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Doug E.
Doug E., Kawasaki Master Technician
Category: Small Engine
Satisfied Customers: 2886
Experience:  Professional mechanic since the late 70's on Kawasaki, Arctic Cat, Jonsered, John Deere and others.
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my 1984 yamaha srv 540 pops back thru the carb just as the

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my 1984 yamaha srv 540 pops back thru the carb just as the starts to engage about 3500 rpm . will ide good and starts easy has about 3500 miles but has low compression i was told it needs pistons and rings or top end rebuild does this sound correct thahks joe

Could you tell me what your compression numbers are? It's best to take a reading cold and then again when the engine is warmed up.

Customer: replied 5 years ago.
compression is 90 psi in both cyliders
That's very low. Be sure the throttle is wide open when testing and repeat the test with the motor warmed up. If it goes down more, I would pull the top end and take a look. You probably need rings. You want a minimum reading of 120.

I don't know if that's your popping problem or not. You may have an issue with the throttle safety switch. If that is worn or out of adjustment, the motor will cut out just as you start pushing on the throttle. I'm looking for a wiring diagram so I can tell you what wire to pull to test that. All of my sled manuals are at home. I'll let you know as soon as I can find that info.

Customer: replied 5 years ago.
if you are talking about the switch on the carb that has the wire going to it for the kill switch mine does not have that switch to by pass mine has tether switch
I know the 85 had the safety switches but have been unable to find a diagram.

I would bypass the tether, kill switch and key switch to make sure they are all good. I think you can check all of them at once by unplugging the main engine connector at the engine block. As soon as I get home I can look that up on a wiring diagram.

Try running it with a little choke to see if that makes it better or worse. If it does run better with a little choke, pull the carb again and check all of the passages. There may be a blockage in the pilot jet system that is restricting fuel flow. If it gets worse, you may have a float level that is too high and causing extra gas to flood into the motor. This would also lower your compression readings because of the fuel washing down the cylinder walls.

Check for leaks on the intake manifold and exhaust manifold that may be causing an air leak. Also look at the crankshaft seals for signs of leakage that would indicate an air leak.

If everything checks out then I would take a compression test when the engine is warm. If the compression drops any lower than what it already is, there isn't much you can do put pull the top end.

That is how I would approach the problem.

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