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Low compression cause bike to run rich?

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04 cr250

Fmf gnarly

New boysen reeds

Clean carb and air filter. 

My bike is still on the rich side. I ride it around for a good 20 min. Rapping it out hard on straight aways and lugging it a hit through tight sections, just giving it a good balance of riding. Before I shut her down I hit the throttle hard on a strait away and let it return to an idle. Then I kill it. While it's hot I pull the plug and it's a but wet and black.  

It was fouling a few weeks back too easy. It struggles to idle even with the idle screw turned all the way on and the air screw all the way out. 

The reeds were worn so I thought that could be a contributing factor so I replaced them. Seems to help a little but the plugs are still wet. I've gone down from a 400 main jet to a 370 and the plugs are still wet. 

I have not checked my compression yet but I read that low compression could cause a bike to run rich. But I've also read it'll cause a bike to run lean. Which is it? 

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Low compression will make the engine act rich.


Carefully leaning out the main jet too much, especially if the bike ran well before on the same jetting.


Check your float & fuel level, too high and it will richen each fuel metering circuit.

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Low comp. will act up as a rich cond. since the combustion of the engine is no longer burning the fuel efficiently.   Do you know how many hrs. are on the topend ?

What needle ? Clip position ?  PJ ?  Temp & elevation ?

I would never advise to go 3 steps leaner at once on the MJ as it could have catastrophic consequences.   

Edited by H4L

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