January 12, 2017 at 3:00 am #6087
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.
Canada Member - 1 Year
Topics: 74January 12, 2017 at 3:23 am #50864
It doesn’t sound like you are reaching full rpm. Your 18 hp should be comparable to the 15 hp.
Upper Canada Chapter
US Member - 2 Years
Topics: 33January 12, 2017 at 3:31 am #50865
Do you know what pitch prop you are using?
US Member - 1 Year
Topics: 21January 12, 2017 at 3:34 am #50866
You could have the wrong pitch prop or the pump impeller could be a napa one that is too thick causing loss of power.
RayJanuary 12, 2017 at 3:37 am #50870
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.
US Member - 2 Years
Topics: 35January 12, 2017 at 10:23 am #50874
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.
RICHARD A. WHITE
US Member - 2 Years
Topics: 129January 12, 2017 at 11:05 am #50876
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.comJanuary 12, 2017 at 11:46 am #50877quote BillW:
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.
US Member - 2 Years
Topics: 33January 12, 2017 at 12:56 pm #50879
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".January 12, 2017 at 1:12 pm #50882
I was trying to spare you all the saga. I’ll try to keep it as concise as possible. I bought the motor 1965 18hp evinrude not running. It had been sitting a long time and it had belonged to the sellers deceased father. The seller said it needed a new coil. The first thing I did was order a coil and rebuild the carb. The lead plugs were missing from the carb, and I replaced those as well. For some, still unknown reason, I could never get the float needle to seat properly, I tried several different needles, different seats, different floats etc, but the thing would squirt fuel when the bulb was pumped.
I finally gave up on that carb and bought one off ebay. The seller had listed it wrong and sent me a much larger carb than what an 18hp uses. I filed out the manifold holes and made the carb work. With this carb installed I got the engine running for the first time but it had a severe missfire.
I bought yet another 18hp carb, and this time I received the correct part.
Back to the missifre, I replaced both coils, installed new points, condensers etc. I set the points via feeler gauge, then fine tuned them using an ohm meter. The missifre was still there. One constant problem I had was the flywheel rubbing the coils. I had the coils as far inboard as they would go and they still rubbed the flywheel. I started to be convinced the flywheel was bent, or the crank was bent, or the crankshaft bearings were bad. The flywheel did have a little bit of run out.
Since I was desperate to make progress I ended up buying a whole new powerhead. I put that powerhead on, and still had the missfire. At this point the only part of the ignition system I hadn’t replaced was the plug wires. The wires ohm’d out fine, and I had freshly terminated them. In another act of desperation I ordered a new flywheel, and the mag plate, coils and the whole shebang from the same seller who sold me the powerhead. I was also tired of fighting problems with my rope recoil (even after replacing the spring) so I ordered another one of those. Finally with all these new parts the motor seemed to run great. Nothing was left of the old setup from the midsection down. I think the missfire was probably from bad plug wires all along.January 12, 2017 at 1:16 pm #50883
This is a shot of one of the pistons from the powerhead I’m usingJanuary 12, 2017 at 1:20 pm #50884
Here are some misc project photos. I took the "old" powerhead completely apart.
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