Vapor Lock

Hi, this is 82 750 Spectre, and I suspect it boils fuel somewhere, but I am not sure. It starts right up cold and warm, but once it gets hot it wont start up no matter what you do with the choke. It also has died on me a couple times, once just sitting at a red light and once when I left it idleing for 10 min, someone commented it was running hot then 2 minutes later it died. I HAVE had some success pushing it and popping the clutch. Could it infact be vapor lock? What should I check? How can I fix it? I guess it could be boiling in the fuel line, float bowl or maybe lack of ventilation creating aq vacuume in the gas tank.

Thanks much,
Dan

PS, I just cleaned the carbs by hand (took em apart and put the back together), the problem existed before this and was not fixed by it. (I did not disconnect the individual carbs from each other)

I've had the same problems with my '82 kz750 Ltd, although I've never had it die on me while running, just harder than hell to start when she's hot.

You'll laugh, but I've had alot of success in putting the petcock in prime position and rocking the bike back and forth. I've got a clear fuel line so I can see when the fuels filling the line again. Don't forget to put the petcock back in the run position when you're done.

As far as it dying while running....boy, I'd check to make sure that the vent hole on the fuel cap isn't plugged. It's a pretty small hole and hard to miss, but if it's plugged you'll have trouble with fuel flow.

Good Luck!

PLEASE don't make me laugh. vapor lock ended back in the 1930's. modern gasoline is blended so that vapor lock is gone. you have a fuel delivery problem.

hoghaterkaw wrote:

PLEASE don't make me laugh. vapor lock ended back in the 1930's. modern gasoline is blended so that vapor lock is gone. you have a fuel delivery problem.

Actually, vapor lock is possible if you have a fuel pump. In sacramento back in the 70's (where it gets Hell hot in the summer), I had a Plymouth Duster that would vapor lock sometimes because the fuel up near the pump was more gas than liquid. The fix was to mount an electric fuel pump back at the tank to push cold (liquid) fuel and bypass the front pump.

I was reading around and it is definetly possible, though it seems to be rare on bikes, common(ish) on carburated cars, non-existant with fuel injection, it is also possible if there is no fuel pump. Wikipedia explained it well. I will have to check the vent hole on the fuel cap, where exactly is it? I did look for it as I suspect that might be the problem.

With modern fuel injection it is virtually non-existent. Older styles could still get it because the 13 psi or so that they used was not enough to keep the fuel from boiling. Are you sure you have good spark as well? Most of these bikes us a PM(permanent magnet) pickup. One for each coil cylinder. These can fail when they get hot and go open circuit, when they cool down they will work again. You may noticed the bike trying to start but not having enough umph. Maybe no spark on one coil?

That possibility also meets the symptoms. I do know that the spark plugs are new. Is there a good way to test the coils?

This is what I would do, in this order:

1. Check spark output on both sides of each coil, ensure strong blue flame on all four.

2. If NG check resistance across HOT coils spark wire outputs. Fix as necessary.

3. If good check for signal and power to coils when symptom occurs. Should have batt (+) on the pink or red wire. Check other wire by connecting test light to positive lead on battery and touch probe end to terminal. While cranking the light should flash.

4. If NG suspect faulty igniter unit or bad PM pickup.

You can check the windings of the pickups by measuring across the two wires on each one under the cover. Compare the two, they should be probably within an ohm or two of each other.'

PS: What bike y/m/m exactly? So I can get my info straight.

Its a 1982 Kawisaki KZ750N Spectre. Does it make sence for the coils to fail when they get hot? Because it is definetly heat related.

What does NG mean. And how should I do these tests?

NG= no good. Yeah coils like to fail hot, the heat can play hell with the windings. This stuff is relatively easy, you mostly just need a spark tester(or old plug), test light and a multimeter. Having the FSM(factory shop manual) or equivalent is good also.

I am having trouble finding the gas cap vent to check it. Should it be just a hole looking thing or will there be a hose?

talk about vapor lock!!! try dancing non stop all night in a concrete basement with three hundered guys and a disco light........ from the book THE CRAIG MCDONALD STORY(AKA TRIANGULAGUNA)!!!!!!!