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Author Topic:   Not enough RPMS
mikee332 posted 08-10-2002 10:45 AM ET (US)   Profile for mikee332   Send Email to mikee332  
I have an 87' 120 Johnson on my 18' OR, it runs fine with the exception being it not turning out nearly the rpms stated in the manual(5-6K) vs my max of 3k. I have a 13 x 19 prop on it. Any idea's???
jimh posted 08-10-2002 11:48 AM ET (US)     Profile for jimh  Send Email to jimh     
Check your TACHOMETER accuracy.
simonmeridew posted 08-10-2002 11:54 AM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
mikee332
Here are a couple of places to start looking:

It sounds like the motor/boat combo new to you or newly installed? How about the tach?

Reason being the number of poles on the alternator is critical to the wiring choice on the back of the tach. Suggest going back through the posting looking for tach setup.

OMC motors have feature which limits RPM if it detects a fault in temp, oil delivery etc. This stuff could be OK yet because of sensor fault it could mistakenly revert to slow RPM mode.

Check compression.

Prop sounds OK, at least for experimental purposes. You might fine tune for specialty water skiing or fuel mileage, but you're in the ballpark.
simonmeridew

Bigshot posted 08-12-2002 01:18 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Well how fast are you going. 3000 rpm's would be just about planing. 5000+ rpm's means 40+mph and that thing will be screaming. You would know the difference in your car.....does it seem like 3k?
JoeH posted 08-13-2002 01:58 AM ET (US)     Profile for JoeH  Send Email to JoeH     
You could also fool with your trim. I couldn't get into my wot range until I took a day and just experimented without having to tow the family/friends etc. Joe
mikee332 posted 08-16-2002 03:24 PM ET (US)     Profile for mikee332  Send Email to mikee332     
Sorry I haven't responded sooner, but have been doing alot of ot at work. Thank's for the suggestion's and hopefully this weekend I will try to put them to use. The boat is new to me as I bought it in April so still trying to figure things out.My guess is the tach, as I seem to be doing 30 35 MPH. I'll keep you posted.
simonmeridew posted 08-16-2002 04:02 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
I think if you're doing 35 mph your tach should be reading higher than 3000rpm. More like 5000 or so.
simonmeridew
Steve Leone posted 08-18-2002 02:04 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
if you have a friend with a boat run alongside and see what rpm`s He is doing. that might be a simple way for you to determine if it is indeed the tach. if you are suffering a power loss (the engine is not producing sufficient horses) this will also cause low rpm`s. if the motor is fine, if you are are not dragging a parachute, if you remembered to take the boat off the trailer before you ran it (just kidding but you know somebody actually did this) you need to aquire a LOWER PITCH PROP. posibly a 17inch but maybe a 15in. each time you prop down it equates to adding approx. 400 rpm`s. make sure the tach is working right as this is the only way to figure out the best prop for your boat. Mac Tools sells a hand-held tach with an antenna that will work on any engine. it is expensive but the latest in tach tech. steve in el cerrito.

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