Can You Test for Compression with Carbs Removed, etc...?

[Frank_Hassa]

Today I got my new $20.00 compression tester in from Wal-Mart (delivered). Came with two fittings, and one of them fits in the spark plug hole.

Before I got to far I got a bit nervous and doing this thing with the carbs off. I've read that you test compression with WOT, but never read anything about doing it with carbs removed. Wanted to make certain that was okay. Seems like it should be, but you never know.

Also I have the RAVE valves out. Doubt that makes a difference, but again wanted to check.

AFAIK, you squirt lubricant into the cylinder, attach compression tester and turn it over a few times. Never did this before.

Does it matter if the other cylinder has the spark plug in it?

Also there's a Schrader Valve immediately above the fitting, and I can't see how that is going to work without something physical pushing against it, and there's a thumb-release valve up near the top at the gauge. I assume that's to release pressure after the reading has been made, but not sure about that either. I can post pics, if my situation is non-standard.

 

[hotrodssnova]

The schrader valve is supposed to be there. The compression of air in the cylinder will force it open. I have tested compression without carbs in the past. Don't recall any difference. Once the engine was finished and running, a mid season test was around the same numbers.
Putting oil in the cylinder isn't necessary for the initial test, imo. I have always reserved that for the second reading if there was a big difference between cylinders.
Remove both plugs and ground plug wires before cranking. Make sure battery is fully charged.

 

[Luv2Camp]

If you put oil in the cylinders it will falsely increase your compression numbers

 

[Coastiejoe]

Carbs being off will not affect the compression test.

 

[hotrodssnova]

If you put oil in the cylinders it will falsely increase your compression numbers

I only use oil if I get a reading that is way off. In the v8 world, if you had a low reading, squirting some oil in that cylinder. If the readings came up, it was rings, if no change, valve, hole in piston, gasket.

In a two stroke, it would only be a head gasket, hole in piston, or worn rings/jug.

 

[Frank_Hassa]

Okay I just did the test and understand the "big picture" of how the compression tester works.

Front cylinder tested a solid 120 psi, and the back cylinder was 125. Given that the gauge is a cheap $20 Walmart gauge and the readings are likely to be inaccurate, is it more likely the real pressure is higher, or lower.

Assuming 120, 125 is accurate, what does this mean. I've read 150 is perfect. I also read that when the readings are in the same ballpark, it means there isn't unnatural wear or a serious problem with one cylinder.

What else do these numbers tell me?

 

[hotrodssnova]

It tells you you have 120 in one and 125 in the other. Both are low. If you doubt the readings, rent one to compare. Squirting some oil in the cylinder. If the compression goes up, post what you get.

 

[mikidymac]

Basically the gauge is telling you the two cylinders are pretty close. Even if we assume the gauge is reading low the two cylinders are still within 5 psi of each other.
Now if it is correct 120-125 psi is at the end of the life of a 580-787 engine. Perfect is 150 psi and below 120 they don't like to start in the water.

I am against the oil down the holes for a compression test. This is a 2 stroke so there is some residual oil sealing the rings so adding more oil will give a false reading. If the ski was sitting for years I might give it a shot of lube when I first turn it over but that is it.

The oil was good back in the day when testing car engines with low compression. If it tested low you could give it a shot of oil and if it came up you needed new rings. These seadoo engines operate differently as the only thing that could cause low compression is the rings or mechanical damage.

 

[hotrodssnova]

My thinking on the oil was if the compression increased, he could believe the gauge readings.

 

[mikidymac]

I would just try a second gauge. It is a shame that our only option today is crappy inaccurate gauges. I am in the irrigation industry and it doesn't matter if it is a $5 or $200 pressure gauge they don't last more that a year and are not very accurate. That is the problem with all this junk being made in other countries.

 

[hotrodssnova]

I agree. On my one ski, xp, it showed in the 150 on both. On my wife's gsx, it read 165 on both. Both are 96 model 787. I use a harbor freight cheapo. On both skis, the readings we're repeatable.

 

[Frank_Hassa]

My thinking on the oil was if the compression increased, he could believe the gauge readings.

I gave each cylinder two, 1-second "spritzes" of evinrude spray oil. The nasty smelling stuff. It smells like fish. And something else.

Anyways, the results were:

Front, no oil - 120
Back, no oil - 125

Front, with oil - 133
Back, with oil - 135

I did each cylinder twice, to make certain the readings were the same.

Does this say anything other than what's already been posted?

 

[WorkInProgress]

I would just try a second gauge. It is a shame that our only option today is crappy inaccurate gauges. I am in the irrigation industry and it doesn't matter if it is a $5 or $200 pressure gauge they don't last more that a year and are not very accurate. That is the problem with all this junk being made in other countries.

You don't know how right you are.

I have restored 2 old original flat fender Willys jeeps and the one is just shy of turning 70 years old. The only 2 companies that make new parts for these original jeeps are made in India and they have such poor reputations of quality that I mostly buy 70 year old new never used parts when I can because I can't trust the only 2 companies making me new parts. Even rubber parts like the hose going from the carb to the air cleaner. I bought that from offshore new parts it literally lasted 2 weeks to look like an old dry rotted tire and even had holes in it.

Then I bought that same rubber hose on ebay new never used 70 years old and it's been on for the last 3 years and still looks new

 

[mikidymac]

Your new readings tell us nothing other than your cylinders are within 5 psi of each other.

You need a second gauge to see if they are really within spec.