RPM's

[GlasstronJim]

Hello everyone I just found a plate on my 1975 70hp motor that says my operating range is 4500 to 5500rpm's. (it says it makes 70hp at 5000) My question is you guys told me 5000 was dogging the motor?

 

[Dhadley]

Re: RPM's

With todays fuel and a top rpm of 5000 rpm with an average load is lugging the motor. You can leave it alone but be sure to decarbon every 50 hours as recommended. Perhaps more than every 50, maybe 35.

The closer the set up is to 5800, the cooler the combustion temps will be. The lower the conbustion temp is the less coking you'll have. Coking (sticky carbon-like substance) causes the rings to stick and eventually they could break.

Yes, the motor makes 70 hp at 5000, that's where the motor is rated at on a dyno by the NMMA. With today's fuel it'll live much longer set up closer to 5800.

 

[GlasstronJim]

Re: RPM's

So is there anyway to turn it up other than a different prop? How do I decarb it?

 

[Silvertip]

Re: RPM's

There is a number of decarb articles on the forum - just search for "decarb". You might be able to pick up some rpm by checking the engine mounting height. If the AV plate is flush with or below the bottom of the boat, try raising the engine one hole. You can raise it higher but make sure you don't go so high that you lose water pressure or begin experiencing ventilation. When you have the optimum setup, then think about a prop change if needed.