yz450f help????

Hello, Im new here, have been reading some posts for a bit and figured I'd join. I own an '05 raptor660 and an '04 yz450f. I need to pick your brains for a bit.

My yz sat for a few months and I went to ride it it was hard to start and once started it did idle with minimal sputter. As soon as throttle is opened in any positon it sputters, cracks, pops, and backfires.

I have torn down the carb and cleaned a ton of dirt out of it, re-installed it and still have same condition.

Jetting is dynojet for stock bike and was running perfect before. Fuel is fresh cam2 purple and has fresh sparky.

I have a service manual on the way and I am an auto mechanic so I hope to be able to fix it but some of you may be able to save me some time and frustration.

Sorry for the long post and thanks for any info!

forgot to add: Checked valve clearance: perfect, performed cylinder leakage test has 5% leakage, checked cam timing: perfect

The carb is still dirty. It needs TOTAL disassembly and cleaning with the PROPER solution, not automotive carb cleaner (which can damage rubber components....seriously!). Check with your local Yamaha dealer for the carb cleaning solution, which (believe it or not) you dilute with water OR gasoline!

I just left the carb and had them do it. End of problem.

Good luck!

I agree, the jets must still have debris in them.

Im kinda leaning that way myself I have a carb acid tank but didn't use it cause I didn't want to remove tps until I knew how to adjust it. Thinking about a full overhaul but the damn fcr kits are spensive.

You don't need a kit. You just need to get the passages clean. All you will need is perhaps some new stock or similar jets.

Good luck!:excuseme:

im not gonna buy unless I find something worn out and if thats the case probably cheaper to buy pieces. The old seals are sealing so I don't really see the need to spend money on them.

Well I guess I'll tear her back apart Saturday and post what I find. Thanks for the help.

My only other concern was ignition timing but I don't see any way for that to be off....you either got spark or you don't at least thats how I see it

Avoid removing the TPS for any reason. Although it's much easier to get them set up on a steel bike than the newer ones, it's still not that much fun.

Besides, the carb body has a gasket between the upper and lower section that is VERY hard to buy. The body is not likely to be wear your problem is, anyway.

The culprit is more probably a varnish deposit in the pilot jet (manual calls it a slow jet), or at the point where the pilot fuel mixture screw seats in the body. Also check the pilot air jet at the carb mouth.

Use the least aggressive method you can to dislodge the deposit. Read:


Also, check that the hot start plunger is free and closes completely.

You can get a PDF manual for free here:


Right on..jusr got the manual in the mail 10 minutes ago...ive been looking for pdf all week...oh well cant read a pdf in the can anyway

...oh well cant read a pdf in the can anyway
There are two sites I know of to download manuals from, one in Europe (the one I posted) and another in Australia (see the Common Threads FAQ's sticky). Most of the manuals on the Euro site are printable, so you can print a page, read it in the can, and then "finish up" with it, if you like. :excuseme:

Got a question....I probably wont remove tps unless I decide to dunk the whole carb. As far as adjustment being a pain I assume you mean having access to the screws but why can't I adjust it while its off the bike?

Because to adjust it, the engine must be running at a specified RPM, and the output voltage is read and adjusted to specs. The TPS is quite sensitive to movement one way or other, and the process can be somewhat frustrating.

(See 6-9 in the manual)

yeah dumbass question was thinking it was adjuted by resistance not voltage

Just tore carb apart and haven't really found much. Hot start plunger o-ring is pretty worn, if this were to leak would that cause my problem???

well there is definently no dirt in this thing and still runs like poo. Do these things have any cdi issues. Only thing I can figure is an ignition timing thing if thats even possible. Unless you guys thing I missed something but I went over it for hours so I don't know. Id hate to throw a $300 CDI on there and be wrong.

Time for a topend maybe? I have an 04 that ran like crap when the compression got really weak on it doing the same things you are describing with the popping and all that.

check the slide plate on the engine side of

the carb. You could have installed it upside

down or stretched the o-ring on the back side

with the carb clean,this is a common oops!

Time for a topend maybe? I have an 04 that ran like crap when the compression got really weak on it doing the same things you are describing with the popping and all that.

well I thought the same thing but the top end is fairly new and cylinder leakage test was perfect.

check the slide plate on the engine side of

the carb. You could have installed it upside

down or stretched the o-ring on the back side

with the carb clean,this is a common oops!

thats possible, the oring fit was good but that doesn't mean it was sealing so I dont know what to do.

My buddy went over the carb once and I went over it once and all seems well. I found a used complete used ignition on ebay for 60 bucks do you guys thing its worth a try?????

Anyoe ever have the decomp system stick?

Just to sum up my new questions:

No more dirt found in carb its been gone over twice all passages cleaned with chemicals and shop air....unless its eluding me but I nearly completely dissassembled carb

1. found o-ring on hot start plunger worn (not much resistance, can this suck air and cause my problem?

2. Can Ign timing be off due to cdi failure or would cdi failure just result in no start (have over 40kv of spark) Found used for 60 bucks, should I try?

3. Can decomp system stick and cause me compression loss? I should've checked when inspecting valve clearence but I didn't??

  1. The O-ring on the hot start isn't an O-ring, it's a lip seal. The lip should face upward to seal air out of the carb. If it leaks, it could be noticeable, but it will almost work without the seal even being there.
  2. The CDI maps the timing, so yes, it's possible for the firmware in the CDI to become corrupted and screw with the timing. Have you tried unplugging the TPS? Did you have the TPS off the carb?
  3. The decomp system is active at all times that the engine rotates at less than 700 RPM. It reduces compression, it does not release compression completely. If it stuck, your engine would be ridiculously noisy (stuck active) or impossible to start (stuck inactive).

Thanks for the input you have been a big help. I did unplug the tps with no change. I did not remove it. And I tested it to be sure and all seems well. The hotstart works (will kill cold engine) seal seemed a little worn but seemed minor. I may just try the used cdi since I can't veryify spark timing. I feel its a long shot, in my mind a failure would result in a no start. But anyting is possible with solid state devices so its worth a $60 try. Worst case I will have extra parts.

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