New to me 04 YZ450f

Hey guys i just picked up a 04 yz450f yesterday. It was 1100 bucks and is in very good shape but needed a head gasket. The previous owner said it was extremly well maintaind and ran fantastic. I replaced the headgasket and the cylinder wall looked perfect inside. Im a diesel guy so im not very good with these little gas engines. i used the wrong timing mark on the flywheel the first time but have it fixed now. i cleaned the carb out and changed the oil and air filter. i couldnt kick start it at all so we pull started it and it would harly turn the wheel cause it has so much compression but it fired and it took a lot of throttle to keep it running. It had a lot of power but im used to utility atvs so not a real fair comparison. it would die instantly if i tried to idle so im not really sure what could be the problem.

If it is timed correctly, an '04 YZ450F should be about as hard to turn over on a bump start as a 200cc two-stroke.  IMO, that's a clue that you need to check your cam timing again.  


As for being hard to keep running, that could also be a symptom of timing, but it's more likely that it has to do with a partial blockage in the pilot jet or aprt of that circuit.


Another thing that people unfamiliar with the bike will do sometimes is install the floating vacuum plate on the slide upside down.  Even though it looks as if it should go the other way, the "square" end with the hole near the edge goes down. 

Well i never took the slide out so i didnt put it in backwards but the previous owner might have so ill check. When i get home ill take a picture of the timing and see what you think. Its right to the best of my knowlage. I think i counted that 13 pins between the upper hash marks right also. When i cleaned the carb i cleaned the two jets in the bowl and cleaned the accelerator pump passages.

Edited by jeffvian

And when i said i couldnt kick start it it would turn over but i meant i couldnt get anything more than an ocasinal backfire

 ... I think i counted that 13 pins between the upper hash marks right also.


When i cleaned the carb i cleaned the two jets in the bowl and cleaned the accelerator pump passages.


The count of pins between the marks at 12:00 cannot be depended upon for timing cams except for the cams being in the correct relationship to each other.  Those marks are an aid to assembly only.  The actual marks that align with the head tell you whether each cam is in the proper orientation to the crank, which is the important element.


The pilot jet is notoriously difficult to clear by anything other than some physical means such as a piece of wire, or best, pushing the shank of the correct size number drill through the orifice.  The metering hole is so small that even a film no more than .001" thick will reduce the effective jet size by 6-7 sizes smaller.

Well i think i have the mark vertical at 12 and the other two to the outsideflat with the head. Could that jet being plugged be causing the problem. How many jets are there. Im not good with these carbs. Why is there like five overflow tubes?

I mentioned the pilot jet because it's not only possibly the problem, it's common after a carb has set around for a while.  There are 5 jets, of which the main air jet isn't removable.  It's in the air bell next to the pilot air jet.  Neither of these are usually involved with trouble.  In the bowl, there's the main jet, the pilot jet, and the starter jet, this last of which provides additional fuel when you use the "choke".


The center overflow tube is actually an overflow and drain tube.  On some carbs, there is a small Allen screw on the bowl next to this tube that can be opened to drain the carb.  Either way, it connects to a standpipe in the bowl that drains gas out if it reaches that high a level.  The other 4 tubes are redundant vents. 


Free manuals from Yamaha:!%20Mods%2C%20Maintenance%2C%20Common%20Issues%20and%20FAQ%27s%20-%20YZ%20400%2F426%2F450%20-%20ThumperTalk&txt=http%3A%2F%2Fwww.yamaha-mo...uals%2Findex.aspx!%20Mods%2C%20Maintenance%2C%20Common%20Issues%20and%20FAQ%27s%20-%20YZ%20400%2F426%2F450%20-%20ThumperTalk&

Alright thank you. I will try recleaning those extra well and letting them soak overnight in carb and choke cleaner. When it comes to these carbs im basically an idiot

Also, here's three more don'ts:


Don't remove the Throttle Position Sensor (TPS) from the carb unless you want to suffer through re-calibrating it,  and...


Don't soak the TPS in anything


Don't separate the upper and lower carb body halves from each other until AFTER you have a replacement gasket for that joint in hand.  They are available only from JD Jetting as far as I know.

I think the cams are as good as they can be. The exhaust cam is right above the head and the intake cam is just barely below. If i moved the intake one tooth it would be above more than its currently below. Ill try to get a clear picture but my phone camera is screwed up. The pins between the cams to help decide if i should move the intake one tooth, is it only the pins between the dots or one onside and the other side one on the outside since the dots are exactly in the middle of the pins. And its 13 pins correct?

The key is in where the crank is.  Be sure that you are aware there are three timing marks.  From left to right as you face the flywheel, the first two should be connected by a horizontal line that makes them into a sort of "H".  These are for strobe checking ignition timing.  The THIRD mark, sometimes called the "I" in "HI", is the TDC mark.  If you have any doubt, you can probe for TDC through the spark plug hole. 



My intake dot is just a little lower but my chain is tight. That one appears to be lose. Could just be my eyes though. My chain might need replaced though. Im using the I on the flywheel. My timing must be pretty close if it ran the other day. I tried letting the jets soak all day today so if it dont run tonight ill probably order a jet kit and just try some new ones. 

Well i pull started it again and it ran but still didnt want to idle. I went and turned the idle screw up some till it would idle but it seemed way to high. I ran it for about five ten minutes and half way through running it the first foot of the header was slightly red. When i was about ready to shut it off i went to push the choke in to see if it would idle and i just bearly touched it, and didn't push anywhere hard enougb to get the choke in and it instantly died. And it was dripping oil out of the valvecover brether tube pretty bad. I drained the drain in the middle towards the back of the engine and put a quart in. Could i have put to much in or put the bike together wrong. This is starting to frustrate me

Edited by jeffvian

Start by going to the common threads topic and reading up on the oil change process. If you drained the engine, but not the frame, then yes, a quart would make it almost twice as full as it's supposed to be.

The glow on the header is normal because the bike was running at a high idle not being ridden for too long a time.

The symptoms you're describing are a classic match to a clogged pilot, which I've already pointed out are notoriously unlikely tone cleaned up by soaking and blowing.

Alright well i ordered a jet kit so ill just replace them all with new ones. Thanks for all the information you have given me

Alright well i ordered a carb rebild kit from shindy or something like that off rocky Mountain atv. I installed the three jets in the bottem bowl today and it still wouldnt kick start so we pulled it. After it was running i rode it and warmed it up then increased the idle some so it would stay alive when i put the choke in. The idle speed seemed pretty good maybe a smidge high. I shut it off and tried kicking it but it still wouldnt fire. I turned the choke on and blipped the throttle twice and kicked it and it had a tiny pop and didnt kick real hard and kinda tried to go but didn . Do you think the carbs just tuned wrong or what? It seems really close now. Im not sure what all the factory settings for the idle and air idle screw in the bottem and i think the air screw in the intake when you take the air box off.

Edited by jeffvian

There is no air screw.  It's fuel and air, so out is richer, in is leaner, and yes, it's on the bottom and it's a pain in the butt.  Extended screws with thumb knobs are popular and available.  


Start at around 1 3/4 turns out from just seated. 

No i have the air fuel mixture screw already with a aftermarket thumb knob. The air jet in the mouth of the carb i screwed out some but im assuming that made it more lean so i need to screw it back in snug right?

The air jet in the front of the carb needs to be snugged into place like any other jet.

Well snugged the air jet up and put air/fuel screw at 1 3/4 turns out and wont pop or anything when kicking it. It wont even pull start. Seems like the more i do the worse it runs. I have no idea what it could be

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