Home › Forum › Ask A Member › 1969 Johnson 25 hp 25R69D › Re: 1969 Johnson 25 hp 25R69D
The 18-20-25’s are not the same motors. If your block has an 18hp plug, unless someone was playing a joke, I’d assume it’s an 18hp motor. The other way to tell is to look for a P/N on the crank and cylinder head (which would be on the inside of the head, generally, but may be stamped on the outside on the port side near the plugs).
To my knowledge, there was a change in the pistons, crank, and cylinder head (and perhaps the port distances) to help get more HP. What may further complicate things is if someone decided to throw a 25hp carb onto an 18hp block. The carbs in the later 18 and 25’s use the same carb casting, but different orifices, so you’d have to pull the high-speed orifice and cross reference the orifice number with what the respective HP motors use.
I will say that if I see one of these showing 90-100PSI, 1st make sure the cylinders aren’t bone dry. That is usable, but in my opinion, getting on the low side for these size motors. I aim for 115-120+ on both cylinders as an OK number. Spray some fogging oil in there and see what happens. It may shoot up 30-40 points on your gauge and put you where you want to be (I’ve seen them go as high as, believe it or not, 150-155 on really nice motors).
The electric start bracket is the valuable part of that motor. Everything else is pretty common stuff.