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XR650L falls on its face after 3/4 throttle

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Okay so last summer i had to rebuild my xr650l engine after piston ring failure. its been bored out to 102mm, running stock cam still and i put an FCR carb on it. i noticed that if i where to take the throttle past 3/4 it would stumble and pretty much fall on its face, until i backed up on the throttle. I think its a jetting issue, but thought i would get your guys input. I run 91 octane in it and, the needle is set on the 3rd clip from the top, main is a 180 pilot is a 42, and the starter jet i cant tell is either a 68, or a 89. i pulled the plug and it looks pretty rich to me. So i know i need to drop the main down but what about the pilot, and starter jets? 

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Edited by rheim954

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I have a 102.41 mm piston in mine with 10.5:1 compression.  I'm a bit rich with a 158 in there.  But it pulls strong right up to redline.  I was going to go with a 155, but Steve the engine builder in SF recommended keeping it rich to aid in cooling and to avoid detonation.  It runs good at altitudes from sea level to 6000 feet.

 

If you really have a 180 in there, you're way too rich and I'm surprised that you aren't fouling plugs.

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Agree on the main jet possibly being to large. That would also somewhat depend on the needle you are running.

I am at 158 and live in Kansas as well. The other thing I have seen cause that is to low a float level. Check that while changing the main jet out.

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When I was rebuilding my top end I know I bought a 158 main. I thought I put it in there but I must have grabbed the 180 the FCR came with instead by accident. I knew it was rich cause you could smell gas in the exaughst. But I was okay with that at the time cause I didn't want to lean out my fresh engine and grenade it. Somehow I managed to not foul and plugs out . What difference would it make if changed the pilot from the 42 to a 40? Would that make it a little bit more responsive at low throttle or just lean it out?

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Agree on the main jet possibly being to large. That would also somewhat depend on the needle you are running.

I am at 158 and live in Kansas as well. The other thing I have seen cause that is to low a float level. Check that while changing the main jet out.

I'll check the needle and see which on I have exactly this evening. And I'll figure out how to check the float level as well. Thanks

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Last but not least. Even more a remote possibility.  If you had the carb apart and the slide out.  Check the slide plate (the plate on the slide that can come off). Be sure the oring under it is still in place and more importantly that the plate is not on upside down. It can go on either way but only one is correct.  The hole on the slide plate goes down. That can cause something similiar

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nobody has asked you which FCR yet for some reason.......yes it matters...

 

Is it the slant slide or FCR MX...either way 180 is fire hose rich ..

 

 

B

Edited by brianhare

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When I was rebuilding my top end I know I bought a 158 main. I thought I put it in there but I must have grabbed the 180 the FCR came with instead by accident.

If you don't have the stock Keihin carburetor, then all jetting advice that we're giving you is invalid,

 

Does the bike have an FCR carburetor, or the stock one?

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Then it`s an FCR MX.........and 180 is freekin way large...

 

42 or 45 pilot and a 158 main to start your tuning...an NVCT needle is most people here use too..but toss that 180 in the trash first off..when you have it 3/4 to wide open the main is the main feed,,,and it`s choking itself with far to much fuel...

 

B

Edited by brianhare

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