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YZ250 FMF Cylinder Ported?


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Hello all!

Long time lurker here, great site! 

Long story short, I pick up a bike with low compression for a little project. I notice "FMF" on the side of the cylinder ask him if he knows anything about it? He replies "the guy I got it from said it was an FMF race bike, but I don't think so. Figured someone just carved that in the side." 

I have never had my other yz250 apart, looking at the ports and comparing them to the few pictures I have found, it does not seem the cylinder was ported. Admittedly, I very inexperienced when it comes to any 2 stroke porting. Stock rebuilds is how I have always rolled. Also, I noticed the "c12 only" but the head only looks lightly milled, if at all. Any expert eyes out that have some input?

I am trying to keep this rebuild reasonable since the holidays are around the corner. There is a gouge in the cylinder from the ring breaking. Last time i sent a cylinder off for repair it was 350ish. If it is a stock cylinder I may try to find a used one in good shape a little cheaper. 

Any help or suggestions would be great appreciated! 

 

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I'd wanna fix it up just to see how it runs.  But if you don't want a top end (loss in low) motor that you have to run race gas buy a stock cyl. The lack of squish radius shows the heads been reworked could have even been setup for a different piston?  Exhaust port timing seriously raised, case matched and hogged. How much the head is cut isn't the only thing determining compression and fuel requirement. If the porting pipe and case volume all work in unison it can be stuffing 270cc into a 250cc space and have a running compression increase.

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Thanks for the help, Highmarker!

Had some time to get back in the garage yesterday. It is a big bore kit 68.5mm, so 265cc. Guess it is worth repairing at this point. 

Haven't rode a ported or big bore bike before, this could get interesting LOL

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Well the saga continues, but completely my fault. 

I was told it was a 2000 model. I just took that at face value, which I know better. Some things weren't adding up as I tearing it down. Graphics and white frame were throwing me off, then the piston size. I forgot the year of the frame color switch and assumed it was right. So i check the vin like I should have done from the start. It is a 96, so stock bore. I'm was just too eager and got ahead of myself. 

I am going to rebuild it anyway and bring it back from the dead, keep it around for friends and such. I might start a build thread to keep me on track and motivated.  

 

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Yup, still a ported cylinder and cut head.... Undecided wether to leave it or go back to pump, found a cheap stock head. Could try that to get the compression back down to pump levels. 

C12 is pricey, especially just for a trail bike! Plus who knows what the current piston this head was set up for. 

Edited by Sc0tty
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