Beware of Auto decomp damage

holy smokes! That bike is going to rip with that stuff.... Which exhaust are you using?

If you do decide to go with the manual decomp, before you drill into the head I would try to find someone that wants to trade heads with you. A number of people have installed the auto decomp in their older bikes and have plugged the manual decomp hole. My '01 WR426 has this mod. The PO says it has a YZ cam, don't know what year. I'm planning on pulling the head some time next month. If I don't have the same problem you do, I'd trade with you if you'll pay shipping.

If you do decide to go with the manual decomp, before you drill into the head I would try to find someone that wants to trade heads with you. A number of people have installed the auto decomp in their older bikes and have plugged the manual decomp hole. My '01 WR426 has this mod. The PO says it has a YZ cam, don't know what year. I'm planning on pulling the head some time next month. If I don't have the same problem you do, I'd trade with you if you'll pay shipping.
You'd be very disappointed.

One, the head from a 426 won't fit either 450, and two, the head from an '03-'05 model (or '06 WR) will not fit '06 or later YZ's or '07 or later WR's.

Bad idea.

You'd be very disappointed.

One, the head from a 426 won't fit either 450, and two, the head from an '03-'05 model (or '06 WR) will not fit '06 or later YZ's or '07 or later WR's.

Bad idea.

I thought the 03-05 model was nearly identicle to the 400 and 426. Oh well.

I thought the 03-05 model was nearly (identical)
nearly identical is an imprecise, subjective term...
nearly identical is an imprecise, subjective term...

is that not allowed on this board?

not saying you can't say it, just that it has its limitations.

The 400 is "nearly identical" to the 426, but only so much of the two engines are interchangeable. There's even less interchangeability between the early 450 and the older models. When the current engine was introduced in the '06 YZ450, it came with nearly 200 part number changes, and is really an almost completely new engine.

holy smokes! That bike is going to rip with that stuff.... Which exhaust are you using?

I’m still using the White Bros E pipe from my 400.

Anybody want to take a guess at the jetting? 478cc, open pipe and -30C.

I'm planning on pulling the head some time next month. If I don't have the same problem you do, I'd trade with you if you'll pay shipping.

Thanks for the offer Pablo, but as Grayracer mentioned, she won’t fit. I’m going with the YZ cam and auto decomp for now.

I’m still using the White Bros E pipe from my 400.

Anybody want to take a guess at the jetting? 478cc, open pipe and -30C.

Thanks for the offer Pablo, but as Grayracer mentioned, she won’t fit. I’m going with the YZ cam and auto decomp for now.

I'd say just remove the jets, as a starting point. :)

Anybody want to take a guess at the jetting? 478cc, open pipe and -30C.

I could be wrong so I am sure Gray will straighten us out But dosen't the increase in displacment make the existing Carb/jets richer by default due to the increase in vacuum signal in the venturi?

You are pulling more air but you are also getting more fuel with the additional air.

I was thinking don't make any jetting changes until you test it once?

Anybody want to take a guess at the jetting? 478cc, open pipe and -30C.

what's your elevation?

I'd say just remove the jets, as a starting point. :)

Haha, that is funny!:banghead:

I could be wrong so I am sure Gray will straighten us out But dosen't the increase in displacment make the existing Carb/jets richer by default due to the increase in vacuum signal in the venturi?

You are pulling more air but you are also getting more fuel with the additional air.

I was thinking don't make any jetting changes until you test it once?

An interesting concept, and I honestly don’t know. I’d hate not to get it close though. Changing the jets is a bit of a pain on a good day. On top of that I have to bring the bike inside (my livingroom, no garage) so the airbox boot will thaw out enough to bend out of the way so I can get the carb out.

what's your elevation?

I live at sealevel, but the surrounding hills are 2000 to 3000 feet high. Most of my riding is between zero and 2000 feet.

I could be wrong so I am sure Gray will straighten us out But dosen't the increase in displacment make the existing Carb/jets richer by default due to the increase in vacuum signal in the venturi?

You are pulling more air but you are also getting more fuel with the additional air.

You could be wrong, but this time, you're exactly right. The effect is more noticeable the bigger the carb is in relation to the engine size. In the case of the YZ450 specifically, it might make very little difference, but it would tend to be as you said, and for just that reason.
I was thinking don't make any jetting changes until you test it once?
Good advice.

I have the JD jetting kit for the WR 400/426/450. It is designed to be used with the YZ throttle stop and higher flow exhaust. It says to use a 170 main for 0-3000 ft. The kit includes different needles for hot and cold conditions, but the jet size remains the same. The jet also remains the same for all different size engines, so I'm guessing the mixture remains relativily constant with different displacements.

You could be wrong, but this time, you're exactly right. The effect is more noticeable the bigger the carb is in relation to the engine size. In the case of the YZ450 specifically, it might make very little difference, but it would tend to be as you said, and for just that reason.

Good advice.

Good call GCannon, and thanks for the back up Grayracer. I'll leave it as is and hope it's close. If it's too lean, :banghead: rather than rejet, :banghead: I'll just stop cleaning snow out of the airbox until the mixture is right.:)

I have the JD jetting kit for the WR 400/426/450. It is designed to be used with the YZ throttle stop and higher flow exhaust. It says to use a 170 main for 0-3000 ft. The kit includes different needles for hot and cold conditions, but the jet size remains the same. The jet also remains the same for all different size engines, so I'm guessing the mixture remains relativily constant with different displacements.

Thanks Pablo, I have the JD kit too. I'll try it as is.

You don't have to take the carb off to change any jets.

I take one of my wife's mirrors from one of her makeup thingies and put that under the carb and change the jets from the float bowl plug hole.

The mirror allows you to see what you're doing.

You don't have to take the carb off to change any jets.

I take one of my wife's mirrors from one of her makeup thingies and put that under the carb and change the jets from the float bowl plug hole.

The mirror allows you to see what you're doing.

I tried that before, and did manage to change the main, but it was tricky and I was worried about cross threading. It took me so long to do it (outside in the cold) that I think it was quicker to pop off the carb.

Yes, direct plug 'n play. Same timing marks, etc... The 06 and 07 yz cams fit, but the word is that the 06 cams give the best power boost. I used both intake and exhaust cams from 06 and the power is quite nice.

I know on the older 426's the intake cam on the wr was the same as the yz cam and only the exhaust need to be changed. Do you know if the exhaust and intake cams are both dirrerent from the 07 wr to the 06 yz? Thanks!

Do you know if the exhaust and intake cams are both dirrerent from the 07 wr to the 06 yz? Thanks!
Yes they are.

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