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1986 KX250 HELP!

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My friend has a MINT 1986 kx250 that ran great until last summer.  The bike is in superb condition, well taken care of and always ran great but now it has a problem where it runs away once started.  It has tons of compression (no reading, sorry),  Just cleaned the carb thoroughly, and all else seems to be in good order.  

The bike starts normally only sometimes but most times it screams to the rev limiter once started and sometimes the kill switch doesn't even kill it, so i have to disconnect the plug.

I feel like the problem is an air leak somewhere that is causing a lean condition.  (crank seals???)  but i sprayed some carb cleaner over the intake boot while running normally and it showed no signs of a leak there.  Also, the throttle is Not jammed opened and seems to seat normally.

 

Anybody have any ideas about what the problem could be?  Need this thing running good asap.

 

 

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Sure sounds like an air leak! Could be the crank seals. They are 28 years old if they haven't been replaced. I wish i could find a nice 86 around here! Was my first KX I bought it new!

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Sure sounds like an air leak! Could be the crank seals. They are 28 years old if they haven't been replaced. I wish i could find a nice 86 around here! Was my first KX I bought it new!

 

 

There was an 86 KX250 for sale local to me. Can't find the ad anymore.  bummer! Was cheapish. needed a top end and some plastics, looked complete otherwise. (topend was there just needed redone)

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Sure sounds like an air leak! Could be the crank seals. They are 28 years old if they haven't been replaced. I wish i could find a nice 86 around here! Was my first KX I bought it new!

That's exactly what I am thinking.  I think If not the intake boot, crank seals are the only other thing to cause an air leak.  They are about $15 each on RM so price is no issue. But replacing them is the problem.  From the looks of the exploded diagram on RM OEM parts page, it looks like these seals are pressed in from the inside of the case.  Which means splitting the case just to replace them.  

Can anybody confirm this??

I really dont want to get too involved so What is my next move?  What options do I have?

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Pay someone to do it lol. Pretty sure they are inside too but can't confirm. Youtube video perhaps? Idk if there's any of 86 motor rebuilds tho Probably not a ton different tho.

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You will need crank bearings also. While its that far apart I would do a top end, new reeds etc. just to be safe. Take your time, get a case splitter and a good manual. It isnt that bad of a job.

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Before you bust into the cases why not do a pressure test?  You can buy stuff to build it from a hardware store for under $10.

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Thanks for all the responses.  I guess I will try a pressure test then.  I found some instructions on how to built the tester and it looks very straight forward.  

 

Does anybody know offhand what size the intake boot and exhaust port is?

 

I'll post back if and when I get any results.  

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Using propane is a better test method. You can direct it straight at any air leak.

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I'd honestly be surprised if the problem was a seal.  I'd check the throttle cable and carb real thoroughly.

 

Large air leaks might let the motor get away from your for a few seconds but it should quickly starve for fuel if the throttle is closed.  The crankcase isn't under constant vacuum either, so you'd probably lose more fuel and air during primary compression than it would draw in.  Reminds me of one time I was screwing around with an RD400 parts bike and it started up with no carbs or pipes on it.  Ran up to 9k RPM or so real quick before it sucked all of the old fuel from the crankcase.  That was loud.

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