1990 90 HP Force Warm up & Prop ?'s

[irnman1827]

Hello,<br /><br />I just put the above motor on my boat and have some questions. My first question about my 90 HP Force is about prop selection. My ouboard is not ideal for my boat. Boat is an 18' Astro Fish & Ski, so I should have a larger outboard. However, my concern with the 90 HP is I am getting too many WOT RPM's. I can hit 8000 RPM. The prop on the boat is what I believe to be a 13" diameter (I did measure diameter) and 15" pitch. The stamping on the prop has in one spot "4214" and in another "15". Will a 13 X 19 3 blade cupped prop get my RPM's back in check? I was afraid if I went to a 13 X 21, I would not have enough RPM's? <br />Also, this motor seems to be cold blooded after every start. I have to let it run for a couple of minutes btw 2000 to 3000 RPM's before shifting into gear or it will die. This is true even if I have been running the motor for 1/2 hour and stop to fish for 1/2 hour. Is this normal? <br /><br />Thanks for your help,<br /><br />Jeff

 

[irnman1827]

Re: 1990 90 HP Force Warm up & Prop ?'s

Hi,<br /><br />One thing I need to add: to keep the boat on a decent plane with current prop, I need to be running about 6000 - 6500 RPM's. I appreciate any advice.<br /><br />Thanks,<br /><br />Jeff

 

[roscoe]

Re: 1990 90 HP Force Warm up & Prop ?'s

Welcome to iboats.<br /><br />Something just doesn't seem right.<br />I don't think a Force motor will stay in one piece run at 8000rpm.<br /><br />I bet the tach is not operating properly. Or...you have the wrong tach.<br /><br />Some Force motors require a specific tach to read the pulses from its 20 pole charging system. I'm not sure if the "90 90hp requires one or not.<br /><br />Does the tach have a selector switch on the back, that allows you to select the number of "poles" in the motor's charging system?<br /><br />If the tach is set to accept signal from a 12 pole system (6 magnets under the flywheel) and your motor has 20 poles (10 magnets under the flywheel, the tach will read off by 60%.<br /><br />This would mean you are actually reaching only 4800 rpm at WOT.<br /><br />And that your planing rpm would be approximately 3800 rpm, which would seem more reasonable for that boat/motor combination.<br /><br />What motor was on the boat previously?<br />And how did it perform, and at what rpm?<br /><br />Theoretically, to get your 8000 rpm number down to where it should be, you would need a 28p prop, and thats just plane rediculous.<br /><br />I would verify number of poles in system and correct tach for the motor before doing anything with the prop.<br /><br /><br />As far as being cold blooded, may need to have the carbs cleaned/adjusted, also do a compression check.

 

[irnman1827]

Re: 1990 90 HP Force Warm up & Prop ?'s

Hi,<br /><br />Thanks alot for the reply. I agree, something doesn't add up. I didn't see a switch on the back of the tach. I bought the boat without a motor a couple of years ago and just recently added the Force motor. The previous owner had a 200 HP Mercury on the boat. I do not know what kind of boat the Force motor was on. The tach on the boat appears to be factory issued. This boat originally came with a 150 HP motor, so I wouldn't be surprised if the tach isn't a correct fit. I did have a marina install the Force motor on the boat, but they probably didn't pay much attention to the tach - other than it was functioning. Could a marina do some kind of "plug in" test to check RPM's?<br /><br />Thanks again for your help,<br /><br />Jeff

 

[roscoe]

Re: 1990 90 HP Force Warm up & Prop ?'s

A decent shop should be able to verify your tach, but they are gonna charge you, and you will be waiting for a service appointment. <br /><br />Cheapest and easiest way to check/verfiy your tach and rpm is to buy a tinytach.com Give them a call and order up one of their little digital tachs. Simply need to wrap the signal pickup wire around a sparkplug wire.

 

[irnman1827]

Re: 1990 90 HP Force Warm up & Prop ?'s

Hello,<br /><br />I have a tach on the way that will work with my motor. I will post the results once I get it hooked up.<br /><br />Thanks for your help.<br /><br />Jeff

 

[irnman1827]

Re: 1990 90 HP Force Warm up & Prop ?'s

Hi,<br /><br />I finally received my tach yesterday and installed last night. I got the boat out on the water tonight - how's 4900 RPM @ WOT sound? Makes much more sense. Thanks alot for your advice, it saved me $100 or so, as well as alot of performance headaches.<br /><br />Thanks,<br /><br />Jeff

 

[roscoe]

Re: 1990 90 HP Force Warm up & Prop ?'s

Sounds good.<br />Your motor is rated for 5000-5500 rpm.<br />So if you were to buy a prop, I would drop down to a 14p or maybe even 13p.<br />Your rpm will be good, and your holeshot would improve.<br /><br />happy boating.