98 YZ400F Carb Conversion



29 replies to this topic
  • SnowMan1983

Posted January 04, 2011 - 07:36 PM

#1

Bike: 1998 YZ400FK

Current Carb: Keihin 39mm FCR

Issue: I bought this bike recently as a bike with carb issues. I have isolated down the problem to a pair of pipes/hoses that run up and over the carburetor in opposing directions, being missing. I figured it would be an easy fix, so I contacted a few different parts dealers only to find that the pipes are no longer sold.

New Dilema: What would be a recommended replacement carburetor for this bike? I have read several posts, and while the YZ450F carbs sound nice, I am looking for something that will snap in so I can ride. Mainly I am looking for a quick decent priced (below $300) fix, that won't hinder the bike too badly.

So far the carbs I have looked at are:
-Stock oem replacement from Ebay, but I don't have to worry about these unreplaceable hoses falling off again, so that option is out.
-Mikuni's flatside smoothbore carb 38mm bore to 43mm spigot
-Mikuni's flatside smoothbore carb for 4 strokes 36mm bore to 43mm spigot
-I keep hearing that the Edlebrocks are a decent replacement carb on this forum, but have yet to see which would work best on my bike.

Also, if anyone has had any experience with any of these conversions, can you please provide your jetting and other useful info?

Any and all help is appreciated as I would like to get this thing running as soon as possible. Thank you.

  • grayracer513

Posted January 04, 2011 - 08:38 PM

#2

The hoses #60 in this illustration?:

https://www.thumpert..._carburetor.gif

...They're just vents, and they're just hose. Unless the fittings are missing, they aren't anything special, and I doubt you can seriously attribute any problems to their absence other than dirt intrusion.

The YZ450 carb comes as close to snapping in as you can get. Basically just bolts on. Just be sure to get as much of the hot start assembly with it as you can.



  • SnowMan1983

Posted January 05, 2011 - 01:32 AM

#3

The entire assembly is missing from it, which includes those T fittings inbetween the "57" clips.

Essentially, what happens right now is that the bike will kick, idle and die. If any gas is applied, instant shut off for the bike. I am waiting on the clymers to get to my doorstep, as I barely got this bike a couple days ago. When it comes in, I plan to break down the carb, but regardless I am still missing those T fittings.

As far as the 450 option goes, that is my backup plan, but everything I have read about it mentions stretching the boots and changing up cabling, amongstsome other things. Since this is my first 4-stroke carb project, I was trying to go as simple as possible to avoid over complicating it. Appreciating the the tips though.

Anyone know where I could maybe find the fittings for the tubes?

  • baxterj787

Posted January 05, 2011 - 02:26 AM

#4

RE Greyracer's last post: They are just vents. The T fittings cam be purchased at any aquarium / pet supply store. Their absence will have zero effect on how the bike runs or responds to throttle.

Sounds like the carb is plugged. Once you get the manual, make sure 1) the Accel pump is actually squirting into the venturi of the carb and 2) make SURE the SMALL flat washer and o-ring are present around the fuel screw.

When you clean the carb, do it right by blasting the jests and the passages where the jets reside.

  • grayracer513

Posted January 05, 2011 - 07:45 AM

#5

The only cable that needs changing/adding is the one for the remote, handlebar mounted hot start. If you buy the carb with the cable, all you need is a lever. Your throttle cables will bolt right up.

The air boot is just not a big deal. Clamp the carb into the boot, then push it forward into its mount and tighten it. In all of the threads you've read, have you seen one that didn't state that the conversion worked great, was easy, and the bike never ran so good?

Either way, if you can start it and idle it, the pilot circuit is "probably" OK. Check it for varnish deposits anyway. That it won't take any throttle indicates that the main circuit from the jet upward is blocked. Whatever you do DO NOT separate the upper and lower body sections from each other by removing the screws from inside the float chamber. You will destroy the gasket, and no one sells it.

  • SXP

Posted January 05, 2011 - 03:27 PM

#6

Your throttle cables will bolt right up.


The throttle cables between the 400s and the 450s are different. Both the push and pull 400 cables (at the the carb end) have similar "barrel" style adjuster with two 10 mm nuts each. One of the cables on the 450 has the 400 style barrel adjuster with the two nuts and the other is, well, you know what that looks like. The 400 cable might work on the 450 carb, I haven't tried it, but I just went ahead and got a 450 cable for my carb upgrade.

EDIT: Picture added below: 400 cable right/top, 450 cable left/below

Posted Image

Edited by SXP, January 05, 2011 - 07:11 PM.


  • mjholeton

Posted May 14, 2013 - 05:20 PM

#7

hey new to the form just bought a 1998 yz400f for $800. I thought i had an ok deal till i pulled the carb off. The main carb body was busted up beyound repair. My question to you is . did you replace your carb or rebuild it ? I need to replace my carb.I was thinking about the yz450f . will any year do? thanks

  • grayracer513

Posted May 14, 2013 - 05:47 PM

#8

Try to get one from an '05 - '09. Get as much of the hot start assembly with it as you can.

  • willcomsto

Posted June 18, 2013 - 09:29 AM

#9

Search, people, SEARCH. Just scanning through the 400/426/450 forums, without the search function, I found five YZ450F carb swap threads, all of which Greyracer answered the same questions. Greyracer: kudos for patience and wisdom!

  • timmytimmytimmy

Posted October 20, 2013 - 07:19 AM

#10

actually there not just vents the t fittings will leak gas and make ur bike starve for fuel i had the same problem i put kno hoses on and it helped alot any one know how these hoses run on the carb stock? i know 1 side feeds into the other then where do the other 2 hoses off the t go?

 



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  • Devil1

Posted January 29, 2014 - 08:14 PM

#11

Resurrecting a thread, why exactly do you need the hot start off the 450? Since the 400 isn't linked to the carb why mess with it onto the 450 carb

  • Jmo3540

Posted January 29, 2014 - 11:29 PM

#12

i have the same question as Devil1*. Curious?

  • grayracer513

Posted January 30, 2014 - 07:37 AM

#13

actually there not just vents the t fittings will leak gas and make ur bike starve for fuel i had the same problem i put kno hoses on and it helped alot any one know how these hoses run on the carb stock? i know 1 side feeds into the other then where do the other 2 hoses off the t go?

 

Actually, they are just vents, and nothing else.  The exception is the one coming from the bowl center.  That one also serves as an overflow drain when the carb floods due to a float/needle problem.

 

The manual or the parts fiche will show the routing.



  • Jonny Thunder

Posted January 04, 2015 - 09:30 AM

#14

I have appreciated the council here for upgrading the YZ400F carb to the 450. I took the advice and ran with it, buying a used YZ450F carb with the hot start cable, the throttle cables and throttle from E-Bay. $175. Did the swap with a friend yesterday and my bike turned over when I kicked it. I ran it up and down the street twice in a state of shock. Thank you, big time. I know there are probably some details that I will need to drift back to this thread to inquire about.

 

For starters (no pun intended) I have the original hot start pull still mounted, but then have the new hot start cable that is mounted to the carburetor. How do I operate the new hot start cable? It has no lever or button currently.

 

I'll go back and search the forum for my other questions before posting again here.

 

Many thanks, again



  • grayracer513

Posted January 04, 2015 - 01:27 PM

#15

You'll need a lever for the bars. One way to get one would be to shop eBay for used clutch lever assemblies from '03 and up 450's.

To operate, you hold it while starting, then release when the engine catches.

  • Jonny Thunder

Posted January 04, 2015 - 01:55 PM

#16

You'll need a lever for the bars. One way to get one would be to shop eBay for used clutch lever assemblies from '03 and up 450's.

To operate, you hold it while starting, then release when the engine catches.

 

Thanks, grayracer. I'l check that out.



  • Jonny Thunder

Posted January 04, 2015 - 03:02 PM

#17

Found it, bought it, $30 on ebay. Thanks for the direction.



  • Jonny Thunder

Posted January 08, 2015 - 04:59 PM

#18

Gray racer,

Can you give me a recommendation on how I should plug the hole from the old hit start button?

Or can I just leave it?

  • grayracer513

Posted January 09, 2015 - 06:37 AM

#19

No, both ends have to be blocked off.  Get a couple of vacuum port caps at an about parts store and glue them in place with weatherstrip adhesive, one on the intake port, and one on the air box.



  • Jonny Thunder

Posted March 01, 2015 - 11:06 PM

#20

Coming back late with a report. I was able to finally ride with my daughter at a local off road park. It was great. We were pretty tentative as the ground was very rocky, but it was great to just be able to kick the beast over. Many many thanks for the advice. I did everything suggested and everything worked out as described. 

 

Thanks!







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