Newly developed flame out problem

I have had my CRF450X a few years now. I uncorked it and rejetted using the JD jet kit when it was new. The jetting has always been spot on with no flame outs and no bogging.

This weekend, while riding some of the really tight trails that I normally ride, I was having flame-out problems. Every once in a while, rolling off the throttle in first gear, it would die.

Any thoughts as to what may be causing this?


Valve job was done this past winter. Valves have been checked a couple of times since and are within specs.

My first instinct is to pull the carb down and clean the jets with a piece of bread wire.

This has almost always cured my bike when it starts to run odd. :thumbsup:


1. partially clogged pilot

2. water in / bad gas

3. old plug

4. vacuum slide plate seal is bad

5. Fuel screw tip is corroded

6. Leaky hot start plunger

Maybe it's just too rich in hot weather. I would lean out the AS a half turn and turn up the idle.

Maybe it's just too rich in hot weather. I would lean out the AS a half turn and turn up the idle.

coob your thinking pingers!:thumbsup: the x has a fuel screw, so in to lean it out

coob your thinking pingers!:smirk: the x has a fuel screw, so in to lean it out

I didn't say which way to turn the air screw:)

coob your thinking pingers!:p the x has a fuel screw, so in to lean it out
I didn't say which way to turn the air screw:)

If you really wanted to do it, you CAN put an air screw on the FCR. Now as to why to actually go through all that trouble for something almost impossible to get to, I am not sure, but it can be done...

For the first gen FCR (and street models) SUDCO makes an adjustable air jet that replaces the one in the intake bell of the FCR. I haven't looked on the newer FCR MX to see if the air jet was the same between those two FCRs, but if it is, you could throw that adjustable jet in.

Only reason I know this is that I used to have the SUDCO FCR conversion carb on the XR and it came with the adjustable air screw. You had to slide the airboot out of the way to get to it (easy trailside change on an XR650R, but next to impossible on a CRF/X) but it adjusted the same as the old air screw did on the smokers.:smirk:

Now I guess I have another project to look at for the FCR-MX carb I have on my XR this coming winter...

Some people call the air screw a fuel screw and vice versa. Either one is correct and it makes no difference 2 or 4 stroke but Kalvin22 is correct in noting that the same direction of turn causes the opposite to happen 2 vs 4 stroke lean vs. rich.

Part 6 is the fuel screw (air screw) while part 43 is the slow air jet.

Not to confuse the issues....anyway the OP might try what I first suggested to help the flame out for me this time of year

Here's another example the fuel screw being called and air screw:

Edited by cubera

The pilot jet was clogged. I haven't had a chance to ride any single track yet to test it though. Thanks for the input.

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