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Nominal cylinder compression in 9.9hp

Since a lot of factors effect the ultimate compression number, for example how hard you pull the starter cord, etc., most people feel that the ultimate goal of compression testing for an outboard is to end with fairly similar numbers between the two cylinders. A less then 10% difference between the two cylinders is a pass. If you have a compression problem you will rarely get numbers within that range when you test each cylinder.

That said, I think the above answer of anything above 90psi is also a fairly good number to see, and yes, that compression tester should work for what you are trying to do.
 
Hi Scott. Good to see your still helping. Everytime I hook my pressurized fuel tank, I think of you.... Wondering if it could blow up someday on me? Just bought two more 1950s motors with tanks. Remember the 1951 super fast twin? Bought it from original owner down at Lake Nebagamon. It's perhaps the lightest and most compact 14 horsepower ever made. I think they skipped one year and then in 53 they bumped it to 15 horse possibly just a different rating same motor.
To put my opinion in on compression on the 99 and 15s I like to see 110 or so. But yes I agree anything over 80 to 90 will get you running but it is not a perfect motor. I have a very mint 1975 that came in at 125. It was a trade-in and a guy threw it in my yard on his way out the driveway. And it's probably time for me to get a new compression gauge like the one depicted here because mine is starting to read low and unpredictable at times but always low.
I just checked it alongside another guy's compression gauge when I tested a 1976 25 horsepower Johnson at the auction and I was showing 90 lb and his brand new gauge was 120???? My gauge is probably 40 years old, maybe 50, but it is a top-notch quality gauge.
 
Anything above 100/100 is great! Actually over 90 on each cylinder will do. Bring close is more important than the actual number. That gauge will work but I’d get one of the ones from auto parts store. I’ve had better luck.
 
Photo of a 51 superfast Twin next to a 5 and a half horse
 

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Yes, the post war Fastwins were really compact and lightweight.
Droid, I'm originally from Hibbing. Duluth since early 66. Dad built WDIO channel 10 in '65. We moved the 70 miles. Started it from scratch and on the air Jan 23, 1966. He was board of directors DLH/Airport Authority, think his name is up on the wall there. Frank Befera. You still at the airport?



.
 
I worked for NWA in MSP (1983 - 2005) until the bankruptcy (2005) then did powersports 12 years.
After that (2017) went to DLH & worked at AAR in the old NWA hangar until Covid shut that down.
Now working line maint for Allegiant Air in Appleton, WI. I've boated this area since a kid so I went full circle a few times.
 
My neighbor Greg B. did kind of the same route but is now retired. He also did some work in the Middle East on helicopters. Pretty good mechanic.
Do you have any experience with the little Yamaha four strokes? I have one a guy is about to drop off with water in the oil. It's an early 2000s four horsepower.
The local dealer RJ sports wants to tear it apart for $100 and give it back to him in a box....ha! But that's not all bad I guess I started out that way when I was eight or nine years old but I didn't get paid for it.... I just got scolded! 🥺
 
Not much on Yamaha, pretty reliable.

Typically work on V4 & smaller OMC primarily staying out of the powerhead. 3cyl & smaller preferred.

Some Mercury, no 6 cyl. But with their switchbox/stator & disintegrated wiring harness issues cost easily goes too high fast.

People think they can do it themselves but frequently that's not the case. And undoing their wrongs takes more time. So I am selective. It's just a hobby at this point. I'm 65, still working so I don't get bored.
 
Yeah me just the same complaining about some idiot working on this Mercury little twin. Spend more time undoing what somebody else thinks is an accomplishment. My mother-in-law helps me sometime and she said it is lack of tools in many cases. And I think it is lack of intelligence! 😂
Guess I'll have to post the question on a Yamaha forum here. It's probably just an o-ring allowing water pressure into the lubrication oil. This is all freshwater stuff up here so I don't have to deal with oxidation.... Unless there has been a mouse nest.... Or somebody threw it out next to the wood pile.
 
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