Kick starter locked after valve adjustment '98 YZ400F

OK, GREAT news. After plugging the hot start, it's almost where it needs to be. It still needs choke to run, but it runs pretty damn smooth on it and I can even rev it up. Actually with the idle turned down a tad and choke on, it pretty much sounds normal. Super throaty, nice throttle response, etc. However, again, with choke off and even with idle turned up, the bike won't idle. It sounds like it really wants to, though, when I plop the choke off after it's going. It'll run for a second or two, then die.

I have more findings, too. The TPS L-B test is good (4.25 kOhm), but I can't get the Y-B test where it needs to be, which is 4.25 * 0.13 - 0.15 kOhm = 553 - 638 ohm. The lowest I can get the resistance by rotating the TPS is to 930 ohm. Would this couple hundred ohm TPS at fully closed throttle make a difference?

Thanks so much, guys, you're helping me get through this. This baby is soooo close to coming back. :excuseme:

If you think the TPS is the problem, unplug it, then run the engine.

Didn't know I could do that. =P And it doesn't appear to be the problem.

I was able to fashion a tiny stubby flat head screwdriver out of some miscellaneous weird tools I had lying around so I can actually adjust the pilot screw with the carb on. I'm getting the feeling this is the remaining problem now because the idle is responding readily to twisting it out, and it's smooth... Soooo, I'm going to get the next size up pilot and go from there. Will let you know.

Thanks again. :excuseme:


So you're just now trying an idle fuel adjustment?

So you're just now trying an idle fuel adjustment?

Lol well, adjusting the pilot screw and idle speed wasn't having an effect, so I figured I ought to address that. So I put in a 48 pilot today and away she went. Kind of. Still took 4.5 turns out and idle speed maxed out, but hey it's idling without choke now!!!

The plan now is to put in a 55 and see where that gets me. Thanks again guys for your help -- you rock! :excuseme:

OK, I haven't got a 55 in there yet (special order $20), but I put in the 50 and still not having results. Idle speed all the way up + pilot screw 4 to 4.5 turns out just to get it idling without choke, and at that point it won't take throttle... Which leads me to believe something is still wrong -- air leak somewhere maybe? Here are my observations in order of what I imagine is most relevant -- guidance appreciated:

1. Float height is set to spec, 9mm, but that's after lightly tapping the floats down with the carb upside down, which means the fuel level could be on the high end. Should I only let the floats sit gently, then measure height from there?

2. Decomp actuator bolt threads are stripped and the bolt won't torque down -- it spins freely. Replacement part is in the mail. Not sure if this would cause a leak into the engine, but obviously the leak would be on the exhaust side.

3. One exhaust valve clearance is on the tight end of 0.25mm (spec 0.25-0.30), but I can put a feeler under it, and going down a size shim would put it over spec.

4. Timing chain has a decent amount of stretch, enough to put the timing mark 0.5-1cm off the notch when the cam marks are aligned.

As I said before, the thing runs "normally" with the choke pulled out. I could probably secure the choke out, then take it out and ride it like this, but I'd rather fix it properly.



when checking the float height you should never tap the float, as it will give you a different reading... if you have the idle turned all the way up you are also off of the pilot circuit and on to the transfer if not further... If you are not getting any difference when turning the fuel screw it makes me think you got something blocked in there as any adjustment should add or subtract to the mixture making it rev higher or lower, whether the pilot jet is the right size or not... The slide seal is known to go bad as well, its a little circular ring that goes between the slide and slide plate. Also, is your slide plat upside down? I have heard you can install it upside down and get it to idle by basically doing what youve done but youll never get it right...

just a few things to consider, hope it helps

edit, I just now saw your note about the decomp... doesnt matter if its exhaust side or not, extra air in the cylinder is extra air in he cylinder.... If its sucking air that could easily be your culprit. get it to idle and spray some carb cleaner up in there, youll know right away if you have a leak or not....

Some updates:

1. Fixed the float height as best I could. Service manual doesn't have any procedure given at all, so I just set it about where the needle valve just touches/closes off, right around the point where the float tang touches the needle valve spring. ~9mm from flange to highest point on float

2. Was a real PITA to fix the decomp bolt. Turns out the internal threads were stripped, not the bolt's threads as I had hoped. Had to do the whole helicoil procedure, but now it's definitely sealed.

3. Put pilot screw back to 2 turns.

It seems to be doing better now, but the idle speed is still all the way up. Makes sense to me what you're saying about being out of the pilot circuit like that. Maybe it will start up and idle normally if I turn the idle back to normal... I'll try that tomorrow morning and see if I can warm it up.

I think the slide plate is on OK -- looks the same as the diagram. Do you think a faulty slide seal would cause these sypmtoms? I'm ready to try anything, and I've become super fast at getting the carb off and on -- 5 minutes each way, no sweat.



PS: I rebuilt the accelerator pump when I first got the bike because the old one was really rusted out bad. The new diaphragm from Sudco had a significantly smaller rod than the old stock accelerator pump in the bike. Could this be contributing to the problem?

I got the bike nice and warm today, and I noticed that the cylinder base gasket was glistening with a tiny bit of oil around the seam. Another goodie left behind by PO. :excuseme:

Obviously I'm going to have to fix that, but just wondering -- is it possible that not-so-good seal could have caused these symptoms? It's very minute. The oil doesn't even drip or run -- it's just a tiny bead around the seam.

Nope. Only an air leak between the intake valve and the air filter end of the carb somewhere would have any effect.

Just another thought since you are messing with the pilot screw alot. When i bought my Kawasaki, it was having alot of the same symptoms. After alot of scratching of my head, i pulled the pilot screw completely out and found that the PO had tightened it in so far that the little tip of it actually broke off inside the carb, thus blocking that passage and rendering the adjustments to the pilot useless. So take a look at the pilot screw itself. When you take it out you should be able to see daylight into the carb. If not, that passage could be plugged as well.

Thanks, I pulled out the pilot screw and noticed that the little red o-ring was kind of tweaked (not sitting fully horizontal), and the orifice looked dirty with light shined in, or at least not perfectly shaped/clean.

So I fixed the o-ring and cleaned the orifice real well with cleaner, a soft wire, and air. It definitely looked like it opened up a lot to me, so that's probably good. I guess I didn't clean it well enough before, and the wire seemed to fix that. Put everything back together and now it doesn't fire up, probably because it's too rich I imagine. :excuseme: (Has 50 pilot jet in it at this point.)

I would just put the 45 pilot back in, but I can't find the damn thing... So now I'm extremely frustrated and to be honest, I'd rather spend twice or three times as much and just have a working MX bike along the lines of this one.

So I put it on CraigsList. *sigh* I feel like I know what the problem is now, but who knows...

If my patience recharges before anyone contacts me about the bike, I'll bust out the magnifying glass and look all over for that stupid jet. Anyway, thanks to everyone for the help -- I learned a whole bunch in the process.

PS: It looked like the chamber underneath the pilot jet had something creeping in around the hole (like an enlarged gasket or something). Not sure what that's about. Could be blockage contributing to the problem, I suppose.

You are killing me with that bike. I have a beautiful 98 yz400 that runs beautiful. The problem is if anything goes astray in that fcr carb these bikes go nuts. I bought mine real cheap because the prev owner could not get it started for a couple of years. It was just the combo of idle turned to low and fuel screw out of adjustment and it would not even backfire, just dead. These people kicked it for ever, dead. I roll started it down the road and ran great, I knew what to adjust after it ran. Been a 1 kicker ever since, unless it's near 0 and I drained all the gas out of it. Then it takes a couple.

Does anyone know the exact procedure to measure the float height? I still don't know if I'm setting it right given that it can be dramatically different based on where you let the floats settle (e.g. barely touching float valve spring vs. fully upside down letting the spring compress).

I have the manual; I even have it printed out and bound. The manual really doesn't have any procedure outlined for setting float height -- the only mention of float height is in the maintenance specifications where it says 9mm float height.

In any case, I think I've found another peculiarity that caused so much grief yesterday. I noticed just now that the 50 and the 48 pilots are very different. In fact, I don't think they're the same style jet. The 50 actually has much smaller side holes, and the head is a slightly smaller diameter. At a glance they look like the same type of jet and the markings are the same.

I put the 48 back in, and the thing runs pretty good, even better since I attacked the pilot screw passage. It responds to the pilot screw turns, but unfortunately still has to have the screw too far out.

I want to put in larger pilot jets, but now I'm confused since the 50 I got is so different. If I order OEM 50, 52, and 55 jets off the '98 diagram, it's going to cost me $50 total ($20 for the 50 and 52, and $10 for the 55). Is it worth doing this? It would suck to blow that money and find out all the jets have those side hole/head diameter differences.

Download the 2000 or later 426 manual. The procedure is outlined on 4-12.

Download the 2000 or later 426 manual. The procedure is outlined on 4-12.

Thanks, perfect. :excuseme: That one has a bunch of other stuff I was looking for that wasn't in my manual.

Hooray. I went to Sudco today and picked up 50, 52, and 55 pilot jets. $5 apiece, and not only that, they're exactly the same as the 48 I have. I'm convinced the dealer sold me the wrong pilot jet for the 50 -- it looks similar to mine, but it IS different. And sure enough, when I put Sudco's 50 in, the effect was as expected.

The bike still needed a little more juice, so I put the 52 in and things are pretty good. Warmed it up, and now it's idling at spec 1800 RPM, good throttle response, and I even got to ride it around for the first time (gently, of course, because the old chain is way tight and all rusted out). :smirk:

Tomorrow I'm going to pick up new chain/sprockets and replace the hot start cable, and I should be able to take her out this weekend!

She sounds great... Thanks again for all your help everyone. I learned a lot while getting this bike going again. :lol:

I can honestly say that the choke issue is still something i have never fully understood, i need more knowledge about carbs, i would imagine it has something to do with something being clogged when the plunger is set back into place that shuts off all the intake air, take it out and clean it. But be careful with the bike at idle for extended periods of time, it is a race bike and is not meant to idle like that, that could be why it gets so hot and smokes and smells funny.

I think that a 52 is WAY too big on the pilot jet for that bike. IDK your waether and such but i run a 42 in my 426 all the way down to 40 degrees and it still runs fine

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now