Acceptable variance in compression?

[ILLandowner]

What is an amount of variance that would be able to be run without a concern, and when is it just not right to keep runnin a motor? I'm talking one cylinder lower than all the rest, which are within a few lbs of each other.

Thanks in advance!

[northeastcat]

Within a few psi is the acceptable amount , anything other then that is the sign of a problem and it should be torn down for inspection and repair.

[pyro]

Trick question:

There is almost always a specific problem that would cause one hole to be considerably lower than the other, and such a continuing problem will usually make the cylinder get worse if you continue to run it, such as a scored cylinder, broken ring, or temperature damage from a cooling problem, lean carb or clogged injector. Ignoring such a condition could lead to a serious failure.

Low compression across the board may be OK, as long as it wan't due to lack of oil or temperature damage.

[maddhatter]

The acceptable difference in compression is 10% or 10 lbs. between the highest and lowest cylinders.

[ILLandowner]

I guess the 117lbs is a tad over the 10% or 10 lbs

:mad:

These were cold readings, that won't make a difference will it?

[WaterBoy]

If it were me, I start the engine and let it run for a few minutes and get warm and then pull another compression check. I have checked mine after it has been sitting for a while and they were off also, then after running it, they were fine.

Just a thought.

Waterboy

[mendo]

I'm with waterboy, warm it up and check it again.

Good and warm is where I get the best, most accurate readings.

[Rickracer]

That may even out your numbers a bit as well. :cool:

[delawarerick]

10 percent is rule of thumb and I agree with Rickracer I had 110 in 5 holes and 90 decarbed and got the 90 up to 105. Decarbing can often remove carbon or unstick a ring. Rick

[ILLandowner]

does anyone have a reccomendation on a product that works the best to de-carb a motor?

I will start motor and warm it up and take readings again. then I'll de-carb and do again ......will post results. Might take me a few days to get time to get it done.

Again..Thanks all who replied!

[rock]

Actually you are pretty close to the 10% rule. If after decarboning you would like to get together and do a leakdown test, get ahold of me.
Rock

[rossi]

According to my Mercury service manual, Variation of more than 15lbs per sq in. between cylinders indicates that the lower compression cylinder is in some way defective, such as worn or sticking piston rings and/or scored piston and cylinder.

Ventura II/Merc

[ILLandowner]

Thanks for the offer to do a leakdown test! I promise we will get together somewhere in the future, I'm just trying to do all I can to help make the decision between this V-6 150 or rebuilding my Inline 150. The V-6 is currently mounted on my enchanter that has not had title transfer and/or license takin care of yet, so I can't travel with it at the moment. This decision for which engine I want on my Glastron Carlson CV16 is holding up the mounting of the 200hp I bought for the Checkmate! If I can find a cheap fix for the low cylinder, (such as decarb) I'm sure the V-6 is in, and out with the ole inline! BUT, if I'm going to have to spend money on a rebuild, it would be on the I-6 to keep boat true to era!

It makes me laugh....I-6 mounted on my Glastron....lost a cylinder, The V-6 150 on the Enchanter...Low cylinder, fresh 200hp hangin by the lifting eye in my garage, Freebie Baja out in the front yard waitin for a new owner!:D :D :D

I need to get my head out of my ass and make a decision!!:p

[Rickracer]

10% of 131 is roughly 13psi, 131-117=14, over 10%, less than 15psi, but I still would figure what's going on to cause the difference. When I built my V-4 the first time, it had exactly 135psi all four after 2 hours, 145 after 10 hours. If your chambers are equal size, cylinder wall finish is the same, bore the same, and reeds functioning properly, they should be pretty much identical. :cool:

[dynobo]

Before I would worry a whole lot about it I would pull the head on that side and take a look. If you don't see any concrete damage (scuffing,scoring,) I'd invest in a head gasket and run the heck out of the thing. I had a '84 200 Merc. on a Bullet and it had 135 across the board except 110 on one cylinder. I pulled the head, didn't see anything major and put it back together and ran it at 80+ MPH FOR 5 more years until I sold it. Far as I know it is still running today. If it was losing power I couldn't tell. Just a thought...........Bobo

[sosmerc]

Run the engine up to temperature and then recheck your compression. But, more importantly, do a leakdown test. If you have a cylinder that is somewhat down on compression AND shows a higher percentage of leakdown or blowby, you definitely have a problem in that cylinder. Expect to see a higher percentage of blowby on the starboard cylinders. I wouldn't get all that concerned unless you find a cylinder that has over 15% leakdown AND is over 10psi lower than the other cylinders. It's not that expensive to pull the heads, and take a careful look at the cylinder walls. Might as well replace the head gaskets at this time and then retest after running the engine up to temperature.

[ILLandowner]

but I have a compression tester, and if a leakdown test is performed just as it sounds, can't I do it myself? Tell me if I'm wrong, but to do one wouldn't I connect to a cylinder, turn motor over, document the reading, wait a certain length of time (HOW LONG???), document reading again, and compare in percentage how much compression was lost?

[sosmerc]

To properly perform a "leakdown" test requires a special tool that is designed specifically for this test. It involves putting a specific amount of compressed air into the cylinder through the spark plug hole while holding the piston at top dead center and then reading the amound of pressure being "held" in the cylinder by a second guage.
You can do a search on this site using leakdown tester as your key search word and you should come up with lots of info about testers.

[ILLandowner]

Thanks for the explanation on the special leakdown tool! I guess I will be taking Rock up on his offer to help perform this procedure once I've gotten legal to travel with this rig.

I just got done running engine until fully warm and took a second set of compression readings.
LEFT BANK 115,106, 117
RIGHT BANK 116, 116,118


Still seems borderline!!:mad: :mad: :mad:

[RB in NM]

As mentioned above , decarbon it.

You'll be surprised how much it will actually help. There's a "shock" treatment that Rich Boger recommends on his web site , which is at Boger Kustom Propellers. Spray sum juice in all holes, leave it over night, maybe a little more, then launch the rig and run the piss out of it fer a while. Bring another set of plugs out on the water with you, as the others are gonna foul.
Then do another warm motor compression check.....may cure yer low PSI issue.


RB