18HP saga continues. Can’t get past 14mph and 3,900 RPM.

[stephenspann27]

Chris I tried that. I can only get to about 7mph on one cylinder.

[stephenspann27]

quote MikeSea7:

Miss understood on the power heads sorry.
These things make us lose hair. How about this it just goes FAT? Cross flow pretty simple on Model 2 strokes I have seen the sleeve be installed backwards and gives you just what you have. But your sleeves are made into the block correct? What was the oil % the engine was built to run on? I think I would try it as nothing could get hurt. One other thing FREE revving will not show up an ignition problem under load. I have been there too.
I am glad these engine are fun and not brain surgeries.
All we need is 1. compression, 2. air mixed correctly with a fuel and 3. good spark

It’s an iron sleeve in an aluminum block but the last two powerheads ran the same top speed.

[stephenspann27]

I found this post. The guy is running 4200 RPM on a 16′ boat. My boat is 14′

http://forums.iboats.com/forum/engine-r … hp-max-rpm

[stephenspann27]

another guy with the same problem as me.

http://forums.iboats.com/forum/engine-r … g-full-rpm

[stephenspann27]

http://forums.iboats.com/forum/engine-r … at-top-end

The above link, a guy with the older style carb was getting a max of 13mph until he leaned the high speed mixture.

[stephenspann27]

Something I was just thinking about. If 18hp is at 4,500 RPM, then 4,500 RPM should be my goal. I hit 4,000 RPM a few times today for a short while when messing with things. At 4,000 RPM I was only going 14mph. How much faster would I be going at 4,500 RPM?

[The Boat House]

Have you checked that the whole exhaust is clear of carbon?
Partial blockage somewhere could cause this issue.

[stephenspann27]

quote Tubs:

Have you checked that the whole exhaust is clear of carbon?
Partial blockage somewhere could cause this issue.

yes I tore the midsection and everything down, sandblasted it and painted it.

[stephenspann27]

I checked my main jet and it’s a #57. The carb the motor came with has a #64 main jet. Not sure why they changed the main jets that drastically for different year models.

http://forums.iboats.com/forum/engine-r … 7351789626

[Mumbles]

If the same problem is on all three powerheads, are you using the same mag on all three? Is the mag plate worn and wobbly causing erratic ignition timing? New coils, points condensers. What about new plugs and plug wires? A new set of NGK B8S might help along with new copper core wires. If the wires seem good, corrosion can build up under the spark plug boot causing all kinds of electrical problems. Maybe pull the wire and terminal out of the boot and then trim a half inch or so off the end of the wire before reinstalling the terminal. It seems strange the issue follows all three motors.

The crank in your ’61 motor has needle and roller bearings throughout and originally used 24:1 mix. ’64 was the first year OMC called for 50:1 in their motors, using proper outboard oil.

How heavy is the boat and would the prop have too much pitch for the load causing the motor to lug? An eighteen should push a light boat around 25 MPH and a 15 about the same.

https://www.youtube.com/watch?v=pxLdVbqhC_g

https://www.youtube.com/watch?v=p96DIKwaR9E

https://www.youtube.com/watch?v=p16PzP5bvRc

https://www.youtube.com/watch?v=wTK67n5P9DE

[Author]

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