RPM range and Idle speed for 91 force 70

[Chris73Cobia]

Okay got deeper into playing with the motor, got the tach installed and working. The motor seems to idle at 900 with no intervention. I think that sounds high, so if someone can tell me where the idle screw is that'd be great, I have a 91 70hp and its the "B" Model, so I have the single large carb. I think it might be the screw near where the throttle hooks up, not sure though.

Also, if someone could tell me the proper max/min operating RPM for on the water that'd be great too, as the prop on there isn't going to be there long, and it will help to know if i need to repitch at all (Probably going to have to go up, because this motor was on a pontoon before).

Oh and does anyone here notice a difference between regular and plus (89 octane) fuel? I was told that this motor prefers the higher octane, but not premium fuel.

Thanks again for all the assistance.

 

[steelespike]

Re: RPM range and Idle speed for 91 force 70

i believe the motor developes its hp at 5,000.I wouldn't hesitate to run it up to 5,500.The Force experts can probably be more specific. 87 octane is plenty on a properly tuned motor.Don't assume on the prop.Check the size,it usually appears on the barrel.Don't make a prop change until you see what it runs at with present prop.Its safe to run beyond 5,500 momentarily to get a reading.

 

[Chris73Cobia]

Re: RPM range and Idle speed for 91 force 70

Yes going to the lake on Thursday to get RPM readings and maybe catch a few cats. The prop I've got is an aluminum, but I cant find the numbers on it, I've measured the radius so i know its a 10-1/2 wide prop, which by the manufacturers I've found should be 10 or 12 pitch, I'm probably going to have to take it to the local marine place and have them figure it out for me.

Hopefully someome will reply and let me know the idle info I'm getting a manual soon, but it's not here yet.

 

[steelespike]

Re: RPM range and Idle speed for 91 force 70

A prop shop could measure it.
If they had the right prop on it a 10 or 12" would be about right.