Anyone out there running low compression motors?

I have a 1984 70 HP Evinrude that I've been trying to make run decent at low speeds. I decided to just start over and do the whole thing with cleaning carbs and link and sync . But first I did a compression check and found that it's only 85 to 90 psi. Much less than the 120 figure I've seen posted as decent. Haven't done a decarb yet to see if that helps but that's next.
The thing is it runs great at higher speed. This thing pushes my 17ft deep v at 32. Great acceleration once it's off idle.
Was just wondering if anyone put there is knowingly running a low compression motor and making do.

Or if I'm looking at making the trip back in on the trolling motor soon lol.

Thanks

As I found out with my inexpensive compression tester, quality of tester can make a big difference in accuracy.

When I tested my old Johnson 30 HP using my lowest-cost compression tester, I only measured 70 PSI in each cylinder; both cylinders measured exactly the same. However, it would start and would run, but I was having issues keeping it going at idle and low speed. The problems I was having with that motor were not likely compression related.

I suggest that if all cylinders measure within 10% of each other, and it's running good at high speed, then your problem is not likely to be compression related.

Low compression will affect slow speed the most. Try a decarb and keep fingers crossed, but if comp stays that low, just run it until it dies. Get a small gas trolling motor for slow speed if you need it. 3-5 hp is all you need...or, time for a rebuild on the 70 !

The type of tester can make a difference, if you are using an adapter with a long hose that has the schrader valve in the middle, it is not going to give you a precise reading, the volume of that hose is effectively added to the combustion chamber, decreasing the compression ratio. It makes less of a difference on engines of bigger displacement, which is what most compression testers are designed for. Your 70hp triple is likely ~250cc per cylinder, where an automotive four stroke is double or triple that.

In summary I wouldn't put much faith in a single test. Try your gauge in another outboard that runs well and see what it reads, or try a shorter adapter if you have one that works.

MrGiggles said:

The type of tester can make a difference, if you are using an adapter with a long hose that has the schrader valve in the middle, it is not going to give you a precise reading, the volume of that hose is effectively added to the combustion chamber, decreasing the compression ratio. It makes less of a difference on engines of bigger displacement...

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Thanks for this! I had not thought of that but it totally makes sense. Yeah this tester is cheap with a long hose and each adapter has a section of hose on It making it even worse. Plus the check valve is right up against the gauge. Used to have a good one but it got run over by someone who didn't do a walk around before exiting the shop. Really haven't needed one for a while so I just got a cheap one at harbor freight.

airshot said:

Get a small gas trolling motor for slow speed if you need it. 3-5 hp is all you need...or, time for a rebuild on the 70 !

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Yeah I've thought of this. I need low speed for night fishing. Where I am it's 10mph or less at night and it really runs bad after a couple miles of 7mph. (Won't go 10, either 7 or 20)

Lots of older two-strokes have pretty low compression. I've had a couple that 90 psi was normal. Mostly motorcycles.

Some as low as 6:1, lower than 8:1 is common. Check the manual.

Lots of thing can affect compression. Cranking speed, throttles wide open or not, air box on or off, type of tester, etc.

As long as compression is within 10% or so of each other, I would say they are good.

What Pappy says is true, but hopefully, your motor hasn't gone that far. Hopefully, it's running a little rich at idle and is loading up, and you only need to lean it out 1/8-1/4 turn and maybe replace the spark plugs with OEM specified plugs.

Still, look for the fuel mist as he specified.

BernieP said:

Thanks for this! I had not thought of that but it totally makes sense. Yeah this tester is cheap with a long hose and each adapter has a section of hose on It making it even worse. Plus the check valve is right up against the gauge. Used to have a good one but it got run over by someone who didn't do a walk around before exiting the shop. Really haven't needed one for a while so I just got a cheap one at harbor freight.

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I also have an older HF gauge and it is actually pretty spot on compared to 2 other gauges I borrowed to compare results. One thing that made a difference was using the rubber tip you have to hold tight to spark plug hole. It usually reads 5-7 PSI higher than the screw in hose type fitting. FYI Harbour Freights new lines of tools are really good compared to their old stuff. ICON=SNAP-ON, Hercules I believe=Milwaukee or Dewalt. The Bauer line is great. HF hires engineers away from the top brands and make the 7 changes needed to get past copyright/patent restrictions.

As long as the valve doesn't leak, hose or not doesn't matter. I have not seen that the press and hold works any better than the screw in gauges, but individual gauges may be a little different.

Happily, all of my gauges read pretty identically. I check them every once in awhile.

thill said:

As long as the valve doesn't leak, hose or not doesn't matter. I have not seen that the press and hold works any better than the screw in gauges, but individual gauges may be a little different.

Happily, all of my gauges read pretty identically. I check them every once in awhile.

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The placement of the valve is what matters. As long as it is close to the very end of the hose, the reading should be close to correct. Anything after the valve just becomes a holding tank of sorts, as Pappy mentioned, the cylinder will eventually pump it up.

Any amount of volume before that valve essentially becomes part of the combustion chamber. Take an outboard with a single cylinder displacement of 200cc, likely with a compression ratio somewhere around 8:1. That would make the combustion chamber volume close to 25cc. Now if you add an 8" piece of 1/4" hose with an internal volume of ~6cc, you've increased the combustion chamber volume by 24%, dropping the compression ratio down closer to 6:1, the gauge is going to reflect that.

In the automotive world where cylinder displacements are closer to 500cc or more per hole, that small amount of added volume makes much less of a difference.

It's the same concept as adding or removing material from the piston crown, shaving the head, or installing head gaskets of varying thickness. Making the piston squeeze air into a bigger or smaller space.

Most testers that I've seen have the Schrader valve mid-length in the hose underneath a quick coupler, with a couple of solid adapters and a couple longer hose adapters for hemispherical heads. The short solid adapters are better, ideally you would want the valve pretty much where the spark plug would be, but that isn't possible with many gauge adapters.

AAAHHHH.... That makes sense.

All of mine have the valve right in the fitting that screws/presses into the hole, and I just assumed that all are like that.

But maybe not. I hadn't thought about that. Thank you for that bit of info.

OK, I'm not following. What does the a potential change to the compression ratio, by adding the volume of the test hose, have to do with a compression test result, which is a measure of pressure?

LDUBS said:

OK, I'm not following. What does the a potential change to the compression ratio, by adding the volume of the test hose, have to do with a compression test result, which is a measure of pressure?

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Compression is defined by a taking a fixed amount of air and pushing it into a smaller space. If you vary the size of that space, you get more or less pressure, given the same amount of air. Most gasoline engines are in the 9-13:1 range, diesels are 16-20:1 or more. They are taking 9-20 units of air, and compressing it into the space of 1 unit.

This is why installing thinner head gaskets or shaving down a cylinder head is sometimes done for added performance, as it increases the compression ratio. Shrink the combustion chamber, get a bigger bang.

If you wanted to get a ballpark guess of the what a cylinder should be making, you can multiply the compression ratio by 14.7 (atmospheric pressure at sea level). My 8:1 example above would be close to 120 psi in perfect scenario, unlikely to hit that exact number in the real world, but it should be close. Drop it to 6:1 by use of a long adapter, and you're looking at 90 psi, even though the cylinder in question is likely capable of more.

As Pappy pointed out, it's a bit of a moot point since it's only half of the equation in a two stroke, but worth noting IMO.

MrGiggles said:

Compression is defined by a taking a fixed amount of air and pushing it into a smaller space. If you vary the size of that space, you get more or less pressure, given the same amount of air. Most gasoline engines are in the 9-13:1 range, diesels are 16-20:1 or more. They are taking 9-20 units of air, and compressing it into the space of 1 unit.

This is why installing thinner head gaskets or shaving down a cylinder head is sometimes done for added performance, as it increases the compression ratio. Shrink the combustion chamber, get a bigger bang.

If you wanted to get a ballpark guess of the what a cylinder should be making, you can multiply the compression ratio by 14.7 (atmospheric pressure at sea level). My 8:1 example above would be close to 120 psi in perfect scenario, unlikely to hit that exact number in the real world, but it should be close. Drop it to 6:1 by use of a long adapter, and you're looking at 90 psi, even though the cylinder in question is likely capable of more.

As Pappy pointed out, it's a bit of a moot point since it's only half of the equation in a two stroke, but worth noting IMO.

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Thanks! I figure the only way to learn is to ask dumb questions. haha.