Typical 99' YZ400F issues

Trying to get this bike above, which was a loner, ready for a race my son wants to enter this weekend here in Texas. (Freestone) The bike as far as I know, has never been raced, just used around a farm on a make shift track.

We paid $175.00 to the local dealer to have the valves checked/adjusted. Was a waste because they were at specs.

The issue is, we can get it cranked on about the 3rd kick, but it will only idle for about 2 seconds and dies. If we try to keep it running via throttle control, it has alot of the backfire I have read so much about on this site.

Temperature here is roughly mid 80's to low 90's. Jet needle is set to 4th position from top, has a #45 pilot with a 175 Main Jet and the air screw/mixture screw is 2 turns out.

I did not find any leaks in the exhaust system. I did however repair a possible air leak at the intake manifold. ( The clamp would not tighten up enough to keep the Carb. from being able to rotate it by hand)

I also have been through the Carb, twice! I found no debris on the inside and all passageways appear to be clear. I cleaned the Carb anyway to be sure.

I am running out of expenses and certainly, out of time, lol, so any help or suggestions would be very appreciated. Thank you for your time.

regards :smirk:

As has been mentioned many times in other places: Does the fuel screw have it's small o-ring AND flat washer present? If "no", then you have likely found your problem.

These o-rings are maybe .25" OD and are easily lost upon disassembly. Sometimes they will hide inside the carb body and then fall out when hit with carb cleaner. Sometimes they will be installed backward with respect to the equally small flat washer and then become mangled.

Hmm You have me wandering baxter,Iam not trying to jack this thread but I had the fuel screw out and the rubber o-ring...Where exactly is this washer you speak of....Should have it came out with the o-ring?Which side of the o-ring is it?on the side where you take it out of the carb or more towards the inside of the carb from the o-ring?

To silent:I have been fighting the same battle and my biggest change was when I unplugged the TPS...It will now idle about twice as long as before and then die. If you figure out the problem please do tell.

Lol, Ricky. And yes baxter, I know all about that, in fact, it is all intact. I have rebuilt many carbs in my time and used to work on bikes for a living, just haven't had many opportunities working on YZ's other than 2 strokes. I was always a Honda or KTM Racer.

To answer your question Ricky, and this is what was in my carb, there is a tiny washer that goes on the fuel screw first, then the spring, then another tiny washer, then the o-ring. (In that order) Alot of times when you pull the screw out, the last washer and o-ring will remain inside the oriface and sometimes has to be dug out very carefully. If you plan on soaking the carb. in solvent, that o-ring will have to come out or it will be destroyed, at least it will be if you use a solvent like ChemTool in the gallon bucket.

Check that the hot start is shutting off properly, it may be leaking air.

A simple way to check is to remove the hose off the hot start assembly and plug it to see what happens.

Problem found! Now this is just "my" particular scenario. All of the helpful tips on this site were looked into and checked. Thank you TT for you existence. I was determined not to let this project beat me and after finding the problem I am amazed I didn't see it sooner.

I pulled the carb once again and decided to give the pilot circuit another thorough check. After cleaning all the passageways again I looked at the pilot jet, with a flashlight this time, and although you could see light through the orifice, it just seemed to be too small to me. I took a tiny dia. wire and ran it through the jet after making a few slight bends in the wire to help in cleaning. Sure enough, the orifice was larger, and you could clearly see more light through the jet.

The bike started on the 3rd kick, it idles and there is no more popping. Runs clean and crisp! Could kick myself in the but for rushing through the process the first time, LOL. So those of you experiencing the problem I posted above, pay particular attention to the pilot circuit.

Regards and thank you for all your comments and interest. :smirk:)

Glad you got it fixed....Most people dont post their final conclusion so Im sure a great many people including myself will thank you...Ride On Brotha

Any tips on where we can chase down some on those tiny washers/o rings? Sounds like it wouldn't hurt to have a couple extra on hand next time I take the carb apart

any tips on where we can chase down some on those tiny washers/o rings? Sounds like it wouldn't hurt to have a couple extra on hand next time i take the carb apart

OEM Yamaha parts:

3TJ-14562-40-00 O-ring

3TJ-14952-40-00 Washer

3TJ-14160-40-00 Coil, spring

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