YZ400 bogging big time, real hard to start

What jets are in the bike. What clip position is the needle on. Where is the fuel screw set. You said that it ran fine for 40 min before giving you trouble. Maybe its jetted rich and is killing the plug over time.

So we need to know what the folowing jets are:

main jet __

main air jet__

Pilot jet__

Pilot air jet__

Starter jet__

needle #__

Needle clip position__

Fuel screw position (turns out)__

DOH NEVER MIND :cry: :cry:

Oh, and I'm running all stock jets and the needle at position/groove #3 (from the top - 1 groove leaner).

Oh, and I'm running all stock jets and the needle at position/groove #3 (from the top - 1 groove leaner).

Oops, I meant I'm running it 1 groove RICHER. I went back to stock and no difference. Maybe it's jetting, but I doubt it because the problem started while I was riding; it was like night and day: running fine and then all of a sudden massive stuttering.

The problem seems to be a lot better with the engine kill disconnected. I just swapped coils with my bro's 250F (same part) and will see how that works out this saturday.

Does anyone know if my coil readings are defnitely bad (readings listed in the above post)?

Please help, I am ready to part this b$%tch out. I really hope it's not the CDI. :cry:

Please help, I am ready to part this b$%tch out.

Dont give up hope yet. You will find the problem eventually.

Its hard for us meare mortals to diagnose these problems sometimes without the expertese that the dealers have. I usually end up doing a hit and miss check kinda like you are doing to find the problem. Granted this one is kinda puzzling cause you have gone thru most everything that there is to go thru.

Do you know someone that is good with bikes. Sometimes it helps just to get a fresh set of eyes on the problem. Could be that you are overlooking something. Ive done it many times myself.

But keep at it. You will be happer with yourself once you find and fix the problem.

If all else fails bring it to the dealer. I did that last year when I couldn't diagnose the spun cam gear. Turns out that the dealer couldn't figure it out either. But if you bring it to them and ask them to test the electrical system at least they have a specific thing to look at instead of scratching there head and wondering what is wrong.

When I brought mine to the dealer last time they for the spun cam they blew air thru one of the jets and declaired it fixed. one 3 minute ride on the thing and I knew they didn't fix anything.

Iguess what I am saying is dont bring it to the dealer and say fix it. tell them to check the electrical or run a leakdown test, or check the timing... stuff like that. dont just let them run amock in your bike. Tell them what to look at and you will probably get better results. Unless you have a really good dealer nearby. Mine are a great bunch of guys but they really dont know much about modern 4 stroke dirtbikes.

Dealers are the biggest scum on the planet. :cry:

I'm not done with her yet. I'm just getting very annoyed by the problem. I have learned a lot by going through every possible thing (as far as I can tell) listed in the manual that there is to check.

I might have to open up all the harness sleeves and see if there's a frayed wire somewhere; that or new CDI.

Maybe it's my kill switch or coil too, but these are my last options. After that there really is no more. :cry:

I'll keep ya posted. It sure will be nice to find the little bastard part that is causing this. :cry: :cry:

Dealers are the biggest scum on the planet. :cry:

I agree, although there are a few exceptions to the rule. Back home in Colorado we have a decent dealer that has good guys that are willing to help you out and are not snots.

But most are full of uppity pricks who think they're hot sh*t because suppossedly they are the "bike guys" (and they get their little factory uniform), while most don't know [@#$%&*!], and usually just look at you like you're stupid for asking. :cry: :cry:

I know you have checked and tried plus replaced a bunch of stuff. If the coil doesn't fix it, try unplugging the TPS.

I have also heard of guys having problems with the plug wire, another coil will cure this in testing.

I know it's been apart and put back together but make sure the rubber connection from the head to carb is good, if needed remove the screw spacers on the clamps to tighten the bubber a little tighter. (No air leak)

I assume you have a stock hot start, verify it does close and seal, if it's for some reason a dubach or other aftermarket make sure it works proper.

I know also you had the carb apart, is it possible the slide plate is upside down? I know that it comes as it warms up but stranger things have happened. You may have fixed your real problem cleaning the accell pump and accidentally caused a new one.

I didn't see if you had a stock exhaust or not, but if it has a spark arrester it may need to be cleaned, also make sure the muffler baffle hasn't broken and is somehow blocking the exhaust (mirror and flashlight).

Just a few more thoughts but hopefully the coil repairs it.


PMK, thanks for replying. I've already tried with the TPS disconnected and connected plus I've tested the resistence on it and it's fine. Connected or not I get the same results.

As far as the carb slide I was aware of the possiblity of putting it in upside down before I opened it up so I've got it in right (square side down).

Hot start seems to seal fine. I took it off and can't blow through it with it shut. I also think my carb boots are sealing fine.

As far as the exhaust I'm using an WB e-series. I bought it new in box off of ebay and have only ridden with it installed a few times. I doubt there is an obstruction but I'm at the end of my rope so it's worth a look.

Thanks for the help, I'm going riding tomorrow so we'll see if I can't find something more out. :cry:

Just curious about one thing with the cams, did you make sure that the lobes are positioned similar to how they are shown in the auto cam swap? It is possible to have the cam sproket slip on the shaft as was previously said. You could align all marks correct and still be miss timed. I doubt this is it but a thought.

Curious about the coil, and I cant say it's been done, but the 250 CDI may run the bike and allow another method of checking. Not positive about the swap though.

One other thing is does the bike have much time with the pipe or was it just recently installed. And how many discs, more than 10 or none?

I really suspect a bad plug wire or cap, possibly the coil with the odd reading.

Another dumb one but you did try different fuel from a different supplier, yes.


Hey PMK, I believe I installed the cams correctly I followed the manual and made sure I was lined up at TDC using the "I" and installed the cams with the outer dots level with the top of the cylinder head. While I was installing I moved the cams one tooth off just to see the difference and it was quite a bit; so I really think I got it right. Plus I took the valve cap off again to check my valves and the cams were still aligned as far as I can tell.

Maybe I'll try swapping spark plug caps too, I'll have to check first though.

In regards to the pipe. On the second day of using the pipe is when the problem started. Was running great/normal and then the problem just started. I am using 12 discs (I think - however many come with the pipe). Do you think I need to lose some discs?

I've got the coils swapped over with the 250F and I'll give that a shot.

Oh, and yes I did try different brands of fuel. I've been able to run it on whatever wherever for as long as I've owned it, so I don't think my fuel is the problem.

Thanks for the thoughts, I'll look into the spark plug cap as well. :cry:

I don't doubt the cams are installed properly, the issue could be a cam sproket may have slipped on the shaft causing the timing to be incorrect even when the sprokets are installed propely aligned. The cam assembly is three parts, a bearing, a sproket, and a cam. They are pressed together in proper position but sometimes do slip. I've also pressed them apart and retimed the cams for some more zing. I doubt this is it.

With that WB pipe, you can inspect by removing the discs and making sure it's intact by looking inside the can/baffle. If it only has 2 rides it might be worth trying the old pipe if the new pipe looks distorted on the baffle. 12 discs is plenty, to few can cause some problems.

Good luck


Oh, I guess you learn something everyday. I sure hope I didin't spin a cam sprocket, although I really doubt it. I don't think I even jumped cam-timing so wouldn't a spun sprocket at least be accompanied by jumping timing?

I'm going riding in an hour so we'll see if I can't find something more out.

Thanks again! :cry: :cry:

sort of went riding today. I tried swapping coils and spark plug caps with my brothers 250F (both same part number). Neither helped at all. So I've eliminated every electrical possibility except for the CDI.

I pulled the CDI out of the rubber sleeve for the first time and noticed there is a crack down the center of it. This could have happened before I ever owned the bike because I noticed later that one of the bump stops on the triple clamp was gone; allowing the fork to make contact with the CDI. It always worked fine so I didn't worry about it. I have a lower clamp off a 426 but haven't gotten around to having the old steering stem pressed out of the current clamp.

I should also mention that the bike seemed to run just fine after unplugging and replugging the CDI harness connector. Maybe I'm not getting a good contact in there (or somewhere). I'm reluctant to think any of my "changes" have really done anything because of the sporadic nature of the problem.

I'm out of ideas, except to look for a CDI on ebay. Or if anyone here has a good used one I could take off their hands?

did you figure this out yet?

MY YZ400F had simlar starting, poping & stalling problems. I replaced with new accelerator pump, it solved the problem.

Did u check ur valve? I have a yz250f and my valves where tight and i was bump starting the thing alot before i adjusted my valves

First make sure it's not a contact problem : clean the connectors, use contact cleaners, look if the mother part of connector pins are tight enough.

Next, try with the CDI from the 250F.

you guys do realize this is a 4 1/2 yr old post, right?

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