto, ditto . . . .

Good Fortune!

Topper wrote:

Kawickrice wrote:

Is your gas tank vent clogged by chance, try loosening the gas cap and see if she does better. If that doesn't work I would look at the carbs and a dirty tank.

Where exactly is the vent located? The tank has been repainted, so it is entirely possible that they vent got painted over.

If you look real close on the bottom of the cap you should see a very small hole which vents the tank to the outside atmosphere. Make sure it is unobstructed. If this gets clogged it will do exactly as Patton said. That is the cheapest and simplest fix so lets hope that is the culprit

Ok, I'll check the gas cap when I get home. Would that get worse as the bike warms up?

Topper wrote:

Ok, I'll check the gas cap when I get home. Would that get worse as the bike warms up?

Not necessarily.

But too tight valve clearances get worse as the bike warms up.

Good Fortune!

Once the bike is hot (and starts goofing), perform a compression test. Turn off gas, drain float bowls. Remove all plugs. Connect compression tester, and crank engine while holding throttle wide open.

Numbers should not be more than 10% difference, and all should be over 120psi hot.

Checking valve gap is considered a part of regular maintenence in the manual for these bikes, so if you havn't in a long time, your problem could just be neglect, and a simple shim swap could solve your problem.

Taking extra plug and connecting it to lead, then holding against head while cranking is good advise to check spark.
Gap the plug to about 1/8th inch, as it will make the spark easier to see, and coils should be able to jump this gap in atmospheric pressure, no problem. Spark should be snapping and whitish/blue, without missing at all (constant spark!).

GL.