Re: 1979 Johnson 70 hp over revving
If you were getting 34 mph at 5000rpm would you let it go, or try and prop it? Running ethanol free premium so i wouldnt think the recommended higher rpm would be such an issue, any thoughts?
Before the 15 pitch I tried the same prop with 17 inch pitch. Ran a hair over 5000 RPM at nearly identical top speed. Acceleration was noticably slower and more labored, especially with a heavier load. Switching to the 15 pitch added "spunk" to the boat and overall it's more fun to operate and accelerate. Top speed is virtually identical, which surprised me a little but I attribute that to both props keeping the motor running in its powereband at full speed. If you are maxing out at 5000RPM I'd definitely recommend trying a prop with 2 inches less pitch. I doubt it will impact your top speed much, and you'll probably notice the same performance improvements I did.
As to gasoline, I'm no expert, but it's not just ethanol in fuel that makes a difference. All gasolines are significantly re-formulated for lower emissions compared to fuels from yesteryear. In fact, you may be compounding the problem with premium octane fuel as opposed to regular. Our outboards are relatively low compression engines that run cooler than automotive engines. In this scenario premium fuel can actually create more deposits in the combustion chamber, not less, and accelerate their associated problems. If your outboard does not have issues with pre-ignition or detonation on regular fuel using premium gains you nothing and over the long term may actually be more harmful.
