yz400f still wont run!!!

13 replies to this topic
  • honda0071

Posted February 11, 2008 - 09:30 PM


i have a 98 yz400f and it is great bike other than idle issue... i have cleaned the carb and drained gas in both tank and carb and only run 91 octane... heres the problem it will not idle with choke off. it runs fine with it on then right when i push it in it dies. whats the problem? i have cleaned carb paying special attention to pilot jet/pilot circuit and it made lil and or no diff.. another prob is that throttle resonse is poor and choppy at about 1/16th throttle im guessin.. so any help greatly appreciated

  • LSmith

Posted February 12, 2008 - 04:47 AM


Open the fuel screw (bottom of carb, front side of bowl) in 1/4 turn increments. You may end up having to increase your pilot jet.

  • USED YZ426F

Posted February 12, 2008 - 08:36 AM


Have you tried adjusting the idle speed with the choke off?

Have you checked the valves for proper clearance? I know that tight intake valves can make it hard to start, not sure if it would cause your trouble though.

Other than that, I would check to make certain that there isn't some type of air leak between the carb and head (possibly going extremely lean with the choke off?). I would think that any air leak severe enough to cause the bike to die would also make a fair amount of noise and be very noticable.

Easy enough to check, just spray some starting fluid between the carb and head and if the idle raises, you have a leak. Definitely do this outside and before the exhaust pipe gets too hot, no sense in risking a fire.

  • honda0071

Posted February 12, 2008 - 09:13 PM


i figured out i am missing a bolt on my header... would this cause this severe of a problem... i might have to check valve clearance which is a big job but i guess ill have to do it... how much does it cost at about a normal shop rate? please any other help appreciated

  • USED YZ426F

Posted February 13, 2008 - 04:56 AM


I doubt that a missing header bolt would cause the problem that you are experiencing. But I may be mistaken, perhaps someone with more knowledge will provide a more definite answer.

Checking the valves is really pretty easy, just get a copy of the service manual and follow the directions. All you need is some feeler gauges and normal shop tools. There are links for the procedure in the common threads:

There are links in to the manual in the common threads as well:

There is no way to tell you how much it may cost at a shop. Different rates in different locations. Different mechs have different amounts of experience. Plus, unless you are very comfortable with the shop, you may never know if the shop is trying to sell you parts, and services that you don't need.

The first time I checked the valves on my YZ426, it only took about 1 1/2 hours from start to finish. And that was reading the procedure linked to above, following it step by step, and double checking it with my service manual. I also had to do some math to double check the clearance values (my feeler gauges were in US units vice metric).

All in all, if you can read and follow directions, checking the valve clearances is a very doable task.

  • LSmith

Posted February 13, 2008 - 02:23 PM


Valves that are too tight would cause the bike to be difficult to start. It may not run perfect either, but your symptoms are indicative of a lean condition. The easiest thing (and least expensive) is to:

1) Adjust the idle up (large knob on side of carb) until it will idle. blip the throttle and if the idle seems to "hang" before it settles, then
2) Open up the fuel screw 1/4 turn at a time. The idle should settle as soon as you shut the throttle -- And quick blips of the throttle should not cause a bog. Once you get to about three turns out from fully closed, you're no longer changing much, so
3) increase pilot jet one size, close fuel screw back to one full turn from fully closed and start back at two again.

Sometimes the pilot jet can have a partial blockage that is really difficult to see and therefore clean out. If the previous owner did not have any of the problems you are dealing with, the it may be that you have a spec of something blocking the passage in the jet. When you pulled it out could you see clearly through it? If not, and you can't get it cleaned, buy another of the same size. Those passages are so small it doesn't take much to mess up the jetting. It is possible you have a small air leak in the intake boot or around the throat of the carb. That is easy to determine though, by the method posted above....

  • honda0071

Posted February 13, 2008 - 07:13 PM


how do u know when the fuel screw is closed or not? and yes i will go buy another pilot jet one higher and one the same and try both.... u think a higher pilot jet will help?

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  • honda0071

Posted February 13, 2008 - 07:41 PM


whats the part number on the pilot jet? are there any other jets i should check or replace?

  • LSmith

Posted February 14, 2008 - 03:53 AM


3TJ-14948-05-00 is the stock pilot. It is a 45 which should be good unless you have an aftermarket exhaust. There is a second jet (slow jet) next to the main that you should check to be sure is clean, but the passage is larger and easier to clean out. It's part # is 3TJ-1494F-13-00

As for the fuel screw, you need a small, short flat blade screwdiver. You can't see the screw unless you turn the carb upside down. It is on the bottom front (closest to the cylinder) of the carb. There is an access hole in the bottom of the bowl. Turn clockwise until it is fully seated. It is not easily accessible. Because it must be adjusted with changing weather, most folks buy one of these -- http://shop.thumpert...oductCode=FMS01 It makes adjustments very easy.

  • grayracer513

Posted February 14, 2008 - 09:32 AM


3TJ-14948-05-00 is the stock pilot. It is a 45 which should be good unless you have an aftermarket exhaust. There is a second jet (slow jet) next to the main that you should check to be sure is clean, but the passage is larger and easier to clean out. It's part # is 3TJ-1494F-13-00

Pilot jet and "slow" jet are two names used for the same part. The pilot is located next to the main on the left side, as the carb sits on the bike.

The other jet is the Starter Jet (3TJ-1494F-13-00), and works only when the choke is used. It should be checked, nevertheless.

These are illustrated on pages 4-10 and 4-13 of the manual. A manual can be downloaded from:


Part of the trouble in identifying them stems from the fact that the parts fiche has the item numbers reversed on the starter and pilot jets, listing starter jet PN's under the pilot jet item number, and vice-versa.

  • honda0071

Posted February 14, 2008 - 06:56 PM


do u think i should go with one pilot jet higher? and how can i do the fuel screw while the bike is on with out this tool? is it possible? and do i do this fuel screw with choke on or off? also how do i do it with choke off if my bike wont idle? please any help greatly appreciated.

  • spraker

Posted February 14, 2008 - 07:16 PM


have you done a compression check?

  • LSmith

Posted February 15, 2008 - 04:06 AM


Pilot jet and "slow" jet are two names used for the same part.

In Alabama, we make up names for things if we don't know what they are -- :applause:

If the bike has a #45 in it, that's what I would start with.

Yes, you can access the fuel screw without the special part. It takes a short, small, flat blade screw driver. You do not have to remove the carb. Total length of the screw driver can only be about 2" and you will have to insert the tip into the screw head by feel.

  • honda0071

Posted February 17, 2008 - 08:17 PM


y would i need compression check im almost positive it has great compression because it starts and runs great except with choke turned off its not the motor prob just jet or fuel screw

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