Rd 15 Compression

[Donald Keller]

I have rebuilt the carburetor an put new impeller in this 1953 25 hp johnson.The compression is so strong it will just turn enough to give a good thump…The only way I can turn this motor over a turn with the crankrope is to take both plugs out…Does all of these 25s have this much compression.??? MY BROTHER weighs over 200 and can not pull a turn with plugs in it.i wich there was a compression release on it.

[olcah]

I would remove the gearbox as the new impeller is probably dragging.  When gearbox is off the motor, the driveshaft should protrude from the gearbox.   Try to turn the driveshaft.  If it is tight you will see if the impeller is dragging.   If the motor turns easier  I would put a compression gage on and determine each cylinder’s compression.  My 1954 Johnson 25 had compression in the 90’s to low 100s.

A normal person should be able to pull the motor over with the plugs in.  It will be easier when the motor is on the boat.

[Tubs]

If you have the wrong spark plugs they may be too long.
The piston may be touching the spark plug stopping the motor from turning.
Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[Donald Keller]

it had J4c plugs when I got it..I put J8c in it….This motor is hard to crank with the lower unit and dri e shaft off…I tried to see if it would be easier.I don’t know…I got 3 of these motors from this guy last week…one is a1952 evenrude …the other is a johnson just like this.one…I have not worked on the other 2 ..but they seem to be the same on turning….I know they been sitting for a while …gas line dried up..briittle.I will go buy a compression gage and check it.

[olcah]

I would put oil in the cylinders to see if it loosens up, even though that will mess up your compression readings, it may prevent damage when you pull it over.

[Donald Keller]

I bought a tester at o rileys…Top cylinder says 90….when I run the nipple all the way in on bottom cylinder it stops the piston from moving…I guess…cause it will not turn over…I backed it off one turn and got like 75….I put an extra washer on both plugs and now I can pull it to turn over…it is hard to pull ..but it will turn over…I don’t know if it would start with two washers on the plugs or not

 

[Donald Keller]

I am wondering if maybe there is some carbon build up on maybe the bottom cylinder….Or would a new head gasket help???

[Donald Keller]

I put some marvel mystery oil in the cylinder yesterday…so it may have effected the compression test today..

[stanley]

There’s a huge difference between trying to pull them over on a stand (d….d near impossible) and standing in a boat.I regularly run RDs and even at 75 have no problem pulling them over.Leverage is the key.

[Donald Keller]

Thankyou…thankyou all for the valuable info .I will put it on. a boat soon as I can …

[Author]

Posts