New to me 2007 YZ450F....... stalls when accelerating hard?

Hey guys,

I picked up a really low hour 07 YZ450f special edition/white on Saturday. Love this bike it looks gorgeous, and has barely been ridden by the previous owner, I believe he told me he only ran it once all last year.It is the original tires (worn out), chain, Yamaha grips... etc etc.. I previously had an 04 YZ450F that treated me great, and never had any issue's at all. When I went to look at the bike his yard was real small so the test run was short but I was able to try and put her through the gears and hard as well. It ran perfect, fired right up both hot and cold.

So I bring this new to me bike home on Saturday, warm it up and drain the oil, pull the oil filter, completely drain it, add new filter, fill up the oil with Motul 7100 10w40 syn oil. I removed the air filter and installed a new clean properly oiled air filter. Next thing I did was remove the factory fuel/pilot mixture screw and installed an aftermarket easy adjust billet screw. Yes I did make sure the spring, washer, o-ring were all in place. I installed the fuel/pilot mixture screw bottomed it out and backed it off to 1 1/4 turns. Bike is now ready to go for my first ride. I gather up my kids and they get there gear on and we all take off on our bikes.

The YZ450F starts up first or second kick every time and is idling perfectly. We quietly make our way through the trail and get out back. Once I get into the open field I start to giver the throttle fairly aggressively, running through 1,2,3 all is well until it reaches 4th then it cuts out as if it's starving for fuel?? It does this several times? Sometimes it'll come back and keep running and other times it stalls completely??

I bring the bike back home and remove the sub frame and then remove the float bowl, there is some small debris in the bottom of the bowl, I clean it out and remove the main jet and blow compressed air through just about everything I can without removing the carb. I put the bike back together and try it again............much to my dismay the problem still exists!!!

I tried removing the fuel line to check for flow and there is plenty of steady fuel flowing.

Any help???? Please!!




Anybody???? Gray I know your out there........ advice??? Hope to hear from some one soon.......anybody see this post??

You say you've tried checking the flow through the fuel line.

Will the flow continue for 30-40 seconds without slowing?

If no, check the gas cap vent and fuel line check valve

If yes, then pull the jet access plug off the bottom of the carb and check the flow.

Thanks Gray, I did solve the issue tonight after work. The spark plug cap was loose and so was the spark plug!!!!!!!! Removed and tightened and she ran perfect!!!

Really strong, very pleased!!

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