Tn-27 compression

[clemg]

What’s a good compression for a tn-27 johnson

[wedgie]

I think 80lbs or so is pretty good for that model. It will run on less. It is important that the cylinders are within 10% of each other .

[clemg]

Both cylinders are close to 68psi

[frankr]

last one I had was about 65psi. It ran, but not like I know it should. Wouldn’t get down and tick them off at a real slow idle.

[wedgie]

That’s good enough ! It may come up after it’s been run a while.

Here is a link with some good info from David Poche: http://www.pochefamily.org/outboard/

[wedgie]

I have one that runs very well. I should pull it down and check the numbers when I get the chance.

[clemg]

I have know clue the last time it ran. I pulled the head off and the top of the Pistons had som carbon on them. I cleaned them off and installed new head gasket. I check psi before and after and may have gained a few psi.

[clemg]

Thanks for all the responses!!!

[reivertom]

These motors will run if the spark and gas are there when they shouldn’t. I have one that should have been toast in the 1970s, but it just kept running. One good thing is that nobody will ever steal it because it is the ugliest motor, possibly, in the country. After yours runs a few times (add some Sea Foam to the gas mix) and things loosen up, it might fool you.

[garry-in-michigan]

Small third port motors are very sensitive to piston to cylinder clearance. fifty to one oil mixes are a real killer on those low horsepower outboards. A few thousandths ware on a PO or Speeditwin is inconsequential as a percentage of the large bore. But on the smaller motors it is a much larger percentage. Some are improved by using a low ash SAE 40 weight oil. This usually requires a hotter spark plug. Tinning the bronze bushings with lead free solder (Contains silver) can eliminate excessive crankcase pressure loss through the bearings. In some cases judicious use of a tail pipe expander can add a thousandth or two to the base piston diameter to decrease crankcase pressure loses. Some early outboards added a piston ring to the bottom of the piston. Modern practice uses closer tollerances for this seal.

[Author]

Posts