2003 yz450f Carb troubles. Please help.

[TMAC25]

Ok everyone my bike has officially driven me crazy. I knew better then to mess with the carb on my 03 yz450f. So anyway, here it is, any help would be greatly appreciated. So I put a boysens quick start on my bike and an air adjust screw and a new cap for the hot start cable to enter the carb onto my bike. Now that I have done this, I simply cant get it to run right, I mean it barley runs at all. I don’t know if I stretched a cable or messed the hot start up or simply cant get it adjusted right, but after some adjusting, I can get it started, but it gets super hot I mean my pipe starts to glow bright red, so I turn it off. While it may run if you hit the gas it will stall or snap and sputter. Please help someone, as I don’t want to have to pay the shop. Thanks all.

[Fastest1]

Sounds lean. Is there a leak? If you can get it running, spray some WD40 around the carb boots (if the rpm's change, there is a leak). That is where I would start. If that doesnt help, I would start removing parts I added til it runs good. Double check cleanliness.

[Ga426owner]

Ok everyone my bike has officially driven me crazy. I knew better then to mess with the carb on my 03 yz450f. So anyway, here it is, any help would be greatly appreciated. So I put a boysens quick start on my bike and an air adjust screw and a new cap for the hot start cable to enter the carb onto my bike. Now that I have done this, I simply cant get it to run right, I mean it barley runs at all. I don’t know if I stretched a cable or messed the hot start up or simply cant get it adjusted right, but after some adjusting, I can get it started, but it gets super hot I mean my pipe starts to glow bright red, so I turn it off. While it may run if you hit the gas it will stall or snap and sputter. Please help someone, as I don’t want to have to pay the shop. Thanks all.

Several things here to look at:

Did you remove the carb to do this?

Did you set the AP timing with the tool provided by the Boyesen Quickshot?

Did you make sure when you installed the adj fuel screw the oring was attached? Did you screw it in fully and back out 1.5 - 2 turns to start? And then fine tune the adjustment with the idle adj screw?

Did you check the plunger for the Hotstart before you installed the cap to make sure it was working properly - is the spring still attached? Did you add a dab of lithium or waterproof grease to the plunger in the hole?

Have you ever completely cleaned the carb and removed all the jets and reinstalled?

my guess:

If you did most of this - your problem is probably in the fuel screw adjustment - or the plunger is sticking

screw the shop you can do all of this yourself and it will help you for future carb endeavors - use your manual, it is fairly easy to do all of this, it just takes some patience.... ?

[x2stroker]

I had a similar problem when I installed a fuel screw. I took the fuel screw back out and determined that the o-ring was damaged. I put in a new o-ring and it ran fine.... amazing how the smallest thing can cause such a big problem. Good luck finding a new o-ring, I had to buy a stock fuel screw for $30 as I couldn't find anywhere that sold the o-ring separately.

[Ga426owner]

I had a similar problem when I installed a fuel screw. I took the fuel screw back out and determined that the o-ring was damaged. I put in a new o-ring and it ran fine.... amazing how the smallest thing can cause such a big problem. Good luck finding a new o-ring, I had to buy a stock fuel screw for $30 as I couldn't find anywhere that sold the o-ring separately.

Ace Hardware has all oring sizes

[Blue4Ever]

Like x2stroker and Ga426 stated it may be in the fuel screw area. There should be a sping, very small washer and a very small o-ring on the fuel screw. If you take it out and you do not see the o-ring look up in the carb to see if it is stuck in there side ways, if so carefully pull it out with a pick tool or a paper clip w/the end slightly bent. Yamaha sells this o-ring seperatly, but if you ask the person at the counter they will try to sell the assym. Just give them this part #3tj-14562-40, it should only be between 1-2 dollars.

[x2stroker]

Thanks for the part number and the Ace Hardware tip! I'll know next time. ?

[grayracer513]

I agree. Your trouble does sound like it is:

> Fuel screw (it's not an air screw) out of adjustment

> Fuel screw O-ring damaged or missing

> the hot start not seating

Drag the fuel screw O-ring out and check it for being cut or roughed up. If that's OK, test the hot start.

First, like all other single cable operated devices, it needs some free play, and the cable has to be routed so that the play is present no matter where the bars are turned. The cable needs to return as freely as it can when released.

One way to verify the hot start is working is to cold start the bike, leave the choke on, and open the hot start. It should act like a thumb throttle, with the engine speeding up when you open it and slowing down again when you close it. If it doesn't make a change in the engine speed similar to that, it's not seating.

And BTW, the pipe will turn red anyway, just maybe not that fast.

[yzf450jack]

You have the black plate thats on the slide upsidedown

[f150jokerstyle]

You have the black plate thats on the slide upsidedown

Talk about bringing a thread back from the dead but can this be done? I just rebuilt my carb for the fun of it and it has all the same parts and it is doing the same thing as the OP described.

[grayracer513]

The vacuum release plate on the slide not only can be installed upside down, it often is because at first glance, it looks like it should be. The top of it's round, and the carb bore is round, so.... NOT.

The "square end" that has the hole near the edge goes down.

[f150jokerstyle]

The vacuum release plate on the slide not only can be installed upside down, it often is because at first glance, it looks like it should be. The top of it's round, and the carb bore is round, so.... NOT.

The "square end" that has the hole near the edge goes down.

Thanks I will have to check that out later today

[f150jokerstyle]

Well needless to say the bike now runs great! Thanks Gray