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Discussion Starter #1
Hello All - New to this forum and could use some help.

I'm working on a friends 2001 400EX that is not starting. Shows good spark, the carb is clean and in good order, so the next thing I went to investigate was the compression.

If I pull the Spark Plug and hit the starter, I can feel a strong flow of air leaving the plug hole. However I used a compression tester and could not even get a reading...

One thing I noticed, if I remove one of the valve cover adjustment plugs, when I turn the engine over there is a lot of compression leaving through the open hole. Same goes for the breather tube that goes from the valve cover back to the oil reservoir. If I remove the breather tube, there is a lot of compression leaving from the valve cover.

Is this indicative of air getting passed the valves?

I had the head off, and there is a small score in the cylinder wall and piston skirt, but it wouldn't appear that it is large enough to cause a compression problem.

Any help would be appreciated!

Thank you,
Casey
 

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If your losing compression through the valve cover holes you got more than one problem. Valve seals are toast and mostlikely a bent valve aswell. Welcome to the forums!
 
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Also you have to have the throttle held wide open and a strong battery charge to get a proper compression test. Under 100 it wont run well if it all. Over 100 it will run but not great until your up closer to the 120 mark.
 

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Discussion Starter #5
Thanks for the info - I also just noticed that the threads for the spark plug hole were striped, I'm not sure how well my Motion Pro Compression Tester hose was seating when I tried to take my compression test. I just ordered a Helicoil kit to repair this, and hopefully take a better reading.

But one thing is for sure, I was definitely getting air out of the valve cover. I gave the head a look over, and there was no immediate red flags. Valves were intact, and fully closed when I removed the head. Valves were also within tolerance before disassembly. Even if the valves were bent or leaking, this would allow air back to either the intake or exhaust ports, but as you said would have to also get through blown out seals to flow through the valve cover.

I'll report back if I find out more.

Thanks,
Casey
 

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No problem! Glad to help! One other thing that comes to mind. Maybe the headgasket is blown into the timing chain area and blowing compression in there? You said you saw the piston skirt so im guessing that means you removed the head. Did you re-torque the old head gasket? If so thats not gunna work. You have to replace the headgasket anytime its untorqued.
 

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Also you have to have the throttle held wide open and a strong battery charge to get a proper compression test. Under 100 it wont run well if it all. Over 100 it will run but not great until your up closer to the 120 mark.
I did that to mine after the wife swamped it. 0 psi until I put oil in the cylinder. Then showed good for the 1st test, after 2 more back to 0.

Yet turn the key and it would start right up, smoking like a coal train. Ran pretty decent too lol.

Did a new top end though and back to good compression.

So even with 0 on the gauge it can still run. I must have had just enough instant compression to run but not enough to show on a gauge.
 

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Also you have to have the throttle held wide open and a strong battery charge to get a proper compression test. Under 100 it wont run well if it all. Over 100 it will run but not great until your up closer to the 120 mark.
I did that to mine after the wife swamped it. 0 psi until I put oil in the cylinder. Then showed good for the 1st test, after 2 more back to 0.

Yet turn the key and it would start right up, smoking like a coal train. Ran pretty decent too lol.

Did a new top end though and back to good compression.

So even with 0 on the gauge it can still run. I must have had just enough instant compression to run but not enough to show on a gauge.

Well i cant lie this one has me stumped. There has to be some sort of coincidence that caused this. You could never convince me an engine with 0 psi compression will ever start. Zero compression would mean that fuel isnt even being pulled into the engine to fire. I have seen the sheader valves on compression testers get stuck and do false 0 psi readings but you said you added oil to the cylinder and got 2 readings then back to zero. Im really racking my brain and cant come to any conclusion that could ever explain this. There is no such thing as "instant compression" compression is compression you either have it or dont. Now when the spark ingnites that can create compression to an extent. But with 0 psi you wouldnt be even getting the engine to suck fuel out of the carb to try to fire. Unless it was fuel injected. Maybe that could be a possibility but still were talking extremely long odds. I have learned to never say never when it comes to an engine. They will do crazy things and prove you wrong in an instant. Still scratching my head though.
 
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Would have been very helpfull to catch that test on video/audio .
 

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Would have been very helpfull to catch that test on video/audio .
Haha believe me I know... I said how the hell is it running with 0 compression.... with the Schrader valve in would just barely bump to like 5 psi for a split second and right back to 0. Took the valve out and the needle would jump to 100 ish. Even tried a different valve insert same deal. But once I put some oil the gauge would jump to 160 first, then 120 the second, then 60 on the third. Then back to nothing.

If I'm correct, it pumps the pressure to the gauge and holds it so you can get a reading right? It wouldn't hold anything at all until I put some oil in the cylinder.

At any rate something is wrong again cause it's smoking again at anything over 1/2 throttle. So I'll do the compression check again lol if it's same results I'll take a video.
 

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I wont add too much because your already talking to the right guy ( Slammed )

If I'm correct, it pumps the pressure to the gauge and holds it so you can get a reading right? It wouldn't hold anything at all until I put some oil in the cylinder.

but yup the pressure gauge is suppose to hold the cylinder pressure. I'd try a deferent compression gauge, I'm baffled on this one as well.
 

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Ok so alot of thinking and questioning my engine builder about this and we are to this conclusion. You say the quad was sunk or swamped i think is how you put it. That would mean water was in your cylinder and most likely on top of the piston. Now that being said if you removed the spark plug and didnt turn it over a few times all that water would go right into the compression tester which could in theory throw your numbers all over the place. When you took the shreider valve out the water came out and it would only take a few drops at the most. He and i both agree that there is zero chance your starting a motor with 0 psi compression. It cant happen. Plain and simple. But you are correct on the purpose of the shreider valve is to hold the reading for you so you can look at it in a few seconds like in the instance of a car engine you will be in the drivers seat so you can run over and check it real quick. It will bleed off slowly and that is normal.

As for the reason your smoking again is because if the quad is sunk there is no such thing as perfectly clean water. The sand and grit will essentially sand blast everything. No matter what you do that sand will be everywhere in that engine and you will be doing many topends until its out. The best way to get it out is splitting cases and cleaning it all out in either a parts washer or solvent tank. You can try the bandaid method of flushing with diesel fuel. And many people will fight me on this but that is a gamble. Has it worked for some people? Yes. Am i one of those lucky people? No! Sorry to the OP for the thread jacking here but this one was really bugging me lol!
 

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Yea sorry for the thread jacking...

Correct it was swamped. Plug was pulled and blew it all out. Then changed oil. Months later started smoking. That's when the test was ran lol

Heck if I know lol... with oil in the cylinder the gauge held pressure. When no oil in cylinder it couldn't pump the gauge up enough I guess...

Yea I did a top end on it and it's smoking again.
Need to split the cases. But I'm Leary as I have never done that before.
Slammed if you wouldn't mind PMing me I would like some advice on splitting the case
 

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I'm sure Slammed is going to give you some great tips. Just take your time. Don't rush it or you'll be doing it twice. lol
 

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Yea sorry for the thread jacking...

Correct it was swamped. Plug was pulled and blew it all out. Then changed oil. Months later started smoking. That's when the test was ran lol

Heck if I know lol... with oil in the cylinder the gauge held pressure. When no oil in cylinder it couldn't pump the gauge up enough I guess...

Yea I did a top end on it and it's smoking again.
Need to split the cases. But I'm Leary as I have never done that before.
Slammed if you wouldn't mind PMing me I would like some advice on splitting the case
Gladly! I will later today. When i get off work.
 

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Discussion Starter #17
Hi guys - So a little update.

I took the cylinder off tonight, and once I got it off, it was clear there was another issue. I noticed right away that the piston was rather stiff on the wrist pin, it took a bit of effort just to rotate the piston. Second, I notice that the small score I saw in the cylinder was actually a pretty large abrasion area, which was confirmed when I got the piston off and got a look at the skirt. The rings are worn down on one side, and heavily marred.

I'm going to assume that this relates to the compression issues I had. One question though, if air was getting by the piston on the compression stroke, would this air then find it's way through the oil channels to the top of the head, explaining why I was getting compression out of the valve cover?

I can confidently say the piston and sleeve need to be replaced, and I have some concerns that the rod may be bent (explaining why it was causing resistance at the wrist pin). Is there a good method to check the rod while it is still in the case?

Here are some photos I snapped:
 

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