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'00 YZ250 Carb Tuning Help

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I was trail riding a week or so ago and fouled out a plug, the guy who helped me out said that i was smoking more than usual and that i should go to a 40:1 mixture instead of my 32:1, which i did. He also suggested dropping the needle 1 notch in the carb, which i did today. I also tryed to dial in the carb using the "Yamaha 2T FAQ-Jetting" thread on here. After setting the idle screw and air screw, i took the bike for a spin in between the 1/4 to 3/4 throttle range and got lots of spooge and fluid leaks from the drain tubes and still alot of spitting and sputtering at steady throttle. Should i expect a smooth running engine at steady throttle? Or is this bike not going to run smoothly unless I'm hammering it? The floats are set to spec, newly packed silencer, 2 hours on new top-end, new air filter, reeds in spec. I'm new at this and definitely suck at carbs, going by the manual and this website. Main and pilot are stock. Ambient temp is aroung 60. Any suggetions? Thanks for any input!

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You should have moved the clip up a notch on the needle to lean it out. What main and pilot are you running. What temps and altitude are you running in.

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I would stick with 32:1. Oil is not your probelm. I'm running 45 pilot 175main and a jd jetting red needle 3 groove. Its not perfect but it can idle and not foul a plug.

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I did move the clip up a notch (therefore dropping the needle a notch) to try to lean it out. I'm running a 178 main and a 50 pilot, which is what is in the owner's manual (thought it would get me close). Temps right now are around 50 - 60 degrees and I ride at 450 feet above see level.

Edited by JFoVStar650

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So I found something pretty weird. I pulled the carb off to go through everything again and while I was in there I pulled the reed valve assembly out since I've had some Boyesen SuperStock replacement reeds sitting on my toolbox for a while now. When I pulled the assembly out, the reed stopper (the part with holes in it) was underneath the reeds (solid piece). The manual and a search on here shows that the reeds should be against the reed cage with the reed stopper on top, fastened down with a couple of screws. Did someone before me (I swear it wasn't me!) put the reeds in wrong?? Or is it just a different setup, like higher performance reeds or something? If they are in the wrong order, that would explain why it runs like crap at steady and low throttle....the reeds aren't opening properly. And explains why if I romp on the throttle, it gets up and goes.....because it has enough suction to open the reeds. Am i thinking about this the right way??

Edited by JFoVStar650

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It has an FMF pipe on it but unsure which model. I havn't changed anything on the bike since i bought it used, except oil and plugs. I don't know about the reeds, but I do know that the pics in the manual show the reeds to be split down the middle, the ones that are on the bike are solid. The reed stopper, cage, and the rest of the assembly looks just like what the manual shows. Besides for the pipe, i think everything is stock except maybe the reeds.

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If my bike starts smoking more than before a blown crankseal comes to mind. First thing to do is an oilchange, measuring 750ml and after one or 2 hours of riding get the oil out and measure again. I

50 pilot is what the manual asks for but I run 48. At what is the airscrew set at?

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Only one way to sort out your jetting is by trial and error. You need to swap some brass and experience what lean feels like and rich feels like for yourself. After a messing with your carb enough you will know the sound and feel of rich or lean. Your bike is capable of running well at all throttle openings and speeds. Its a pain in the ass pulling the carb off 20 times but thats what it take to get it dialed in. I would drop a 175 main and a 48 pilot in tune the air screw for best response and see what you get. If its boggy and sluggish its too rich. If it crackles and hesitates its too lean. Don't run it if it does not feel right. If your idle surges and hangs high its lean. If it bogs down and wants to stall and smokey its rich. You could have a bad clutch side seal as well but I'm not sure how common a problem that is.

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It would be good to know what pipe you have too. The FMF SST is know for being hard to jet for and melting pistons. So watch out if it feels lean dont't run it.

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Thanks guys, I'll start swapping brass and see what happens. Does the reed valve asssembly not seem odd to you guys? If it looks like someone put the reed valve and the reed stopper in the wrong order should i try to switch it and see what happens? Or could that be detrimental to the motor? Obviously i would shut it off immediately if it sounded horrible....

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Do you have the instructions for the Boyesens? If you follow those it will guide you in the proper sequence as the Boyesens delete one of the factory parts.

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I cant see how the bike would run with the reeds assembled like that. Boyesen pro series are two stage and have a fiberglass reed and a carbon fiber secondary reed? The stock reed stop is metal and curved it would hold the reeds wide open the bike would probably not run.

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The current reed stop is definitely not curved or metal. Both pieces are fiber and lay flat on top of one another against the cage. I think I will order the OEM reeds and curved reed stop from ThumperTalk just to cancel out that possibility. It that doesn't change anything, I'll start swapping brass, and also try to post a video if I can figure it out..... Thanks for the help fellas, much appreciated!

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If both pieces are flat against the cage I would say its ok then. Why not put in the new reeds you have?

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Because I bought them as stock OEM reeds, and that's not what is on the bike. I would probably need the curved, metal reed stop to use these "OEM style" reeds that i have. Also, i got ahold of the guy that sold me the bike.....it is an FMF Fatty pipe and Boyesen reeds.....although, like I said, the Boyesen reeds must have taken the place of the stock reed stopper and all that.....

Edited by JFoVStar650

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