correct preasure in cylinders -Johnson 9,9

[wejsunek]

somebody knows what is correct engine preasure ? and a minimum for a good working? in metric unit pls....
andrew

 

[JB]

Re: correct preasure in cylinders -Johnson 9,9

Actual measured pressure is immaterial, andrew. What matters is that the cylinders are withing 10-15% of each other.

 

[Moody Blue]

Re: correct preasure in cylinders -Johnson 9,9

I've often wondered this same thing for some time.

JB, there must be general specifications, say 100psi etc for reasonable operation. Can't imagine a motor running well at 40psi, can it? It couldn't produce rated HP at such a reduced compression spec and I imagine it would run like crap also.

Am I right or wrong on that assumption?

 

[JB]

Re: correct preasure in cylinders -Johnson 9,9

The probability that both cylinders would fail at the same time in the same degree is remote, MB. About the only way that both cylinders could measure that low and still be within 10% is a head gasket failure between the cylinder bores.

Because guages vary so much and technique varies so much makers refuse to set spec on compression except to call for a close match. If either cylinder has failed there will be a severe mismatch.

Specs on compression are, of course, common on 4 stroke engines that can have valve failures. Those concerns don't transfer to 2 strokes.

 

[lowkee]

Re: correct preasure in cylinders -Johnson 9,9

According to my trusty (read, brand new) compression tester, 120psi (8 kPax100 .. is that metric?) is pretty much the minimum working pressure. If it's over 300psi, you're running the world's only diesel 9.9

 

[Rick.]

Re: correct preasure in cylinders -Johnson 9,9

lowkee. Are you suggesting 120 PSI as a minimuim or are you saying your gauge doesn't go lower than that? Rick.

 

[JB]

Re: correct preasure in cylinders -Johnson 9,9

According to my trusty (read, brand new) compression tester, 120psi (8 kPax100 .. is that metric?) is pretty much the minimum working pressure. If it's over 300psi, you're running the world's only diesel 9.9

That spec has nothing to do with a 9.9HP Johnny, lowkee, particularly a rope start 9.9.

 

[Bob_VT]

Re: correct preasure in cylinders -Johnson 9,9

An outboard will survive and run at 70 psi. This is normally an indicator on a 2 stroke of severe wear or the common..... severely clogged and carboned rings.

That is one of the primary reasons for a decarb proceedure on a 2 stroke.... to free the rings and restore compression.

 

[lowkee]

Re: correct preasure in cylinders -Johnson 9,9

lowkee. Are you suggesting 120 PSI as a minimuim or are you saying your gauge doesn't go lower than that? Rick.

The red line on the tester begins there (and goes lower). The tester itself goes down to 0 and the green line begins at 70psi (5 kPax100) and goes higher (there is a progressive crossover between the two colors at 70-120psi).

I have never bothered with a compression tester before on a small engine, so I'm merely guessing based on this tester's readings. 2 strokes (if yours is one) will normally run pretty much forever. I've worked on easily 500 of them and never replaced a set of rings and have only seen a few which were locked up ever reach the scrap heap (Mystery Oil is godly).

You can try a decarb, but if you have low throttle response (why else question compression?), I'd look more at your carb(s) than compression.

If you want to try a set of rings, I doubt they will run you much. Buy a hone, crosshatch the cylinder and pop in the new rings. Keep in mind, all new engine break-in rules apply with new rings!

Most 2 cycles I've seen have a nice removable cylinder, which makes it simple to replace rings (again, if ever I find one needing them).

Hope that helps, and let us know what you find.

 

[1968dart]

Re: correct preasure in cylinders -Johnson 9,9

Just tested my 1977 Evinrude 15 ...both cylinder's where close to 125...this motor is like new. Just proud as punch about having this motor given to me. Still haven't tried to start it yet..going to change the water pump and maybe put in a carb kit. I don't know if it really needs it but it sat forever so I probably will.

 

[jbjennings]

Re: correct preasure in cylinders -Johnson 9,9

Boy, that is nice compression! Congratulations on the freebie!
I'll bet she's smoooooooooth.....
Later,
JBJ