91' Kx500 trouble running. HELP!

[91kx500]

I have a 1991 Kx500 which hasn't ran in about 3 years due to a blown piston and me being away in the military. I'm rebuilding it and getting it in running shape now.

I got a new top end and piston along with all new gaskets for top and bottom end. Long story short it's rebuilt and mounted on the bike. I've done 3 top end rebuilds so I'm not new to this but I'm stumped on this problem: The bike will not start without having 1/4-1/2 throttle open. When the bike finally turns over and starts I have no throttle control and it will only run for about 5-15 seconds and then sputters out. I know the problem can be one of three things; spark, air, or fuel.

1. Spark - There is spark but I'm not sure what my best gap setup should be, any suggestions?

2. Air - I'm not sure what would keep it running and I've messed with the pilot air screw anywhere from 1-5 turns out in 1/2 turn increments with no luck. I ordered a new intake boot because of dry rotting (possible leak) and new reed valves because I can't tell if they're bad. Not too sure how to tell if they're still good. Would the reed valves or boot cause this problem? Both parts on order.

3. Fuel - Petcock knob is broken but fuel still flows. On order. How do I tell if my float is shutting off fuel flow too early? Could this be my problem?

Please name everything you can think of so I can order parts or continue my troubleshooting.

[evo550]

Piston in backwards?
Carb passages blocked?

[cnrcpla]

To check if the float height is set correctly, you will probably need a manual, or find one online. Once you get the proper float height, adjust accordingly by setting the small metal tab to the correct hang height for the floats.

PS. I think it's an air leak. Does it idle, or rev really high when you first start it up for the 15 seconds it runs?

[SachsGS]

Almost sounds like you've got a big air leak some where. Did the piston grenade?

[91kx500]

Almost sounds like you've got a big air leak some where. Did the piston grenade?

Used piston and top end but plenty of compression.

[91kx500]

Piston in backwards?
Carb passages blocked?

Didn't know you could put a piston in backwards...
Carb was completely dissasembled and cleaned 3 times. Found some junk but it's clean as a whistle now. Needle is set to the third rink. Carb is a 39mm Kehinin. Not sure what the main jet size is, no marking on it but I've been running it for years before this rebuild. Same with the pilot jet.

[91kx500]

To check if the float height is set correctly, you will probably need a manual, or find one online. Once you get the proper float height, adjust accordingly by setting the small metal tab to the correct hang height for the floats.

PS. I think it's an air leak. Does it idle, or rev really high when you first start it up for the 15 seconds it runs?

I set the float height so that it shuts fuel flow off when the float is level with the part where the bowl and the carb meet up. Should I go lower on the tab so the fuel cuts off at a higher fuel level? Once it starts up it will set at a higher type idol. I peg the throttle out and it barely goes what a normal 1/8 throttle would be.

[cnrcpla]

Yeah, that does sound like an air leak, it wont rev up because it isn't getting enough fuel. Is the rubber boot between the reed and the carb ripped? Sometimes the tear in invisible but can cause this. Also, is the gasket to the reed cage and the engine block in good shape and seated correctly?

I would say it also sounds like a carb problem, but you cleaned it all out. If the floats are set to factory settings, I wouldn't set them to dump more fuel, as you may end up making it piss gas out the overflow. But, have you cleaned the float jet hole where the fuel actually flows into the carb from? Sometimes that is that culprit. If you can easily blow air through it than your all set there. If not, that clean that part out. It sounds like the bike is being is starved of fuel by either an air leak or insufficient fuel flow in the carb.

[arnego2]

How about the coil? Did you test it? Coils  can get back give you some problems which seems to get jetting.

[91kx500]

How about the coil? Did you test it? Coils  can get back give you some problems which seems to get jetting.

Should I have to even test the coil? I'm getting a steady spark. And when you say test the coil do you mean a voltage test or an ohms check? Give me a little walk through on what you mean so I can see if that has anything to do with it.

[91kx500]

Everyone here has great ideas and is full of info. I really appreciate the help and all the knowledge people are sharing. I hope everyone is learning as much from this as I am. If anyone rides in VA or is close to it lets get some riding in once I get this beast running. I have no doubt you guys will help me fix the bike. I'm going to take pictures monday when I go into my shop and show you what I'm working with.

[arnego2]

Coil testing is done over the resistance and even with a good looking spark the coil could be on its way out. They have 2 circuits both have to work. Ohm values should be in the manual.

If you have another coil at hand just connect it and try to start the bike.

[91kx500]

Coil testing is done over the resistance and even with a good looking spark the coil could be on its way out. They have 2 circuits both have to work. Ohm values should be in the manual.

If you have another coil at hand just connect it and try to start the bike.

I have a few gaskets and other parts on order that I think I'm going to try first. They needed to be replaced anyway.

I'll get a multimeter and test the coil pack. Let me ask you this. What is the coil doing for my bike and why would it make the bike start and cut off. Still trying to learn all the parts for my bike. I don't have a spare one but if all else fails I can order a new one.

[TMKIWI]

Check the high speed stator output.

[Stusmoke]

Does the plug show rich or lean?

The plug is just a little wet. Not burnt or anything. I don't think the bike has had a chance to run long enough to show story with the spark plug. But that's what I'm seeing, that little bit of fuel.

The plug isn't oil fouled is it? Could be sucking in tranny oil through the wet side crank seal. After it bogs and dies, does it start back up again with no worries?