400ex, Got,fuel, fire and comp. but wont run?

[Bugman53]

I honed the cylinder is its perfect, rings are new, valves are good.
I build race motors for a living and its got me stumped.

Things I’ve checked so far.

Stock timing key and is not sheared.

cam is in time. (Im going to tear it back down and put the degree wheel on it but i don’t think that is the problem) Lobes facing down with the marks lined up with the head. Kind of odd the cam seem 1\2 tooth off but its way off if I move the chain a tooth is either direction. One think I did not check is if the cam gear is facing the right direction. Will that make a Difference? IF the gear is 180 out but the cams are still facing down?

Cam chain is good.

Bottom end is solid.

Its getting fire. HOT blue spark

Its getting fuel( will not even hit when i spray starting fluid into the intake)

any ideas?

[Bugman53]

Alright update

I pulled the cam and it was missing the plunger for the decomp lever. Pressed the cam apart and pull off the decompressor off the cam.

It got 150 psi comp 5% leak down cold.

The 3rd time ive checked the cam timing its spot on.

Piston is at TDC, the cam lobes facing down, Valve lash is correct. flywheel is lined up at T mark, Cam gear is lined up with head with Center mark facing up.

When i turn the motor over it pulles a vacume from the header. It is pushing through the intake. BUT IT IS IN TIME>>> WTF. Its like the motor is turning bakwards.

It hit for a second but that was it.

[400exrider707]

A 400ex will NOT run with a stock cam with the decompression mechanism still installed, if the plunger and/or spring are missing from the head. I know from personal experience. I pulled my hair out for over a week trying to figure it out. Put those in and it fired right up.

Also the 400EX can NOT be 180 degrees out of time, its impossible.

[JOEX]

Originally posted by 400exrider707
A 400ex will NOT run with a stock cam with the decompression mechanism still installed, if the plunger and/or spring are missing from the head. I know from personal experience. I pulled my hair out for over a week trying to figure it out. Put those in and it fired right up....
Even if he took the decompression lobe off?

[400exrider707]

Originally posted by JOEX
Even if he took the decompression lobe off?

If that is off it should run, but I've never tried. I know with an aftermarket cam it will run without the decompression mechanism, but I'm not sure if this is the case with a stock cam and no decompression mechanism. Might be worth trying to put the stocker in with the full decompression setup, seeing you've already got this much time and energy put into it.


You need Fuel, Air, Compression, Spark. I know the title says you have compression, what is your compression?


Your symptoms sound exactly like what happened to my EX when I tried starting it without the decompression plunger and spring.

Also I know you said the valves are good, but are they at the correct clearances?

Also when was the last time this was running, what was changed that it doesn't run now?

[jmh460]

when i put a stage 2 cam on my 400 the marks on the cam gear didnt line up with the head at TDC, i tried moving the chain one tooth either way also and that didnt work either so put the chain back on the origional tooth and when i turned the flywheel to line up with the F the marks on the cam gear lined up with the head. i know im not a mechanic but Ive head that something wont run at all unless it is in time so manybe if you move the flywheel to the F position it might work...im probably wrong but.....just a thought

[400exrider707]

Originally posted by jmh460
when i put a stage 2 cam on my 400 the marks on the cam gear didnt line up with the head at TDC, i tried moving the chain one tooth either way also and that didnt work either so put the chain back on the origional tooth and when i turned the flywheel to line up with the F the marks on the cam gear lined up with the head. i know im not a mechanic but Ive head that something wont run at all unless it is in time so manybe if you move the flywheel to the F position it might work...im probably wrong but.....just a thought

An advanced timing key would do that, as well as an over-stretched cam chain. Putting the flywheel on F while having the marks lined up on the head is not what I would consider "in time"

[Bugman53]

Got it.

I played with it a wille and it finaly started.

IT was the decompression lobe. It took 20 min. to fire up but its runs Great now. For some reason it took that long to fire up. I was pissed and keep cranking on it for 20 mins and it hit a time or two and then fired up and runs great.

It sat 2 years before it was brought to me.

[400exrider707]

Originally posted by Bugman53
Got it.

I played with it a wille and it finaly started.

IT was the decompression lobe. It took 20 min. to fire up but its runs Great now. For some reason it took that long to fire up. I was pissed and keep cranking on it for 20 mins and it hit a time or two and then fired up and runs great.

It sat 2 years before it was brought to me.

What do you mean it was the decompression lobe?

[jmh460]

Originally posted by 400exrider707
An advanced timing key would do that, as well as an over-stretched cam chain. Putting the flywheel on F while having the marks lined up on the head is not what I would consider "in time"

Well apparently it is "in time" because it runs great, i never thought that a timing chain could stretch that much but it sounds like u know what ur talking about

[400exrider707]

Originally posted by jmh460
Well apparently it is "in time" because it runs great, i never thought that a timing chain could stretch that much but it sounds like u know what ur talking about

yes I was just throwing ideas out to get it going, hard to say from behind a keyboard not looking at the bike. Also it doesn't literally stop running from it being stretched, when the chains stretch far enough the chain tensioner cant pick up the slack and a lot of times they will jump teeth throwing it off of time, and its quite common on the 400EX motor.