68 9.5 rude just wont rev out.

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I fool with a lot of small engines from outboards to chainsaws, snowmobile... I have found in my experience that an engine with low compression, especially a multi cylinder engine with very close readings is very rare. Usually one cylinder will be extremely low and the other is good unless it's been straight gassed. I have a relatively inexpensive compression gauge and on engines smaller than 100cc per hole, it tends to read lower than what performance would indicate. My point here is; if you're reading is 60-80psi but the engine starts and runs well and both cylinders read within 5psi of each other, I'd say compression isn't an issue. It takes a long time to wear a cylinder out, usually a bearing craps out first or some other failure. Compression readings are subjective and I feel similar numbers between the holes is more important than the actual number.

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Weldorthemagnificent said:
I fool with a lot of small engines from outboards to chainsaws, snowmobile... I have found in my experience that an engine with low compression, especially a multi cylinder engine with very close readings is very rare. Usually one cylinder will be extremely low and the other is good unless it's been straight gassed. I have a relatively inexpensive compression gauge and on engines smaller than 100cc per hole, it tends to read lower than what performance would indicate. My point here is; if you're reading is 60-80psi but the engine starts and runs well and both cylinders read within 5psi of each other, I'd say compression isn't an issue. It takes a long time to wear a cylinder out, usually a bearing craps out first or some other failure. Compression readings are subjective and I feel similar numbers between the holes is more important than the actual number.

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Click to expand...


While I can get on board with what your saying, But everything else checks out.

And while rare I have had another motor from last year that had similar issues and was a bear to start.

It was a 25hp that was at one time a rental. It would start with much effort, but it would act up while running and on some days early on it would have good power, then it slowly fagged out.

Compression was 100/100 near the end and I finally tore it down to find the head gasket blown on both holes.

I shaved the head to where calculations told me I should be near 155/155 with a good power head and only got 135/135.

The motor runs but I’m pretty sure it would benefit from rings as testing with oil ups the compression to 150/153.

In general I take things like this to be a motor that was run and serviced poorly resulting in worn rings.

One cylinder off more than the other I would figure that the motor got hot.

Then again I have a 2006 15hp Johnson with the paint baked off of one cylinder and it has 140/140 compression.

Never been touched as all other areas are factory paint to include the edges of the gasket.

And as pappy pointed out, keep pulling the cord till the needle stops climbing and cylinder volume doesn’t matter.
 
Pappy said:
I don't want to take this thread off track but the comments on compression gauges and hose length and shrader valve location is a bit bogus.
Shrader valves are located at the spark plug end of the hose.
Click to expand...


Not on all of them, less than the majority I'd say. Most have a foot or so of hose coming from the gauge, with a fitting on the end that holds the Schrader and a female quick coupler, another short length of hose with a male coupler on the gauge end, and various adapter fittings on the cylinder end. The uninterrupted hose between the cylinder and Schrader is what adds to the combustion chamber volume, on many gauges that is several inches worth.

That extra hose before the schrader would be the same as if you doubled or tripled up on head gaskets, compression would go down, as the combustion chamber is now effectively larger.
 
In fairness I do believe you are mistaking the pressure relief valve at the gauge for the shrader valve? A decent quality gauge usually has this feature.
Any gauge I can recall has had the shrader valve located at the cylinder head end of the adapter hose or if a single hose at the cylinder head end. All adapters each have had a shrader valve located in them. Even in doing a bit of research online to make sure I was not off base this seems to be the common norm.
I have three testers. The MAC I use most often. A Craftsman that was given to me new in the package, and an old rubber tipped push-in gauge that was my father's. Even the Craftsman has the shrader at the cylinder head. This is the only way to achieve an accurate reading.

Please name some gauges that have the Shrader valve at the gauge with no shrader in adapter hoses. Would love to look these up. Since you clearly said less than the majority have this feature it should not take much time to find some?

Why don't we take a poll and see how many folks in here have a compression gauge and see where the shrader valves are located? That may support your claim as well.
 
Pappy said:
In fairness I do believe you are mistaking the pressure relief valve at the gauge for the shrader valve? A decent quality gauge usually has this feature.
Any gauge I can recall has had the shrader valve located at the cylinder head end of the adapter hose or if a single hose at the cylinder head end. All adapters each have had a shrader valve located in them. Even in doing a bit of research online to make sure I was not off base this seems to be the common norm.
I have three testers. The MAC I use most often. A Craftsman that was given to me new in the package, and an old rubber tipped push-in gauge that was my father's. Even the Craftsman has the shrader at the cylinder head. This is the only way to achieve an accurate reading.

Please name some gauges that have the Shrader valve at the gauge with no shrader in adapter hoses. Would love to look these up. Since you clearly said less than the majority have this feature it should not take much time to find some?

Why don't we take a poll and see how many folks in here have a compression gauge and see where the shrader valves are located? That may support your claim as well.
Click to expand...

No. Definitely not the pressure relief.

I do not know about others, I only have a couple of them myself, both of which have the Schrader in the middle.

There are several that I suspect to be the same way, without having them in hand it's impossible to tell. Very often the Schrader goes underneath the female quick coupler. If you are using the short steel adapters, that is of little consequence, but those don't always work, nor do they always come with them.

https://www.harborfreight.com/deluxe-compression-test-kit-24-pc-64917.html?cid=paid_google|||64917&utm_source=google&utm_medium=cpc&utm_campaign=&utm_content=&gclid=CjwKCAjwlbr8BRA0EiwAnt4MTnEhxQ7T9SBziBr0GgX1XklkUiOempHYHdOmo7csdtVxuDuBOihjgxoCBXYQAvD_BwE

https://www.northerntool.com/shop/tools/product_200310615_200310615?cm_mmc=Google-pla&utm_source=Google_PLA&utm_medium=Automotive%20%3E%20Automotive%20Diagnostics&utm_campaign=Mityvac&utm_content=3316154&&ds_e_product_group_id=734370533237&ds_e_product_store_id=&ds_e_ad_type=pla&gclid=CjwKCAjwlbr8BRA0EiwAnt4MTr6Lw5zoRxqIDjcvi-s5a7IOMluHrSyDTm99tH5Ab9y1OcCTfjva1hoCW-oQAvD_BwE&gclsrc=aw.ds

https://www.summitracing.com/parts/otc-5606?seid=srese1&gclid=CjwKCAjwlbr8BRA0EiwAnt4MTl8WHGFFFF9AXf0HOGDbbKUlQWjJ_h_ggwk0sJp1a2eW_bWRM5AzBBoCUgAQAvD_BwE

This thread goes through some of the challenges of using automotive gauges on small engines. It's all sort of beside the point anyway, which is that you shouldn't put much faith in compression testing small engines when an even but slightly low reading is found.

https://www.arboristsite.com/community/threads/compression-tester-issues.310133/page-2
 
It seems as if ya want something that's right ya gotta make it yourself...

Many years ago I gutted a spark plug and epoxied a snifter valve for a piston pump at the very end....its like a shrader valve with a weak spring. Ran a hose to a liquid filled gauge...just because...still have it and is my goto when I need one.

Actually one can simply turn over the motor by the flywheel. One can tell a lot just by doing that.
 
That is not the point of this. The point is that when you make a blanket statement that for the most part so far is factually untrue it can be taken as fact by folks on here that are here to learn.... when in fact it is not.
In these forums we try and stress that if you are not absolutely sure of what you are about to type.....don't!
I looked up the Summit Racing unit. It is made by OTC. OTC supplied OMC technical schools with test equipment. Since I worked for OMC for a couple decades plus I was able to frequent the schools. I know for a fact their gauges have the valve located properly at the cylinder head end of the hoses and adapters. Just to make sure I am emailing them as I suspect the comment on the Summit site was by a consumer.
The Northern Tool one is a MityVac unit. In looking at the MityVac site I see pretty clear photos showing the location of the valves at the cylinder head side. Can email them as well but the drilled passage shown in the photos is the same size as the valve/threads.
Harbor Freight? Have one nearby and can check it in person.
All three examples clearly show the pressure relief valve located at the gauge. That is not the Schrader valve.
 
This is my tester from Napa. The Schrader valve is at the bottom of the hose. No valve in the adapters but I've rarely used them.
Back to the outboard motor. According to Leroy's ramblings, a compression of 65-70 is fine for this motor. Mine was in the mid 80's cold with a bit of oil in the cylinder. Have you actually water tested this motor on a boat?
9e04621f4be4839ffc6968acd71d1a33.jpg
a034a4d9c7c71cb888f8cb040d1337f3.jpg


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Pappy said:
That is not the point of this. The point is that when you make a blanket statement that for the most part so far is factually untrue it can be taken as fact by folks on here that are here to learn.... when in fact it is not.
In these forums we try and stress that if you are not absolutely sure of what you are about to type.....don't!
I looked up the Summit Racing unit. It is made by OTC. OTC supplied OMC technical schools with test equipment. Since I worked for OMC for a couple decades plus I was able to frequent the schools. I know for a fact their gauges have the valve located properly at the cylinder head end of the hoses and adapters. Just to make sure I am emailing them as I suspect the comment on the Summit site was by a consumer.
The Northern Tool one is a MityVac unit. In looking at the MityVac site I see pretty clear photos showing the location of the valves at the cylinder head side. Can email them as well but the drilled passage shown in the photos is the same size as the valve/threads.
Harbor Freight? Have one nearby and can check it in person.
All three examples clearly show the pressure relief valve located at the gauge. That is not the Schrader valve.
Click to expand...

I know exactly what you mean by the pressure relief. That is not what I am talking about.

Well, I can admit that I may be wrong. The sample size that I have experience with is clearly not the majority.

I used the OTC gauge in college. The ones that we had at the time had the Schrader in the middle, but we often used the longer adapters since many engines have deep spark plug wells. You wouldn't use them on small engines.

That still does not change the fact that you need the right gauge with the right adapter to get accurate results. While that style of gauge may not be common like I thought, they are out there.
 
Arborist site?

I quit that place when I was still in short pants.

Comparing a 50cc saw reading to the readings of a 31.8ci boat motor is proving what?
 
RaisedByWolves said:
Arborist site?

I quit that place when I was still in short pants.

Comparing a 50cc saw reading to the readings of a 31.8ci boat motor is proving what?
Click to expand...

I agree about AS. At one time there was a few smart guys on there.

I thought those 9.5s were 15ci? That's only 125cc a hole, where automotive engines are closer to 500+. Even at 32ci, that's still half.
 
Sooo.....

Just to put an end to this saga I have an update.

I had been telling Capitan Bone head that sometimes a hull/motor combo just wont work together. He of course insisted that because he had used this motor on several different hulls that I was wrong and that "I" had done something wrong when I worked on it.

After spending way too much time working on this motor and going back over it 4-5 times it turns out I was right, it was not a good fit for his new hull.

He added a jack plate and not the boat performs as it was expected to with this motor. :roll:
 
That's interesting. If you get a chance, I'd be interested in seeing a picture of this setup. Might be worth trying on my boat to see if it helps it plane easier.

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