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LB_Moto

98 yz 400 is driving me crazy.( wont start)

21 posts in this topic

White bros e exhaust with jet kit is only mod.

So I bought this bike and it ran on starting fluid for a few secs just ran ruff after. Cleaned the carb set fuel s rew etc etc then it started up and ran decently at idle but made weird noise that's apparently normal. Still felt bad on idle but cleaned again and it idled good.

So I pulled the clutch in put it in first and it died. It hasn't started but once sense then and the choke worked free and it died b4 I could get to it. What worries me is before you couldn't push the kick past the comp without the decomp but now you can push it past with just your hand it does take some effort. It doesn't sound like it has good comp and will not fire on starting fluid. I can't test it as I have no attachment that fits. Also it is getting great spark.... any ideas of what is going on? I'm checking valve clearance and looking for something to do with the decomp now...

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I had a '98 YZ400F, and it didn't make any weird noises, so if yours is making a 'weird noise', it's not normal.

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Also check the compression cable to see it is releasing properly. It could be the compression release is not closing off fully.

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I don't have a book for this bike and i'm not sure if it's the same as the 426... its hard to see in the picture, but the exhaust cam is a little off from the intake cam, i know you can count the pins on the chain, but i don't know where to start/stop. i think it has 13 and it should be 14? I don't know if the pictures with the micrometer mean anything, but i think they should be symmetrical... obviously they are not. Any input would be greatly appreciated.

IMAG0204.jpg

IMAG0202.jpg

IMAG0205.jpg

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Provided that the engine is at TDC in the picture of the sprockets, the timing is correctly set. The asymmetry of the cams is typical.

Check the valve clearance next.

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look at the outer dots on the cam sprockets - they are level with the top of the head - that means they are timed correct - as Grey said - IF it's set to TDC.

Yes - put your feeler gauges to use and see if your valve clearances are good, if they are within spec you should by all rights have compression, if they are so tight you can't slide any in, then one of your valves is being held open - and you won't get compression.

If they are in fact still good or close to good, then you may have something stuck to a valve seat that is keeping one valve partially open, or there is something wrong on the piston/cylinder end of things such as a bad ring or score on the cylinder wall.

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If one or more valves are extraordinarily loose, that can indicate that it's bent or being held off the seat by carbon or something else, too. Check them to start with.

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The exhaust cam is a little off from the intake cam (it's hard to tell in the picture) but it's only slight. It will be a few days until i have time to check the clearance. I really hope this comes to something simple.

Thanks to everyone who has replied so far, when i get a chance i'll post up new results.

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Update:

The bike is in time I am happy with that (thanks for the link with the book!) And the valve clearance is:

Exhaust .008 and .01

Intake: less than .008 (smallest I had)

At any rate the valves are good. Now I'm worried. Also a good look at the manual decomp shows it is not a problem. (has fair clearance.)

I suppose the next step is to find a comp tester that fits my hole to verify 100% it is in fact a compression issue - I'm almost totally sure it is.

*sigh*

I didn't expect to have major problems with this bike.

Edit:

If anyone has any ideas/thoughts/impressions/etc ... feel free!

I've never had to take a motorcycle motor apart before past checking the valves, it's quickly looking like this will be my first. What worries me now is what if i don't find anything wrong with the piston/rings? or worse... lol.

Edited by LB_Moto

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The exhausts are fine, but you need to hop down to the auto parts store and grab another set of feelers. Intakes are supposed to be set at .10 - .15mm (.004 - .006") The problem is when they are tighter than .004", and you don't know that yet.

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The exhausts are fine, but you need to hop down to the auto parts store and grab another set of feelers. Intakes are supposed to be set at .10 - .15mm (.004 - .006") The problem is when they are tighter than .004", and you don't know that yet.

I concur. The intakes are usually the first ones to tighten up too.

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One more thing, even if the valves are in spec... this doesn't mean they are sealing well.

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I thought that valves would move down (the gap would get larger) as they wore? Perhaps i don't understand what happens as the valves need to be shimmed.

I bought a smaller feeler and will measure them later today.

Edit:

intake valves are out of spec... both less than .004" i borrowed a smaller gauge... it only goes down to .004, lol. Can someone post a link or explain why the clearance gets smaller, and not larger as i thought it would? i googled it for a little bit but didn't turn up anything useful.

Edited by LB_Moto

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Over time with the valve face smashing into the valve seat there is wear resulting in the valve moving up closer to the cams (tightening the gap), also, the valve stem may? stretch over time as well, causing the length of the valve to be longer, resulting in tighter clearances.

When the valve and cam clearance become too close, the valves open earlier and close later, messing up timed combustion process, and lowering the compression ratio when it is needed the most. And if the valve to cam clearance is so bad that the cam is pushing down on the valve, then you will have a permanently open valve, which obviously results in very low compression.

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Over time with the valve face smashing into the valve seat there is wear resulting in the valve moving up closer to the cams (tightening the gap), also, the valve stem may? stretch over time as well, causing the length of the valve to be longer, resulting in tighter clearances.

Ah, I was thinking the gap would get larger because the spring may compress over time, but that makes perfect sense.

So hopefully all i have to do is get a smaller gauge and some shims! awesome.

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So hopefully all i have to do is get a smaller gauge and some shims! awesome.

Hopefully... but the valve may still not seal properly from being out of adjustment for so long. You may get lucky since you have SS valves.

Don't be surprised if your local Yamaha dealer doesn't have the right shims... if you run into problems finding shims, find a Honda shop. The shims you need are the same as the CRF450R. The Honda's have all kinds of valve issues compared to the other brands, so chances are they will have what you need.

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