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mnhunter79

MAX RPM ?

20 posts in this topic

Does anyone know what the max RPM when wide open throttle should be for a 1995 evinrude 90 hp? mine varies from 6000 to 7000 rpm wide open. Seems a little high. My previous merc. 60 hp was 5500 rpm wide open.

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5500 to 5800rpm is max.

6000 and 7000 is too much you will need a higher pitch prop or you have to throttle down

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Are you shure your tach is working properly. A 1000rpm variation at WOT doesn't sound right to me.

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Tachs don't mark a little above normal, they are completely off (eg 3000rpm at idle), don't mark anything or be right on.

A small pitch prop, or with damage to it (cavitation), or a spun hub can give you much higher rpm than normal.

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It idles at 1000 rpm. i have a 13 3/8 by 17 pitch prop. the boat is a 16.5 ft warrior fiberglass. I also ha a stainless prop that is 13 by 19 pitch but with that one it seems to take a bit to get on plane, but i can get 2 mph more out of it. I think i might be trimming the motor up to much at WOT cause the bottom of the motor does not go below the bottom of the boat very far.

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Way too high... Your going to kill that thing... I'm surprised your not hitting a rev limiter. But maybe with that year, they didn't have them yet. Get a different prop, or like other have said, get off the throttle a bit. Engine height may have something to do with it, but if your getting enough water pressure, the height is probably less an issue than you may think.

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Wow.. it idles at 1,000 RPM? Normal idle speed is 800 in neutral and 600 in gear. Does it sound like it's idling high?

This is how I like to run - trim all the way down until you're up on plane. Start trimming up until boat starts porpoising. Then, trim back down just until porpoising stops.

Are you running somewhere in that neighborhood?

marine_man

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Could be your running lean at idle and WOT. That'll bring up the RPMs. A dirty carb will do that. So unless someone was adjusting the idle air or idle speed I'd take the high idle as a warning to clean the carbs.

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Are you running a stainless or an aluminum prop. My Lund would never porpoise regadless of prop, tilt it up too high and the prop will start to unload.

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What speed are you indicating at 7K? With a decent prop, you will be around 40 mph at 5300 rpm with that motor and boat combo. So if you are truly turning 6500 to 7000, you will be pushing past 50 mph - UNLESS you are simply blowing the prop out.

Blowing prop out - Do you get a corresponding increase in speed for that last thousand rpm, or does that last thousand not increase the speed much (possibly decrease)?

On the other hand, I'm not convinced that you are truly turning those numbers. I can see 6000 with a 17" pitch, but not 7000 without cavitating.

What do you get with the 19" stainless? Speed and WOT rpm?

Tim

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It runs about 36 to 38 mph from 5200 to 7000 rpm. when the rpm goes from 5 to 7k the speed decreases from 36 - 38 mph down to 32 - 33 mph. Ive never seen rpm's fluxuate so much. It just don't seem right. How do i know if the tach is hooked up correctly for this motor?

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It runs about 36 to 38 mph from 5200 to 7000 rpm. when the rpm goes from 5 to 7k the speed decreases from 36 - 38 mph down to 32 - 33 mph. Ive never seen rpm's fluxuate so much. It just don't seem right. How do i know if the tach is hooked up correctly for this motor?

That makes perfect sense. it doesn't sound like a tach problem.

You go from 5200 to 7000 and slow down because the prop has, in essence, lost it's "grip" on the water. It is ventilating (different from cavitating) and is turning in a mixture of air and water. Typically this happens when you continue to trim the motor up as you accelerate. Ideally, you want ventilation to start right after you've hit the top limit of your trim range.

If, right now, you start ventilating and you have barely trimmed up at all, then your motor is mounted too high on the transom. But, if you can trim up (this is all at WOT) and you get a nice amount of bow rise, then your motor height is probably okay.

Next, this all depends on a decent prop. A nicked up prop will increase the problem. Even a perfect Aluminum prop will ventilate before a decent stainless with a bit of cupping. I'm partial to the Turbo line of stainless props on that motor, Soderblooms carries and repairs them.

Do you know where your motor is mounted on the transom? Trimmed all the way down, where is the anticavitation plate in relation to the keel?

Tim

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Do i have the correct size and pitch prop on this boat / motor? 1995 90 hp evinrude, 4 cyl. 16.5 ft warrior fiberglass. prop is a 13 3/8" by 17 pitch.

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Sounds like you figured out the problem. Losing grip and the motor unloads. I used to have a 13 1/4x21 aluminum prop on my boat and if you trim it above 3/4 on the gauge the rpm would spike and lose its grip. Now I have a 13 1/2x19 stainless prop and I can trim it up almost into tilt without any blowout or unloading, gained many mph with the stainless prop also.

A 13 3/8"x17 sounds like the right size for an aluminum prop. Stainless props are more aggressive than aluminum props so sometimes you need to drop down an inch in pitch. Seems like a 13 3/8x17 stainless 3 blade wuld work well.

Solas also makes top notch prop. Turbo and Stiletto are made by Precision propellers Inc who also makes some props for Yamaha.

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I also have a 13" by 19 pitch stainless prop for it. Ill have to try both props and play with the trim to figure it out. Thanks for everyones opinions>

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Wow.. it idles at 1,000 RPM? Normal idle speed is 800 in neutral and 600 in gear. Does it sound like it's idling high?

This is how I like to run - trim all the way down until you're up on plane. Start trimming up until boat starts porpoising. Then, trim back down just until porpoising stops.

Are you running somewhere in that neighborhood?

marine_man

This is exactly how I run my boat. Have a 16.5 alumacraft with a 75hp 17 pitch prop on it. If is stay around 32mph it stay's around 5000 rpm. If I try to juice more it goes up. I go full throttle to get her up on plane, but then ease off a little. The extra two mph aren't worth the rmp wear to me.

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It sounds like your'e getting closer. Tim had some good advice and I'll add a couple of basic setup steps to check out.

1. Check your prop SHAFT height in relation to the hull. On the trailer and with the motor trimmed vertical, put a board on the bottom of the boat and measure to the center of the propshaft. With your boat and motor you should be about 4" or a little more. With the motor too low you get drag and torque steer, and to high you'll get ventilation and possibly lose water pressure. (Remember the props you have are designed to run IN the water!)

2. Watch your trim angle! More trim does not make you go faster, rather it slows you down. Your propeller is most efficient when it is in the water pushing the water backwards. If you trim too high you put that energy into throwing water up in the air. Looks cool for a while but doesn't accomplish much. Try trimming at WOT while turning left/right. You will notice when the prop loses grip in the turns and where the straight line speed stops increasing. This is your maximum trim angle.

3. Use the stainless prop. Due to the thinner blades they are more efficient and will give better all around performance. As Tim mentioned, it may be cupped at the trailing edge of the blade which will give better grip on the water.

Good luck with the setup!

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It sound sliek everybody hit the nail on the head here. It's not a tach or prop, per se, problem. You are simply trimming your motor up too high. I would bet the same thing happens when you corner/turn hard. All of a sudden your RPM's jump and speed is lost. Once again, you've lost grip. An SST prop will tend to allow you to trim higher and less likely to lose grip due to it's rigidity. An aluminum prop has more flex and at WOT would most likely lose grip sooner than the SST would. I know your 19" takes longer to get up on plane, but if it isn't severe I would run that prop and save your motor. Or just make sure to pay attention to the noise of the engine. When the RPM's kick up, then trim down right away until you "feel" and hear traction again.

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You also don't want to run a prop that lugs the motor down too much either. Pick a prop thats gets you towards the upper end of the manufactuers recommended range for best performance.

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