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Is this SX250 Possessed?

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We have a 1997 SX250 that was once raced by NZ's Shane King.

With the throttle closed, the motor suddenly revs up as if someone was blipping the throttle.

This also occurs while riding it!

We have stripped it down expecting to find a blown base gasket or crankshaft seal.

The seals are all perfect, we have replaced the gaskets, piston rings etc.

The ehaust is sealed in the barrel, the reeds are fine, the intake manifold is good, we have stripped and checked the carb, and even replaced it with another.

The bike continues to start and rev up and down by itself with the throttle closed, and cable slack.

Any suggestions????

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You;re right in looking for an air leak. The other possibilitie is the center case gasket. Take a leak down tester and pressurize the cylinder to 10 psi, then go around the engine spraying everything with soapy water. Watch for the bubbles :ride:

 

If possible, see if you can reset it to stock jetting and a premix ratio of 40 or 50:1 with a good premix.

Edited by baseballpro3112

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Sounds too simple, but air boot to carb possibly?

 

That's what I thought too, but he said he checked the carb manifold

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You;re right in looking for an air leak. The other possibilitie is the center case gasket. Take a leak down tester and pressurize the cylinder to 10 psi, then go around the engine spraying everything with soapy water. Watch for the bubbles :ride:

 

If possible, see if you can reset it to stock jetting and a premix ratio of 40 or 50:1 with a good premix.

Thanks I will check the main case gasket for leaks as you suggested! We already set the jetting to stock.

I will also try fresh fuel...

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As with any leak on anything they can be a pin, as above pressure up the cc and soap it down  but first try running it and sprayingwd 40 over everything if thers a leak there will be a noticble sound.

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SORTED!

As is often the case it's frequently the most likely or obvious things!

It turned out to be the crankshaft seal on the gearbox side, we originally discounted this as we expected to see excessive smoke out the exhaust if the seal was suspect.

HOWEVER this is not our bike and we decided to check the gearbox oil and found that there was basically none! Topped up the oil and tow started the bike! There was extreme smoke!

Pulled the clutch out and found the seal was completely out of the crankcase flopping around on the shaft!

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Thanks for the update. It is always nice to find out what the problem was. So often it gets fixed and those of us that were following the thread never get to know what happened.

Thank You.

Larry

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