400ex decided to stop starting

[Divergent12]

Bought a used 400ex. Worked great for two weeks. One day was out riding for a bit then turned it off. 10 minutes later went to start it back up and while it was cranking over the noise coming out of the exhaust changed from a very definitive compression sound to a less definitive compression sound. But it still had compression, you could feel it putting your hand behind the exhaust.

Took it to the local Honda dealer, they said it was a broken connecting rod. They are apparently idiots. Wanted me to pay for a whole rebuild.

Thought it might be a stuck valve, took the caps off, and all valves move freely as they are supposed too. BUT, while spinning the crank (spark plug was removed), I noticed that at at one point in every rotation it got VERY HARD to turn, like two hands and a lot of force to get it to go, like it was a compression stroke, except the spark plug wasn't in.

Any ideas? Thanks in advance for your help!

[DragonGunner]

Well if it was me, I would take the head and cylinder off and see what going on with the crank and piston.

[Divergent12]

Not what I wanted to hear....

Been doing some research, and it sounds similar to the decompression spring/pump not working.

Is this a possibility? How do I check that?

[KKiowaTJ]

That would be rare. Only true way is as mentioned, take it apart. It's obvious something(s) are not correct in the motor. Not what ya wanna hear, but money ahead instead of nickel and dime diagnosis's. Good luck either wah

[Blodg]

It can't be the decompression spring because you said it was hard to turn over by hand even with the exception spark plug out. Definitely need to pull the head off which isn't that hard to do. But what you find may or may not require a lot more work.

[Divergent12]

OK, thanks for the advice, I'll let you know when I get in there.

And it's only hard to turn in one spot, the rest is cake, lol

[DragonGunner]

OK, thanks for the advice, I'll let you know when I get in there.

And it's only hard to turn in one spot, the rest is cake, lol


Had a blaster like that…..crank was shot.

[Divergent12]

Took the head off, everything looked fine. Turned the crank, turned easily all the way around. Springs aren't stuck, watched the piston move properly. Looked at the head, nothing looks bent or out of place to me. Everything moved easily. Put the head back on, turned the crank, same thing, gets to one point and it's very hard to turn. Spark plug was not in it. What is the torque specs for the head bolts?

[CJM]

the large 14mm headbolts are 33 foot pounds, the small 8mm headed bolts make HAND TIGHT no more or you will strip the threads.

[Divergent12]

Thank you. I only have one large head bolt, it's in the middle and its 12mm. All the others are 8. It's a 2006 model.

I also took pics of inside the head, but. Can't seem to upload them from my phone. I'll get them online from work tomorrow.

[samm3020]

the 12mm bolt I believe is 16-17 ftlbs....the smaller 8mm headed bolts a 7ftlbs...if ever in doubt just google what your looking for ( honda 400ex cam cover torque specs).... I hardly work on these motors without a torque wrench....i've never stripped/broken a bolt when torquing to spec which is good enough for me.

It's not to hard to do once you get the head off i'd pull the cylinder off as well....4-6 more bolts I believe and you can check the crank....sorta sounds like rod bearings starting to seize....1 thing is for sure if your riding that motor and the rod locks up it could take out the whole motor (my brother had a yz400f do that)....new base gasket and head gasket is cheap insurance

[Divergent12]

I will take the head off and look into it, but I do not believe it's a rod bearing. When spinning the crank, it does not make any grinding or any worn bearing noises. It also turn freely and easily when the head is off. When the head it on, it feels like a compression stroke, even though there is no spark plug.


And I can't get the files to upload, keeps telling me it failed.

[samm3020]

Just re reading all of this trying to rack my brain...(unfortunately I've rebuilt quite a few friends and project 400 motors)....when you put the head back on I'm assuming the cam chain and cam were in.....is it possible your just having to compress the valve springs on every rotation....honestly if your just bare head on there no cam box....make sure the cam chain isn't binding...other than that it's got me curious as to whats going on.

[Divergent12]

Hey! Got it to work!

17181106481718110649

I'll also check for cam and chain binding. Didn't see it when I was in there, but wasn't looking specifically at that.

[Divergent12]

Goin on vacation for a week, I'll revisit this when I get back. Thanks for the help so far!

[Divergent12]

Hey! Got it to work!

17181106481718110649

I'll also check for cam and chain binding. Didn't see it when I was in there, but wasn't looking specifically at that.

I should clarify, I got the photo upload to work! Not the quad.
Back from vacation, haven't cylinder the head off yet.

[Divergent12]

Discussions elsewhere concluded that the timing chain might have jumped and the hard position I get to while turning the crank is the valves coming into contact with the piston.

I turned the crank and linedup the T mark and was told that the rocker arms should have a "Little" bit of play in them.

Here's my results:
When the T mark was aligned, there was no play in any of the rockers at all.

Here's what I noticed. While turning the crank I observed 4 timing marks. The first two were not marked with any sort of label. The marks were probably about 5/8" to 3/4" apart from each other. As I continued to turn, I came across the second set of marks. The first of the two was labeled with an "F" the second a "T." Their distance apart was about the same as the first two, but the distance from the last mark of the first set and the first mark of the second set was probably an inch to an inch and a half.

As I was turning the crank, and I started to get to that "really hard to turn" sport, I noticed it was the exhaust valves opening, and that the exact spot where I had to grab it by two hands was at the "F" timing mark. It was hard to tell if the piston was at TDC at that timing mark or not.

So I agree that it probably jumped time. My questions now, is this an interference engine? Would the exhaust valves be bent?

Where can I get instructions on replacing the timing chain and properly setting up the timing?

Thanks!

[DragonGunner]

First please get a service manual, they are such a big help, I rebuilt the transmission with no knowledge of doing one using the manual. The "T" mark can show TDC, if you line up the "T" and valves don't move, then turn counterclockwise one more turn until the "T" lines back up and your valves will now be loose and can be checked, this is TDC now with valves closed, not open.

[KKiowaTJ]

A crf450 cam chain would be a better upgrade. Fits the same, a lot stronger. But with no manual, you have nothing to go off of. Was the best $ I spent for my 400.
It's a pretty straight forward motor, but with no manual or a cheap pdf, you are taking a chance. This way you can tear apart and go through and check all parts to find the flaw. Amazon.com is where I got my manual. Good luck

[Divergent12]

I am back with an update:

Got a manual, took the valve cover off again, and found out the spring in the chain tensioner broke. I'm guessing that it caused ti to skip a gear or two.

So got that taken care of and re-timed it; put the crank on the on the "T" Mark, aligned the camshaft sprocket marks parallel with the head, with the lobes of the cam facing down and the third mark of the cam sprocket facing up and perpendicular to the head. Got the chain on and the tensioner tight making sure all the marks were still in their right places.

Put it all together and go to start it up. And it just turns over. Cranks and cranks and cranks. When I five it some gas, it'll backfire a bit, either out the carb or out the exhaust. Sometimes both...

Now when I got this atv, it had a definitive compression sound when starting it. Now it does not. I can feel pressure when I put my hand on the exhaust, but it doesn;t sound like it used too.

If this is not the stock cam, could the timing be off? How do I found out what cam it is and find out the timing for it? I know it's had a rebuild, but I don't know what was done to it in that rebuild.

[DragonGunner]

All cams are timed the same way so your ok no matter what cam. You can take the plug out while cranking and stick your thumb over hole to see how the compression is, backfire is lean condition. Carb may not be feeding the cylinder gas….follow trouble shooting for that, quick way is dump a little gas down spark plug hole and throw the plug back in and hit the starter, if it fires up shortly then dies and doesn't start then your not getting gas to the motor.

[ride red 14]

I have built several 400ex motors over the years, many of which I bought to rebuild and sell. I have had 2 that I can think of right off hand that I bought knowing they had skipped timing due to bad tensioners or stretched chains. in both instances they had bent valves. might not be the case with yours. but that's where my bet is. put a compression gauge on it and check compression, that will tell you if it is compression or fuel related.