WOT RPM for 74 115 Johnson

[Bartman]

It's time for a new prop. Can someone please give me the WOT RPM range for a 1974 Johnson 115? I know everyone suggests propping (is that really a word) to run in the upper RPM range but I'm CONCERNED being it is a 33 year old motor. Those poor rods and pistons have to be tired. I really don't know the history of this motor. It has 110-105 PSI on all cylinders cold and runs good. She is mounted on a 1985 Stratos 375V 18.5' and runs 45 MPH @ 4800 R's with the current "dinged up" prop of 13" x 19".

Another question is the current prop nut is a nylock and I was wondering if I should use a castle nut since there is a hole thru the shaft for a cotter pin? Any idea what the treads are? I would guess to be a 3/4-10?

Thanks in advance,
Bart

 

[Ranger 330V]

Re: WOT RPM for 74 115 Johnson

batman...I'm not by any means one of these pros on here and hopefully they will chime in. you will do more damage lugging that engine than you will running it in the upper rpm range. its not the fact that you are in the upper range at wot as the fact that if your not you are lugging the engine through the entire range. if you are below 5000 rpm at wot you are lugging the engine pretty bad. what will happen is you'll get coking on this engine which is carbon depiosits. these 4 cylinder engines are notorious for doing that when lugged. your wot needs to be between 5300 and 5800...not that you'll spend much time in that range, but if your wot is in that range the rest of your rpm range will be much easier on the engine. thats why you prop for that rpm range. and yes that was correct terminology. how long have you been running in that range?? you probably need to do a decarb...search the faq for the correct way to do this, use seafoam...thats what everone on here swears by. 18ft is a pretty big boat on that 115, I would say a 17 pitch is in order...you need to be turning more rpms than you are at wot. the carbon buildup will eventually make a ring stick and break and ruin the engine.