compression test

[ChaseyMay]

ok today i got a new toy since my wifes fourwheeler took a crap on me ( new toy a compression tester) , so i decided to check the 1100 since i already took the carbs off so here they are, 1# 0psi 2# 120psi 3#almost 120psi and 4# almost 120psi also, but what i dont understand is i have compression on the #1 cylinder with the tester in, i put my hand over the carb boot and it really pulls my hand in, so now im confussed, if i have to replace number 1 rings should i just replace them all? or can i get away with one, or am i kinda wasting my time trying to fix, since its been on fire before and setting for 2 years thanks for any help
Jeremy

[John]

Squirt some oil into the cylinder, hold the throttle open, and recheck the compression. If it goes up, then rings are probably to blame, if there is no increase, then that usually means a valve problem, either bent or burned.

[Yamaniac]

I don't know for sure, but aren't you supposed to do compression test with the engine warm? I think I remember reading that on here before. Guru's please correct me if I am mistaken

[John]

I never thought to mention a hole in the piston will give you 0 compression. You can look into the spark plug hole and see the top of #1 piston. Try that.

[GrandPrix46]

I had a Grand Cherokee that had 0 compression on the #1 cylinder, turned out to be a golf-ball sized hole in the piston. Hope that's not your problem. Rings and valves would be the only other possibilities, not the head gasket since the other cylinders checked out good.

Compression check is supposed to be done with the engine at operating temp, FWIW.

[ChaseyMay]

thanks for the help guys, im still feelin sick over this, b.c i just got another tank this week, but back to the problem at hand, i thought there might be a hole in the piston also, but lookin threw the spark plug hole no hole in piston, so i guess im gonna just have to do a tear down later, also i dont have the carbs on they are apart on the bench, so i was checkin the compression with carbs off, but if i didnt have any compression y would it want to suck my hand threw the carb boot? also can i still get rings, valve or vavle sets for this? and is this a hard job to do? i have experiance in rebuilding but not a 4 banger..
thanks for any help
Jeremy

ps you guys are right your supposed to check them at normal temp, but this bike hasnt been ran in 3 years due to it catchin on fire

[ccogswel]

Jeremy

Try this first. You'll find out real quick where your problem is at. First rotate your crank shaft to where your valves are closed for the number one cylinder. Then take your compression tester and screw it in but leave the guage off. It should have a air chuck fitting on the end. Take a air hose and connect it to your compression tester hose. And listen for any air leaks. If you have air coming from the exhaust you have a bad valve the same for the intake side as well. Or if you hear air coming from the crank case you either have a hole or worn rings. Or maybe even a blown head gasket.

Good Luck!

[prometheus578]

Rings can stick due to sitting and not seal properly. That may account for the low 120's when they should be 142's.
As for cylinder#1: Sticking rings won't cause a zero compression reading. Check your valve clearances before doing anything else. Too tight of a clearance (meaning no clearance at all) will leave a valve open. If you have a larger clearance, say around .020 or something bigger, you may have a bent valve.(which would give you no compression)
If the valve clearances check out Ok, you may have a burnt valve. If you have a compressor, this will be an easy check.
Bring #1's piston up to TDC.................

Oh. I see that ccogswell has stolen my thunder while I was typing!
One step he left out....
On the end of the checker hose that screws into the plug hole, there is a valve, like a tire stem valve. You must remove this valve first, or the air from the compressor won't get into the cylinder. Don't lose this valve! It looks the same as a tire valve, but this one has a much weaker spring to it. (A compression tester with a "tire" valve in it will register about half of what actually pressure is.)
Don't ask me how I know this

OK... the Mac Tool guy told me this...when he was selling me a replacement compression checker valve after I lost the original one. And yes, I had tried using a tire valve....

[ccogswel]

Sorry Prom.
Didn't mean to steal your thunder

My compression test hose doesn't have that release valve on it. Mine is on the guage itself. I didn't know they made them any other way.

[ChaseyMay]

thanks for the help guys, il do the check early this week, today is my wifyes bday , so we gonna go out , but il do the check this week, and i was wondering if it is a vavle, wouldnt a 79 750 specail vavle work? thanks again guys
Jeremy

[prometheus578]

"Not that valve..."

Not the pressure release valve...
there is, inside the hose.. inside the fitting that goes into the plug hole, a tire-like valve stem thingie. (Schrader valve?)
compression checker hoses have them, leak down tester hoses don't.
It's a one way valve as I'm sure you can figger out.
"But why?", you ask.
Doing a compression check the piston pushed air out into the hose. This air goes past the valve to the gauge. if there was no valve in there, the air would rush back out and the gauge's needle would swing wildly. (I used to swing wildly, but am too old now) The valve is needed to hold the air so you can read the gauge. You have a pressure relief valve so that after your done reading the gauge, you can release the pressure trapped in the line, etc.
This valve must be removed to use the gauge for a leak0down test. In this instance, the air supplied is coming from the hose into the cylinder. It's going down the hose in the opposite direction. if the valve was still in there, it would block this air.
There has to be a valve between the cylinder and the gauge.
If you have something different, We'd all like to see a pic.
(I'm not especially fond of looking at pictures of men's hoses and valves... But sometimes, it's OK provided that one doesn't make a habit out of it)

[ccogswel]

Oh I see. I didn't know that. See mine has it all in the gauge. My hose doesn't have that valve in it. But I looked and it is in the bottom of my gauge. I guess it's good that this all came up. I did not know that about that being like that on some compression testers. I just learned something new.

Thanks.

[Flatlander]

Isn't the valve there also to hold compression so that the hose and gauge don't become part of the compression chamber? Isn't that part of the reason why you turn it over several times?

[Flatlander]

You may be getting 140's. So don't be too alarmed about the other cylinders, ChaseyMay.
And do another test with one in the end of the hose, ccogswel, you might see some difference.

[Flatlander]

Typo.

I ment to sat "You may be getting better than 120's", not 140's.