TN28 compression testing

[bonzoscott]

I am looking for opinions on required compression for an old motor. The figures I got initially were 70-72 in both cylinders - decent, right. I've been struggling to get this old motor running for a while and had the carb apart several times finding debris in it. I've been through cleaning tank, cleaning filter, and finally bits of the cork were coming through (after coating with dope) from poking the needle valve through it. Anyway, I've been able to run the motor at higher RPM and after today's run, it got hard to pull. I first checked the new plugs. There is some carbon on them but they were very dry, which I thought was odd. The dry plugs and hard pull is what lead me to check compression again. Now I had 50 and 51 psi. Any opinions? I then sprayed some sea foam in and got 75 and 80 psi. I've read from some you should check compression dry, but then some say to put oil in. Is the motor still poor if you add oil? I feel I have a lean condition. I made a new intake gasket, but it's been on and off three times. Thanks in advance.

 

[HighTrim]

Re: TN28 compression testing

70 psi and up the motor should run fine and idle down, any less and you will be struggling.

What is the needle settings on both high and low? Is there adequate spark that will jump a 1/4" gap on an inline tester? Did you go over the ignition syste,? Coils arent original are they?

 

[JDusza]

Re: TN28 compression testing

How's water flow through it? Before you give it too much higher speed, make sure it can stay cool. Can you get it in a barrel?
Now, following High Trim, you need compression, spark and fuel. Compression you understand, somewhat. Test for spark at each plug as advised. If good, you're on to the fuel system. If spark isn't good, fix that before moving on.


btw, a wet compression test with higher numbers than a dry compression test can indicate worn and leaking rings.

 

[bonzoscott]

Re: TN28 compression testing

The best I can figure, everything is good - new coils, points condenser, perfect gap, 5/16 or better spark. I'm not sure it's pumping water. I have a new impeller. It's ran for only 1-2 minutes. But that's what I was looking for - improved comp numbers with oil means wore out. I just don't know what caused it to drop in compression numbers other than Lean Condition. Thanks for the input and I will try to nail this one way or another!

 

[HighTrim]

Re: TN28 compression testing

You did not mention your needle settings?

 

[F_R]

Re: TN28 compression testing

With that thing peeing out the copper pipe elbow, it should be obvious if it is pumping water. Gotta ask this----you did put oil in the gas (didn't you)?

 

[cajuncook1]

Re: TN28 compression testing

bonzoscout,

This link has a lot good information regarding your motor. It tells you how test your coils, compression, clean the carburetor and make a new float and etc..

http://pochefamily.org/outboard/

 

[bonzoscott]

Re: TN28 compression testing

Needle settings as read on tank: 3/4 turn out from seat. Yes, I put oil in the gas. 24/1. I thought that little hole in the elbow is where I should see water pee and it is not, but like I said, new impeller. I guess I'll be tearing her down again
Correction - 16-1 oil - 8oz to one gallon