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XR650L with CRF450R carb, float bowl issue?

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Hello everyone,

So the biggest performance bang for your buck IMO is a FCR carb swap in your XR650L. I have an '03 that I swapped in a 40mm carb from an '08 CRF450R. The swap was easy, and I noticed a big increase in throttle response.

However, intermittently, the bike falls on it's face when I give it throttle. If I let off the throttle, it will barely maintain speed, and if I twist it again, it just bogs. Like it's not getting fuel. After cruising slowly or stopping for a while, then the bike runs fine. It's sporadic and definitely is instigated by wheelies. Fuel flow is OK with a full tank, so I'm thinking the float level needs adjusted? It's almost like...if the float bowl is full, it runs great. After the float bowl is depleted, it falls on it's face.

Any suggestions, or other ideas?

Thanks for any help!

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Accelerator pump could be causing a rich bog. I had an FCR on a 1994 XR650 and it ran really well, I forget what jetting I swapped in though.

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Float level can be checked externally.  Put a clear line on the overflow, and loop the line up above carb level in a gentle bend.  Open the overflow screw, and the fuel level in the line will equal the level in the bowl.

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5 hours ago, heart_of_darkness said:

Float level can be checked externally.  Put a clear line on the overflow, and loop the line up above carb level in a gentle bend.  Open the overflow screw, and the fuel level in the line will equal the level in the bowl.

Thanks for the replies everyone. I don't think it's the acc pump. The bike "normally" runs good. The problem is just intermittent. I'm going to give the float bowl some more fuel and go from there.

 

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Follow up for anyone else experiencing this in the future. I adjusted the float level tang a few mm's, giving it more fuel, and that's all it was. Runs great now, even when riding long wheelies. My logic is the stock float level setting on a CRF450R carb doesn't keep the bowl full enough for the volume of fuel the XR650 pulls.

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All you did was make it run richer by raising the float level , that isn't a cure for an intermittent problem. What you did was get the crap out of your float bowl that was getting sucked into the jets when you took it apart.

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Are you sure your FCR-MX is off of a verified 2008 CRF450R? Because that year they made a 41mm carb.

If so, then the float height should be 6mm instead of the previous 9mm.

I bought the FSM for my 2008 to make sure I was setting it up right. They made some changes to the fuel circuits on the '07-08 carbs.

Like JoeRC51 said, you may just be masking the problem. Better to get your constants as they should be, instead of just randomly bending your float tab until it works... that way you can know what the problem at least isn't, and narrow it down to what it truly is.

If you adjust your float to 6mm (or 9mm if it's a pre '07-08), and the problem persists, you know you need to address another issue to get it sorted.

IMG_6437.JPG.4cdecd319fd4eefcb974ed7f561a8e0b.JPG

 

Edited by CongoXR650L

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Guys, I really appreciate your input, however, I thoroughly cleaned the carb inside and out when I first got it off ebay, even though it was very clean to begin with. I do not believe it was a dirt/debris issue. I changed pilot and main jets several times to no avail. Only after I dropped the float level tab a few mm's is when the problem went away. I've ridden it several hours, and I mean rode it hard, and it is now running consistently strong. Even with the front wheel up in the air for extended periods, it is continually running smooth. I'm convinced the float bowl was starving for fuel. I just wanted to follow up with this for anyone in the future having the same issue.

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