Compression question

[genetoo]

It took a long time to get it through my head that the very 1st thing one should do when getting a old outboard that you know nothing about ; before working on it was to check the compression.No matter what one does to it if the compression is low it will be hard to start ,won,t idle down or won,t start at all. This is typical with 2 cycle engines.Each model & brand outboard has compression stats from the factory where it will run as it,s suppose to.

My question ,a TN -28 What is a good compression reading with this particular outboard, will start ,idle & run well.? I know a lot of these particular outboard were damaged by mixing the wrong oil or wrong ratios back in there day making it hard now to find a good one these days.

Gene in Mn

[Buccaneer]

The HD and TD Johnsons will idle so-so with 65 psi, good with 70 psi, and great at 75 psi.
I presume the TN’s are the same.

Prepare to be boarded!

[lloyd]

70-75 PSI are the magic numbers for these motors. 77 PSI is the highest I have ever measured on one. Be sure to check compression when the block is cold and dry and crank the motor over until maximum reading is reached. Usually 3-4 pulls on the recoil. Checking compression when the block is still warm from running may give a false low reading and adding oil or fuel to the cylinders will give a false higher compression reading. Good news is that they ran on a 16:1 gas to oil mixture so finding one with acceptable compression is not hard these days. Most of them were put out of service by worn out primer pump seals and or rotten cork floats long before the cylinders wore out and lost compression.

[Author]

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