18hp Performance problems, still.

[Hanr3]

History, brief.
'57 Johnson 18hp, top speed on a 14' v bottom is 7.3-7.5mpg via gps.

Compression is 95 top piston, 110 bottom piston.
Sprak is great, I syncronized the timing to the carb per the repair manual.

I rebuilt the carb, no difference.
Bought a used carb, rebuilt it, now she starts and idles great. I can get it started now on the first couple of pulls. However top speed is still 7.3-7.5mph.

I beleive the prop to be the original. From what I understand it has a rubber bushing. I have broken a couple shear pins, could I have broken that rubber bushing? Could my problem be the prop? How do I tell. Woudl it still work even if broken?

What else could it be?

 

[Daviet]

Re: 18hp Performance problems, still.

All it takes is compression, spark and fuel. It sounds like you have all three. A two cycle engine devlopes horse power by rpm, do you know what rpm your engine is turning?

 

[Hanr3]

Re: 18hp Performance problems, still.

No, how do I test for rpm's on these engines?

 

[Daviet]

Re: 18hp Performance problems, still.

I use a hand held tach that attaches to a spark plug wire. Check with a local shop and see if they can help you out.

 

[Glenn Romain]

Re: 18hp Performance problems, still.

Are you sure your running on both cylinders ? and what are your carb settings and did you make sure to adjust the carb float ? I have a 1963 Johnson and im getting 26 to 27 MPH out of it on a 12foot Mirocraft thats wide and deep.

 

[jbjennings]

Re: 18hp Performance problems, still.

THe spark testers with the lights are useless. You're only getting spark on one cylinder. If you pull one wire from the plug, the engine will run like it always does. If you pull the other wire, it won't run at all. Correct?
FInd what's causing your spark problem and set your carburetor correctly and you'll be in good shape. My high speed adjustment usually winds up somewhere close to 3/4 from lightly seated.
JBJ