Johnson 88 SPL 1993 Motor Height RPM Issue

[SteuryLover]

Hey everyone, thanks for the help so far. I finally declared defeat with my low RPM issue and dropped the boat off at a local shop. I have not taken the boat out yet, I will do in the morning. Here is a quick recap.

Boat will not go above 3000 RPM while under load in the water or on the dyno. I attempted to rebuild the carbs, changed old cracking coils as well as a # of other parts that were replaced as the problem. Having faith in everything but my carb rebuild I could not solve this issue on my own. So i dropped if off. This is what they said.

Rebuilt Carbs - new gaskets and so on...
Checked timing
Check linkage
Put in in the water while still on the trailer they were able to get it to around 3000-3400rpm
They came back, put it on the dyno and they said they got 5000+ but did not have an specific # for me.

They claim motor should be good to go, needles to say i am a little nervous about the test run. If it runs correctly (5000+ RPM) I will feel as though I am dreaming! So here is my question

The boat would only go 22MPH at 3k RPM before I brought it to the shop. The boat would plane (18' center console deep V 1200lbs dry) no problem but the cavitation plate was under water. First off, will the motor raise with speed if the motor actually gets up to correct RPM and speed, hopefully 35+ MPH. Second if it is still a couple inches under water will it decrease the RPM dramatically say 1000RPM, or just slightly say 2-300 RPM? Thanks for everyones time on this.

 

[emdsapmgr]

Re: Johnson 88 SPL 1993 Motor Height RPM Issue

This is considered a small engine for your boat. Despite that, you should be able to run the engine at normal operating rpm's and it should run fine on the boat. You did not mention what prop pitch you were running. If this is significantly overpropped, it will lug the engine-similar to what is happening. I would think a 17 might work, probably a 15 pitch would work better. I'd target 5500-5700 rpm as optimal. Even 4500-4700 is considered "lugging" the engine. If the engine is mounted too low on the transom, it will have a great effect on its performance. When on plane, at top speed, check the water coming out from under the transom. It should flow just under the large flat plate just above the prop. If that antiventilation plate is submerged, it increases drag, even causes excess spray. If that plate is underwater at planing speeds, you need to raise the engine on the transom till it is right at or slightly out of the water. If you do have a height issue, fix that problem before you get into prop changes.

 

[SteuryLover]

Re: Johnson 88 SPL 1993 Motor Height RPM Issue

It is a 17" prop currently. The boat is rated for 150hp, but it is strictly a fishing boat and I was told 90hp is a good outboard. I was looking at 115hp outboards, but after some research and conversations the main difference I was told is top end above 5k, below 5k I will not see a positive change. I was also told in the case of the V4 Johnson, the 90 has a slightly better hole shot then the 115. I will see about raising my motor, it is on the lowest hole currently. My motor also does not have power T&T so I am sure that is making things a little harder on me. Thanks for the reply.