Top speed with 18hp Evinrude on 14′ aluminum

[stephenspann27]

I pieced together an 18hp evinrude, well it’s a ’65 evinrude from the powerhead down, the powerhead is from a ’61 johnson. I have a ’64 14′ lone star aluminum boat that it pushes about 16.5mph with me in it, and a trolling motor/battery. I put my motor on my father in law’s 14′ flat bottom jon boat and it ran the same speed. I’m wondering if i’m reaching max RPM or not. I’ve purchased a tiny tach but I haven’t had time to install and test it yet. I was expecting 18-20mph with this setup. My father in law has a ’92 15hp evinrude and his motor runs like a scalded dog compared to mine. I realize his motor is prop rated, with through prop exhaust etc, but it’s got my wondering if my motor has a problem, or if I need to change the prop. My motor starts easily and idles well. Compression is good on my powerhead.

[wbeaton]

It doesn’t sound like you are reaching full rpm. Your 18 hp should be comparable to the 15 hp.

Wayne
Upper Canada Chapter

uccaomci.com

[fleetwin]

Do you know what pitch prop you are using?

[ray-schaber]

You could have the wrong pitch prop or the pump impeller could be a napa one that is too thick causing loss of power.
Ray

[stephenspann27]

The prop doesn’t have any markings on it. I’m curious too if I’m reaching max RPM, by ear it i’d guess about 4k rpm instead of north of 5k. I’ll try to get the tach on this weekend and do some testing.

[billw]

There are a lot of possibilities but I would start with the compression. I have found that 18s can be all worn out and still idle like a watch. They will start instantly and yet have no top end. A good 18 should have 120 psi compression or better.

Long live American manufacturing!

[RICHARD A. WHITE]

My 1956 Johnson 15hp electric start, with me, (260 lbs), will do nearly 22 mph and I am sure probably faster…I run a 14 foot Starcraft Seafearrer..
I have yet to try any different props…yet 🙂

http://www.richardsoutboardtools.com
classicomctools@gmail.com

[stephenspann27]

quote BillW:

There are a lot of possibilities but I would start with the compression. I have found that 18s can be all worn out and still idle like a watch. They will start instantly and yet have no top end. A good 18 should have 120 psi compression or better.

The seller listed the powerhead as having 118psi per cylinder I believe. My gauge showed 100 I think. My gauge might be reading low. I need to try a few more gauges. Both my old powerhead (which I never had running correctly) and this powerhead have score marks on the sides of the pistons that you can see, if you remove the plate by the fuel pump.

[stephenspann27]

[fleetwin]

Well, you certainly did a nice paint job.
Like others have said, compression reports seem to vary, but I would think you should see 100PSI minimum. Why is it that you replaced the original powerhead? You also seem to say that the engine ran the same way with its original powerhead, do I have it right? You mention seeing score marks on pistons of both powerheads, needless to say these could be an issue, or not. Most two stroke OMCs will show some minor scratching/scoring on the pistions, usually not an issue. But, if there has been aluminum transfer, and the rings are stuck in place that is another story. You mention the engine starts easily and idles nicely, which suggests the powerhead is "OK".
You went through a lot of work cosmetically, would be a shame to mess that up over something simple. I guess I would start with something simple like having a look at the mag plate and carb linkage. Is the mag plate advancing fully, perhaps the interlock is interfering. Is the carb butterfly opening fully? Is there a chance that someone swapped out the carb from a 25hp, thinking this would make the engine run faster. Perhaps they just changed the high speed jet, thinking this would be the answer to their performance dreams. I would mention a blocked exhaust, but I’m thinking you would have seen a problem like that when you had the midsection apart.
Don’t forget to check the prop pitch, the engine will just bog down if it is over propped.
Tell us more about the history of this engine, why you changed the powerhead, and how it ran with the old powerhead. I don’t want to mislead you based on my "assumptions".

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