prop or jets

[sanjuan]

I have an older 70 horse johnson that runs well at Denver altitiude 5,000 ft, but wont develop the RPMS at higher altitute,, above 7,000,, one thought is I need a different prop at the higher altitude, This is a 60 something engine with the three seperate carbs. I haven' been able to make any impact by changing fuel mix... Any ideas

 

[fireman 757]

Re: prop or jets

Shouldn't be the prop. I think it is like us flatlanders and just is starved for oxygen at that altitude. Don't know what to tell you t do. Adding more air from your carb adjustment (if you have an air adjustment on that model) might work.

 

[sfels]

Re: prop or jets

That's the biggest drawback to carbs vs injected. Altitude changes like that usually require rejetting the carbs. Adjusting the mixture may give you some relief, but probably not enough.