04 YZ450F won't start!

I've searched and searched, and done everything I thought of, but still no luck. Here's what I've done:

1)new OEM cylinder and new 13.5 comp JE piston

2)cleaned jets

3)new timing chain

4)checked valves (twice) I do have one intake valve that is very difficult to get a .038mm feeler in. But if I were to go down a shim, my gap would move to ~.08, so I left it.

5)installed header pipe gasket (not on before but Gray said it should be there)

Only bad thing that happened (not sure of any effect) is that I chipped a little bit of the insulation off component of the stator when pulling the flywheel.

it did start eventually when I completed the job long enough for me to break it in (and ran great for the most part). I can sometimes get it to start with lots of throttle and kicking the sh*t out of it. When I do hold some throttle on, I often get kick back (normal?). But I still have to hold throttle almost wide open to keep it running. Messed with idle screw to see if that would help, but still can't seem to get it to start smoothly. I know this is a lot, and it's probably something stupid/simple, but I've already missed over a month of riding, please help!!

Edited by jbodin18
If I could delete this I would. I'm an idiot. Was using wrong valve measurements...gonna try again.

Check the plate on the engine side of the throttle slide, especially if you had it out. The square edge goes down, not the rounded one.

Well, I've been totally through the carb after no start and finally correct valves. I had a slow gas leak from bottom tube so I figure its the needle valve and plan to replace. but check this diaphram out...is that supposed to be perforated like that. If so, is it supposed to be spring side or opposite spring? If its torn up for some reason, what would cause it (maybe installed upside down)? I had the post side away from spring. Pic of cover is Raven aftermarket, don't think it actually improved anything though...been on for years. Thanks.

IMG_5664.jpg

IMG_5663.jpg

Nope. Not supposed to have holes in it. The side you have showing is the bottom, and also the spring side. That will cause an external fuel leak, too.

so if I understand I had it in backwards. I had diaphram with post down, then spring, then cap. right? And what would have caused the diaphram to tear like that? And would this be the reason i can't start?

I had diaphram with post down, then spring, then cap.
That would be correct, except for the spelling. (the diaphragm is #31 below) Wear causes the tears usually, and it's unlikely that it has much to do with the difficulty in starting other than the fact that it will reduce or eliminate the accelerator pump shot, which could mean the engine wasn't getting a good prime. Here, however, you said you had a steady leak from the overflow, which should indicate it was flooding.

Is it possible you had a leaking accel pump the whole time, and the gas was running down the outside of the overflow tube?

1240_carburetor.gif

anything is possible at this point. I ordered a new diaphragm and will be in next week. I'll post results. Thanks.

Grey will probably roll his eyes for my posting this, but also look at # 23 (fuel screw) and make sure all four pieces are present (adjuster screw, spring, small flat washer and small flat o-ring). That little o-ring makes a big difference.

Also, the o-ring that fits between the float seat and carb body (above #26) can deteriorate over time, acting like a worn out float valve. Gas will leak around the outside of the valve instead of passing through it. I bought a float valve seat from a 2002 CRF450R and the o-ring was the same.

Also, the o-ring that fits between the float seat and carb body (above #26) can deteriorate over time, acting like a worn out float valve. Gas will leak around the outside of the valve instead of passing through it. I bought a float valve seat from a 2002 CRF450R and the o-ring was the same.

The only problem with that is that this is an '04. The needle seats from '04 onward are not serviceable. They're pressed permanently into the carb body and can't be removed. They very rarely leak, but if they do, it's a new carb...

I stand corrected. The Peanut Gallery rests it's case.

Replaced diaphragm. Started up second kick. Then flooded. Replaced needle valve. Runs great now. Thanks for the help! Guess the flooding issue was just bad timing after doing top end. Wasted a lot of time messing with engine.

Wasted a lot of time messing with engine.

Even though the stuff I touched is what I usually check first when something doesn't work after I work on anything, at some point you have to walk off, then come back and look at the thing as if you never saw it before and don't know any more than what you see in front of you.

Great way to look at it. Thanks again. Going to the track today!

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now