help with trouble shooting motor

[BBD]

Hey guys i have a mid 60's johnson 18hp tiller motor and it runs great at low seeds but when you give it full power it dosen't want to go the rpms are not going up any body have anything i can try?

[HD]

Sounds like you need to clean the carb.

[deadeye]

I would go with a possible bad coil.  I have a 1955 Johnson 25 hp and had the same issue a while back.  Ok, it was a long while back but still had the same symptom.

[BBD]

k i cleand out the carb and that was a no go as for the coil would that make it run fine at low speeds and not get full power at full throttle?

[deadeye]

Probable the frequency it need to charge and fire.  At low rpm it has time to generate enough spark but at high rpm it just needs more spark. 

[BBD]

And that would be caused by a bad coil? any clue what a rough cost for them are? and also are they hard to replace i could figure it out but jw how hard or how long it will take.

[BBD]

And that would be caused by a bad coil? any clue what a rough cost for them are? and also are they hard to replace i could figure it out but jw how hard or how long it will take.

[deadeye]

The guy that fixed mine was an old timer.  (He died a couple years ago at 100 years old).  If I remember correctly he tested it using a process simular to this one.

For a quick check on the points-

Use a 12v test light, hook the ground side to the points lead (disconnect it first) and touch the other end to a power source. Remove the plugs and slowly rotate the flywheel (clockwise). The test light should go on and off as the points open and close. This is not a perfect test, but it will quickly tell you if they are working at all.

Quick coil test-

Check resistance with a meter. Primary (small wire to ground) should be very low... just a few tenths of an ohm. Secondary (coil wire to ground) should be approx 1500 ohms. You can also BRIEFLY power up the primary wire and brush the ground lead against a good ground. while doing this, hold the coil wire close to a good ground, you should have a bright blue spark capable of jumping abt a 1/4" gap. This imitates the points opening and closing, and the coil will throw a healthy spark if it's good. Once again, not a perfect test, but it can quickly eliminate some things if you have a bad component

[deadeye]

One more thing to check would be gas flow from the tank.  Maybe enough for idle but not enough flow for full throttle.