YZ450F Revving problem.

Hi I just recently purchased a 04 YZ450. My first couple of rides went great but today when I started it up and started riding I could tell it wouldn't rev up as high as normal. The only thing that I have adjusted on it is the idle because during riding when it would get hot if I just got off the gas and let it idle it would shut off. Other than that I haven't done anything to it. It is completely stock. I sat and let it idle for a few minutes and lowered the idle back down but when I would rev it up it would sound like it was hitting the rev limiter too soon. It also would do that when I would get on the gas while riding too, it would get up into decent RPM's and then start popping like it was hitting the rev limiter. Does anybody have an idea of what is wrong or what I could do to fix it?

I just noticed and saw that I used the wrong gas for my bike. I have 92 octane for it and regular 87 for my lawnmower and stuff and I grabbed that one instead. Could the lower octane fuel be the reason why it won't rev like usual?

I don't know if the low octane gas would do that to the rpm's ... It certainly isn't good regardless, and you should drain it out and put in some of the good stuff right away ... What you're probably getting is a whole lotta ping and that's not good ... Bikes should never backfire when on the gas, that's just bad ...

Sounds like a jetting issue to me. Has the temperature changed between when you were riding it before and when the problem began to occur? Are you riding at the same place (same elevation)? Was the humidity the same every time you rode it? All of these can affect the jetting. As for the gas, I don't think it is your problem, but you should probably stick to the 92.

I just drained the gas and put in some 93 octane and added some octane boost. And it got alot better but not totally back to premium running. It still seems to sound like it is hitting the rev limiter early and the throttle response seems a little sluggish. I am riding in the same place I have been riding. No change of elevation, and the humidity was about the same too. The temperature was about 7 or 8 degrees cooler today but I dont know if that is enough to make a difference with the jetting. Any suggestions on what I should do?

Check the condition of the blue wire running up from the neutral switch. This wire is grounded by the switch when you are in neutral, and part of what it does is to lower the rev limit. If the wire looks OK, try disconnecting it (up under the tank), as the switch itself could be shorted.

Another thing that will cause this problem is a clogged air filter (shame on you if it is), a blocked jet (stuff happens), or a simple bad spark plug.

Ok... I just got done checking the Air Filter and it is fine. I checked the Blue wire and it seems fine too and I disconnected it and no change from engine performance. The jets are ok also. I haven't checked the spark plug yet but just incase I am going to replace it tomorrow as soon as I can get over to the parts store. I have a feeling it is the spark plug because before I bought it the guy let it sit up for about 7 months and with the amount of riding I have done the last few days could have done it in. If this doesn't fix it what should be my next move because it still sounds like the rev limiter kicks in early and backfires a little bit when I am on the gas?

Have you cleaned the carb? If the bike set for 7 months it could be gummed up from the old gas, if the bowl wasn't drained during storage. Have you physically checked the jets?

I haven't cleaned the carb yet... and i dont know if the bowl was drained before storage. As far as the jets go when I bought the bike the guy said before he put it up for storage he checked them and said they were fine. could it be the fuel screw? I don't know where it is set right now and it could be running too rich. what is the recommended setting for a stock yz450?

what is the recommended setting for a stock yz450? 2 1/8 turns out

Ok today I cleaned the carb and jets... I also replaced the spark plug with a CR9E (NGK of course) rather than the stock CR8E thinking it would do a little better since it is a hotter plug but it ran worse... popped and backfired really bad. I think it is the plug so I will be pulling that plug out and putting in a new CR8E instead and see how that does.

The CR9E is one range COLDER than a CR8E. Japanese and Euro plugs run in reverse of American ones.

The CR8E is the correct plug.

87 octane shouldn't do that to any bike. Most of the newer bikes like to run on higher octane but i just bought a lightly used '04 yz450f and run it on 87 with no problem. Is there any varnish in the carb or places equally important because if the previous owner let it sit..... you never know try running a tad of fuel system cleaner. Mine wouldn't idle when i bought it (sat all winter) and i just rode it for one tank of 87 with a tiny bit of fuel system cleaner idles like a champ.

Ok.. Everything is good to go. I cleaned the carb, the jets, put 93 octane in it with some octane boost, and changed the spark plug back out to a CR8E and ran it for 2 hours and it runs like a champ! I think it had alot to do with the plug. From it sitting up for 7 months the plug was real gummed up and so was the carb but its rippin up the track now. Thanks guys!

All right! Ride the wheels off of the thing and have FUN!

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