Compression test... hot or cold?

[MtnEX]

When you guys check your compression, are you checking it hot or cold?

And how much higher would it be when hot vs cold?

Thanks.

[CJM]

I usually check when cold, not sure the exact numbers of hot vs cold tho.

[TRX01]

you want the machine to be cold so that way you get the exact compression, when the machine is hot the piston rings expand makeing it higher compression but obviously it wont stay that same compression as it cools down, so do it when the machine is cold

[MtnEX]

Well, the advice goes back and forth on the hot and cold thing... and then the service manual says hot...

So I am just trying to get the general concesus among 400EX owners on which way to test.... and also what compression figure the is the cutoff number for stopping and rebuilding?


Service manual says hot and 100-128 PSI I think.


On the hot/cold thing I can see the argument on both sides. I can see checking it hot because that's how it runs... but I can also see checking it cold, because that is how it starts.

Overall, I guess it would be ideal to be "in range" both cold and hot?

I won't be able to resist checking it hot just to see what the difference is.

[CJM]

I say check both, I did mine before I rebuilt and it a cold it had barely 90psi..

[honda400ex2003]

both ways is the best, then you can compare the two. they should not be too much different.

different site but same principle

http://www.dootalk.com/forums/index.php?showtopic=183683

steve

[MtnEX]

Originally posted by honda400ex2003
both wasys is the best, then you can compare the two. they should not be too much different.

different site but same principle

http://www.dootalk.com/forums/index.php?showtopic=183683

steve

That's pretty specific there... I like that...

[MtnEX]

OK so this 400EX I have here has like 90-95 PSI cold... 120 PSI hot.

:huh :confused:

[MtnEX]

Originally posted by MtnEX
OK so this 400EX I have here has like 90-95 PSI cold... 120 PSI hot.

:huh :confused:


So what is the opinion on that guys?

[honda400ex2003]

im not really sure lol i have never done a comp test before lol. seems low though for the cold one. steve

[CJM]

I think its to low.

I did a comp test on my 00 before I redid anything and cold I got 90psi. I redid the whole topend and got 120-130 iirc. Same thing with a friends, we didnt test it before but after 120psi cold.

Manual states 120psi iirc too.

Try a new head gasket, its not to much work.

[MtnEX]

My factory service manual says warm it up to normal operating temperature.

It's not the first time I've found a mistake in a factory service manual though.

So let's put it to a vote...

90 PSI cold ~ 120 PSI hot...

Time to rebuild it or not...

[honda400ex2003]

rebuild gets my vote. too far off for me. i would be afraid to check after that comment. lol steve

[MtnEX]

I showed him the blue smoke.

He's a believer now....

Took it down the pavement cycling from acceleration to deceleration... and it puffed smoke on acceleration and deceleration.


They always say smoke on acceleration is rings/cylinder... and smoke on deceleration is valves related.


I don't think he has valve problems though... unless the rings/cylinder are worse than I think... because it doesn't smoke on startup... and compression does increase like 5 psi or so if I squirt a teaspoon of oil in the spark plug hole and test again.

[honda400ex2003]

i have not heard that method. ill keep that in mind. i have heard that under hard accel and start up is valve but decel and when warm is rings. almost but not quite the same, ill add that to the arsenal of stuff in the bag to think about. also you inbox was full. sounds like you are good to go now. congrats. steve

[honda400ex2003]

also do both while you are at it. for the 50 bucks for the seals it is not worth having to tear it apart again lol. steve

[MtnEX]

He wants to pay me a little to fix it (I'm outta work) but I've never done it.

He was saying tear it apart, and if nothing looks real bad, he'll buy one of those drill hones, a brush, gasket set, valve seals and rings.


I reckon that might work and I might be able to do it with the help of the manual. But I got no way of checking bores or pistons and stuff.

What if his cylinder is worn, ya know?
I have no way of knowing if it needs oversize rings, or an overbore, etc...


I think I am going to pass... not sure it's a good idea. I think he has just talked himself into this for the convenience and a cheap fix... thinking about that instead of the "what if's".

[CJM]

Measuring the piston and bore are how to tell really.

BTW adding oil to the cyl will always increase the compression-even good compression.

[MtnEX]

Yeah, and I don't have the tools or experience for that.