Yz250f carb on yz400f

Hi all

I have a 99 yz400f that I bought in January. It sort of rank but not well at all. upon inspecting the carb I noticed it was missing the black plastic clip that controls the accelerator pump. I looked for it online, but nobody but NOBODY sells that part. Also, the carb showed quite a bit of corrosion from sitting, and seems unrepariable.

My friend has a 2001 yz250f carb. I put it on the other day, and managed to get the engine to sputter for a second, but then it started leaking gas out of the overflow (a separate issue entirely-I'm gonna try to spray out the bowl with cleaner, and if not maybe buy a new needle)

I have a JD jet kit for the 400 with various needle and jet sizes. I put in the jets that JD recommended for the 400 at my elevation, just to get a ballpark estimate. Right now it has a 175 main jet and a 42 pilot jet.

The kit also came with those O rings that can richen the AP timing if that's something I should consider.

So, my question is, is there any way to get the 400 to run decent with the 250f carb, and what steps should I be taking to make it work?

I know the 450f carb works, but Id like to spare the expense of buying one.

Thanks

I'm gonna follow along... I have a 99 yz400 and the stock carb is giving me trouble.. I have an extra carb from an 04 yz250 and was thinking of putting iton but it didn't look like the throttle cables would work.

The 250F carb is a smaller, 37mm unit.  You may have to tinker with the pilot air jet to get it to work correctly, and that can be a challenge.  Personally, I'd avoid it and go with a carb from a 450.

I put it on and after hundreds of kicks, it sputtered once and that was it. I used the jetting from the 400f as a baseline sand had the fuel screw half way out. Granted there could be other issues with it, as even with the stock carb it was always excessively hard to start. I think I'm just gonna look into a 450 carb swap at this point , but of anyone else has any ideas on the 250f carb I'm open to suggestions

"Fuel screw half way out" ?

Turn it in until it seats and then 1 1/2 turns out.

Would it being out of adjustment by 1 turn possibly cause it to not start at all?

Also, I'm using a Flexjet fuel screw that was meant for the 400f-could that be part of the problem?

Edited by hondarider1031

If you're only one turn off of 1 1/2 turns, you aren't "half way out".    Neither that, nor your fuel screw should keep it from starting.  But you might try it a little farther in.

 

I would suggest that you pull the pilot jet and see if you can pass a fine wire through the metering orifice at the small end.  It could be wholly or partially plugged with dried fuel residue.

ok, thanks for the advice on the fuel screw.

 

I'll try that, and order some other miscellaneous parts the bike needs

 

Also, In your opinion, what size pilot jet would be a good baseline? currently it has a 42

 

EDIT-also, any ideas what to do about the hot start that originally came on the 400? the 250f carb has one built in, so do I need to somehow block off the intake for the 400f hot start? 

Edited by hondarider1031

You can get a 05+ 450 carb for 100-160 all day on ebay. The 99 yz 400 is a great bike with a totally outdated and poorly designed carb. Do yourself and the bike a favor, get the later carb. I'm telling you my friend, it is dollar for dollar the single best upgrade you can do for that bike. If your not bald already your going to pull your remaining hairs out with that 250 carb

He's right.  Everyone who does the carb swap says so.

 

A 42 pilot is as good a place to start as any, but that will only be the start of things unless you get really lucky.

You can get a 05+ 450 carb for 100-160 all day on ebay. The 99 yz 400 is a great bike with a totally outdated and poorly designed carb. Do yourself and the bike a favor, get the later carb. I'm telling you my friend, it is dollar for dollar the single best upgrade you can do for that bike. If your not bald already your going to pull your remaining hairs out with that 250 carb

Wish I could find one :(

Wish I could find one :(

keep your eyes out man

keep your eyes out man

Hopefully soon....

Alright-thanks for all the help

I think my plan is to see what I can do with the 250f carb, and If the bike's still not running in the next couple weeks, I'll order a 450f carb.

 

Just curious, why does it have to be an 05+ carb? I thought they updated it when they came out with the original 450 in 03.

Oh and G-bro, I'm 14 with a fine head of lettuce and I hope to stay that way :thumbsup:

Doesn't have to be an 05+ but the later the better. I have an 07 on mine and it's a vast improvement

The 250F carb is a smaller, 37mm unit.  You may have to tinker with the pilot air jet to get it to work correctly, and that can be a challenge.  Personally, I'd avoid it and go with a carb from a 450.

For the record, are the years for yz450F carb upgrade  2005 - 2009? 

 

I only see them on ebay for $250 and up.

The '05-'09 carbs are preferable because they have a leak jet, which makes the slightly more tunable in one respect, but any of the '03-'04 carbs can also be used.

Oh ok-that makes sense

If the 250f carb doesn't work out I'll start looking for the 450f carb

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