1973 135 hp OMC Head swap to '74 115 questions

[DangerBoy]

Hi,

Just joined this group and this is my first post.

I have a 1974 Johnson 115 outboard. My family has owned it and I have pretty much done all the maintenance on it since new. It has been bored out and had oversized pistons installed. Not sure of the overbore size. I also have a seized 1973 Evinrude parts motor in the boathouse which I picked up years ago. I took the bubble back and tuner off of it and installed it on the '74 which gave me a mild performance gain (I think).

In doing some reading of some older threads on this site I discovered that the '73 135 heads were the highest compression available on those motors. I am now wondering if it would be a good move to swap the '73 heads into the '74 as well. What kind of performance gain would I be likely to get from doing that? Would the performance gain be noticeable more at the low end or top end or throughout the whole range?

I use the boat mainly for water skiing so low end performance is my main priority. Also, I've got the '74 idling and running as good or better than it ever has before (really smooth) and I don't really want to sacrifice that. Will switching to the '73 heads affect the idle very much? We are running premium fuel so I'm assuming that pinging won't be much of an issue with the '73 heads.

Thanks very much in advance for your input.

[martinc]

The 73 heads were made for 1973 gasoline. The 74 heads are very simular however they have less compression due to the scallop in the top where the spark plug is located. If you like what you have, stay with it because you proberly couldn't tell the difference by the seat of your pants. If you decide to go with them, watch you jetting. Ole man Detonation is never too far behind. Thanks Martin

[EMDSAPMGR]

Agree with Martin. The heads will fit. The thing to be concerned about is to keep the combustion temps cool with the high comression heads. Possible that the 115 has 1 3/16" throated carbs. If you swap the heads and do no more modifications, the smaller carbs may not flow enough fuel to keep the cyl combustion temps down. The 135 likely has larger carbs-perhaps 1 5/16". If you switch the heads, I'd also switch the carbs. You need to keep the combustion cool, and that's by flowing more fuel into the engine. The 135 carbs may overfuel somewhat, but you can rejet it once you get it running. You'll gain hp too.