Still fouling plugs...Help!

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Dec 12, 2002
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#1
OK, here's my situation. 1980 RM250. Bike runs well, but feels blubbery on the bottom, yet when I sit and idle the bike and blip the throttle, it has a lean rev as the rpms come back down. I don't know how to describe that. The thing is, I foul plugs constantly. The bike is extremely easy to start with a fresh plug, but ride for a bit and it becomes more and more difficult and eventually won't run without a plug change.

I have checked compression, installed new boysen reeds, replaced the right crankshaft seal. New intake manifold, head and cylinder gaskets. I am sure I don't have an air leak or an oil leak, but my plugs foul constantly.

The jetting is close to stock but the needle and needle jet are no longer available and I used a slightly leaner counterpart that was available from Sudco. Float level has been set to factory spec.

Do I ignore the sound I am getting when blipping the throttle and go leaner on the pilot circuit? Mess with the main? (main feels OK by the seat of the pants measurement)

Where do I go from here?
 
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#2
From here it sounds like an air leak,regardless of your figuring, you need to do a leak down and compression check! The jetting sounds fat on bottom, ride it where you have issues and reference your plug,wet,wayyyy rich!
 
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#3
Well, unless there is a hole in the cylinder, the only place left is the left side crankshaft seal. I hope I can get that one without splitting the cases. I would think it would sound lean on top if that was the case though.
 
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#4
read the boyeson reeds instructions, it says that it will cause your bike to run a little richer.
 
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#6
You replaced the right side crankshaft seal? Is that the one behind the flywheel or the one inside the crankcase behind the clutch?
 
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#7
I already tried the hotter plug, that didn't work and I knew itwould only be a temp fix if it did.

I replaced the right side crank seal. The one behind the primary gear that drives the clutch. In response to the post about an air leak, I have ordered the left side crank seal and have pulled the flywheel. When the seal arrives, I will replace it. Where else could I be leaking air?
 
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#9
Head gasket is brand new. Intake boot is brand new. Crank seals (at least right side) were known to go bad on these old RM's. Crank bearings are fine, no play or roughness. The left stator plate looks like it has never been off, so I kinda assume the seal is 27 years old. Every oil seal on the rest of the engine was bad, so why not this one?

There is no rubber boot around the cable at the carb. Could that let in enough air to make a difference? I can't imagine it would.
 
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#10
Letting air in the top of the carb is not a big issue, dirt is! If the crank feels tight, 27 years old, replace all the seals! How is the boot from the carb to the airbox?
 
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#11
easy way to check if you got a air leak problem is to spray starter gas around the parts you suspect might be leaking when idling the engine, and if it revs up you found a leak. pretty nifty trick

altho im by no means any expert but wouldnt an airleak mean it runs leaner ? and you should be able to see that on the plug pretty easy, and if its a bad leak it would actually melt the plug (the electrode). again im no expert but this is what ive been taught
 
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#12
The airbox boot was also replaced last week. Lots of the rubber on this bike was stiff and nasty, so I replaced it.

Asscobra, the contention is that although the bike is jetted rich and showing signs of it at the plug, there may be an air leak that makes a lean surge when idling and blipping the throttle.

My new left side crank seal should be here next week and I'll find out then I guess. Anyone have any suggestions for how to remove that seal without scratching the crankshaft? The right one had a feature that makes it easy, but the left seal doesn't have that.