High RPM'S at full throttle

[Huskie]

Just got my rig back from the shop, he has it running good he said, except that the rpm's are very high at full speed, what should it be at full throtle? He also said the prop shoild have more pitch, what will that do and what is the cost of a new prop? Will get it in the water this afternoon and see for myself. Any thoughts or ideas from anyone, thanks,

[DTro]

Every motor has a optimal RPM range at WOT (wide open throttle). If you are running higher than that you run the risk of damaging your motor. Think of it as redlining you automobile. This is normally caused by not matching your prop to your boat setup. Your prop may not be necessarily the "wrong" prop, just wrong for your boat.

If you post the specifics of your setup, it would be easier to make recommendations.

[cold one sd]

If you have a tach, just keep the rpms within range. If you don't have one go to a higher pitch prop. I have a low pitch on my 15 4 stroke and I just watch the tach when running on plane by myself. When there are two on the boat it won't over rev.

[Ralph Wiggum]

As far as increasing the pitch, what that will do is lower your RPMs at WOT (wide open throttle). Like Dtro said, post some specifics and there are a lot of knowledgable guys here that will help.

[upnorth]

Every step up in pitch will lower your RPM by about 200 rpm. I check your rpm against what it says in your manual for max rpm.

[Huskie]

Thanks guys, it is an older, (1991) 70 Hp Johnson that I had rebuilt about 4 years ago, bought it used, do not have the manual. I have it on a heavy tracker, same vintage, 17' magna fish. One problem, the motor is undersisized for that boat it could take up to a 115HP. Has difficulty getting the bow up at top speed, makes for a wet ride. Can't afford to go bigger at this time, size matters.

[upnorth]

So what kind of RPM are you running at WOT. I have an "87" 70 horse evinrude and if memory serves me 5800 or 6000 is tops for WOT on my motor.

[PerchJerker]

The lower pitch prop (which over-revs your motor at WOT) will help your hole-shot, which may be a consideration if your motor is undersized for your boat. As mentioned in another post, just don't run WOT - back off enough to keep your rpms where they should be.

Also, does that motor have power trim (I am wondering because of your comment re: the bow doesn't go up at top speed)? If you have power trim, trim your motor out away from the boat when going fast, this will raise the bow (trim it in to help holeshot). If you don't have power trim there should be a manual pin that adjusts the angle of the motor on the transom, try moving that up (so the motor's lower unit tilt's away from the boat) and that should help your ride.

Good luck.

[Huskie]

Had the boat in the water yesterday, ran fine except it was a little sluggish between 1800-2200rpms. At WOT the rpms were at 5800, when I back off a bit it drops to 5000 or less, redline is at 6000. I do have power trim and trimming up does help bring the bow up some, but not much, lot of weight in the front end. I think I will just run with it this way for the rest of the season. If I keep bringing it in, fishing season will be over!!!! Thanks guys.

[cold one sd]

Transfer some of the weight to the back if you can. I have the opposite with my boat and it is nose light. I may have to try a prop with less lift or run with the front livewell full. When I am taking it easy when it is rough, the bow comes up and bounces too much at moderate speeds and I don't want to run faster and pound the boat.

[marine_man]

Quote:

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redline is at 6000

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Is anyone 100% sure this is correct? A lot of older 2-stroke motors redline was around 5250-5500 RPM... 6,000 on a 2 stroke seems a little high to me...

I'm not saying you're wrong... but has anyone seen anything from Johnson / OMC on that particular motor of what the RPM range is? Valv - do you have an old service manual or anything that would help?

marine_man

[Hookmaster]

I think you're right Marine Man. When 4-strokes first came out I noticed their redlines were higher than the 2-strokes.

[Huskie]

The tach on my boat has the redline at 6000. Without the manual for my motor, I am not sure if that is correct or not. Where could I check that?

[Ralph Wiggum]

It might be on the label on the motor.

[upnorth]

do a search on iboats.com someone may have posted it there.

[marine_man]

Just because your tach has a redline at 6,000 RPM doesn't mean that's where your motor redlines... multiple tachs work on multiple motors...

I'd try and hold it at 5500 RPM or prop it accordingly to keep you there unless you find out otherwise...

You could also call a marine dealer and ask them to check in their service manual... it should have it in there.

marine_man

[JWB475]

I would suspect your are probably hitting the revlimiter at WOT. While hitting the revlimiter on occasion may not do any damage. If you spend the entire summer crusing around at WOT, and you are actually hitting the revlimiter, you could seriously damage your outboard.

Even though the motor is a 1991, I would contact the manufacturer and find out what the operating RPM range of your motor is (they should be able to tell you), if you are running to high of an RPM, either re-prop your boat or adjust your throttle.

[JWB475]

Actually I stand corrected... I found your engine model specifications on-line in about 5 seconds using GOOGLE:

Manufacturer: Johnson

Year: 1991

Horsepower: 70.0

Cylinders: 3

Type: 2 stroke

Gear Ratio: 2.42:1

Min WOT RPM: 5000

Max WOT RPM: 6000

Spline Count: 13

If the above is correct, you should be fine at 5800 RPM's...

[marine_man]

Good work JWB475...

marine_man

[Huskie]

I will retrace your google search. Thanks alot, getting different ideas from different people. Also will back off the throttle anyway.