[Prox6616]

I recently was given a 1985 yamaha vmax 540 here's the problem it fires on almost every pull but will not actually run for more than 2 seconds. I have rebuilt fuel pumps and assured they are pumping I have also soaked and clean carbs all passage ways. Compression is at 135 per side. And I have a very nice looking bright blue spark. Again it fires won't do more plugs are getting wet and It doesn't seem to be any different if I prime cylinder or just pull on it. Any help greatl appreciated I am completely confused thank you

 

[trumpet]

Could you possibly hook this motor up to an auxillary fuel source, ( old lawnmower tank or rigged up bottle of fuel ) and see if motor will continue to run that way. If it does then you have a fuel delivery problem or a venting problem. ( fuel cap ) Just my.02

 

[yamadad4]

sounds alot like a crank seal issue or a bad impulse line.

 

[Prox6616]

How check crank seal if u can and impulse line?

 

[yamadad4]

hard to check the seal if it wont run very long. a visual inspection for cracks or abnormalities might reveal something. i have seen alot of them spit the spring ring out and its visible behind the clutch. the mag side seal is buried behind the ignition system, so its a bit harder to inspect. you can try the reverse of the typical seal check and spray ether behind the clutch and see if the sled fires or runs on the ether. careful here, you dont want to melt your pistons. as for the impulse line, remove it and vacuum test it. its the hose that goes from the fuel pump to the crankcase.

 

[Prox6616]

Thanks I will give her a try and lt u know

 

[marczoid]

Good chance its crank seals on a sled that old.

 

[Prox6616]

Another question can you bypass the throttle overide system on these

 

[yamadad4]

to by pass tors: find the pair of wires on the carb, unhook the bullet connectors and plug them back into themselves. the carb wires dont matter, its the other pair from the bars that need to be reconnected. just a warning: make sure there are no other underlying problems with the throttle cable or have a safety lanyard/tether attached!

 

[Prox6616]

Ok I have no wires to carbs I think previous owner remove piece of harness there is a three wire plug by airbox with nothing in it could his be where harness was connected

 

[yamadad4]

what color are the wires in the plug?

 

[Prox6616]

Appears to be two black wires and a black with yellow

 

[yamadad4]

does the kill switch on the bars work? is the plug by the airbox in a triangular shape and one of the black wires may have a white stripe? typically the tors switches are connected with bullet connectors and should have a black and a black/yellow. that sled i believe had a sensor on each carb that ran back to the throttle switch. man those sleds were the chit back in the day, i miss that old girl. anyway, confirm wire color and plug shape. i need to dig up a wiring diagram unless you have one available.

 

[Prox6616]

Ok not sure if one has white on it I'm not home currently bu it is a triangle shaped plug It does have a sensor on each carb and the handlebar kill switch seems to be working fine. I have not been able to find a wiring diagram for it