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TOPIC: RPMs changed on last outing - Any Ideas?

RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113542

Hi All;

I took my 73 Crestliner out for the day on Saturday with the intent of testing RPMs at light load so I can choose the best new prop.

What happened is this:

First hour or two the max RPM at WOT was 4,800 at 25 or 26 MPH. This led me to think I need to go down from a 17 pitch prop to maybe a 15 pitch.

After changing to second tank, I looked down and noticed RPMs were up to 5,500 (and we were cruising at around 30 MPH.

Here is the info:

Engine is 1973 Evinrude 65HP Long Shaft with less than 80 hours on it from new (boat was used by my Uncle very little - lived most of its life in a dark dry garage). Until recently, the boat was last out of the garage before 1998 (last registration expiration).

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113544

Here are the variables that I think are possible:

1. Choke was not fully open till later in the day (possible bad choke solenoid?)..... I can test this next time out by putting choke into manual off position on engine.

2. Different batch of gas in the two 6 gal tanks? It is possible that 1 of the tanks had 8 oz. of Sea Foam in it - can this cause lower RPMs?

3. One tank flows better than the other?

Not sure what to think,

When I got this boat as a gift from the best Uncle in the world :blush: he recommended it be gone over by a good outboard mechanic, so I had a local place with a good reputation do the work. He:

Took it apart, and checked out the pistons, etc. Said it was as clean as a new motor inside
Put in a new water pump impeller (for the heck of it).
New plugs.
Changed fluids in lower unit (said old oil was clean, but thought best to put new in.

Timed it and tank tested, then gave it back to me.

Since then it gave me a problem after it would run about 45 minutes (turned out to be the power pack - the 43 year old original power pack was still on it). Now runs like a top.

Any ideas?????
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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113550

  • Kerry
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If the second tank had the Seafoam in it, it may have cleaned out a bit of gunk in the system. Or, simply bad gas in the first tank. Continue the Seafoam anyway, never hurts.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113553

I hope that you are correct. I want to re-prop. Anyone have an idea of how a OMC Viper 13.875 x 17 would do on this boat? I am trying to get a bit more speed out of her.

Boat is 73 Crestliner Tri-Hull open bow runner.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113560

On that boat, I'd stay with the 17. I could be wrong but 5500 rpm is about top for that motor if I remember correctly. Your performance seems about normal to me for that combo. I grew up with a Starcraft Capri 15' tri hull with a 1972 65hp Johnson which is very similar in looks and weight, to your rig and performance was just about exactly the same as what you were getting on your second tank. That is not a light boat.

If you want more speed, maybe consider going to an 85hp, which is probably what that boat is rated for. The prop that is on it is the best combination of power and speed. Changing the prop is only going to gain one thing at the loss of the other and the possibility of overreving the motor is there.

One other option is to bump the tilt pin up a notch. Should be able to go one more without cavitating at high speed, maybe. Also take off the cav plate. Probably gain 2 or 3 mph doing those two things, of course it's going to take longer to plane so if your skiing and tubing, put it back down. Lots of times, whatever you do to try to increase speed past a certain point is going to effect something else. Power trim and tilt can also help to optimize motor position under any circumstances, which is why it's so popular.

Don't take this the wrong way, that's a beautiful boat, but tri hulls are a compromise. They can do many things and do them well, fish, ski, room for the family, handle somewhat rough water, probably the most versatile design out there, but they are heavy.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113574

Thanks ggreggc;

I know it isn't a boat built for big speed, so no harm.

Do you think I will gain anything by going to the Viper 13 3/4 x 17 prop, or is it not worth the $ for this boat?

Prop on there now is nicked up a bit on the edges (and 43 yrs old), so I want to put a new one on.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113577

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5500 is correct top RPM. Likely your 43 year old speedometer is off, that 17p should be closer to 40mph. Buying the Viper will not gain much more than a replacement aluminum, maybe one or two mph. It will not nick or ding as easily. I also agree to remove the fin.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113584

I would think that 40 mph is pretty optimistic on that combo. That boat must weigh in the neighborhood of 800#. But who knows. If you have to change the prop anyway, it probably wouldn't hurt to go with the other one as long as it's not priced out of sight. Unless you plan to pull up two slalom skiers at once (which it probably won't do unless they are in the 100-130# range each), you probably won't notice much difference and gain a little top end. Actually what am I thinking? Hardly anyone seems to waterski anymore anyway.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113599

ggreggc wrote:

I would think that 40 mph is pretty optimistic on that combo. That boat must weigh in the neighborhood of 800#. But who knows. If you have to change the prop anyway, it probably wouldn't hurt to go with the other one as long as it's not priced out of sight. Unless you plan to pull up two slalom skiers at once (which it probably won't do unless they are in the 100-130# range each), you probably won't notice much difference and gain a little top end. Actually what am I thinking? Hardly anyone seems to waterski anymore anyway.


Hey, I just skied behind my '71 Kona last week. Nobody wants to spend the time to develop the skill needed anymore. Now they want the easy to do and flashy wakeboard.

Will

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113600

Your right Will. I've skied since I was 10. I've really lost a step though, arms just have a hard time taking it anymore. Of course if I would quit still trying to touch my shoulder on the water like I used to be able to.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113605

Greg, I've learned not to think I was 25 years younger than I am. But I had fun.

Hey, Crestliner, how's the wake on that hoat? If she throws decent, might make for good time tubing for the kids! How does she handle heavy chop? Gotta tell ya, looks awesome. Here's to good times on the water. I grew up with my dad on the lake, and now I'm gonna show my kids a great time too.

Will

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113621

I am jealous. I have the exact motor. I ran a 15P until I finally bought a tach. Went with a 13P. My boat weighs 1200 lbs though. 5400 RPMs at WOT with a full load of people and 25 mph.

FYI the manual says 5000 to 6000 RPMs.


Pic taken today. We are headed skiing.
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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113622

Re:RPMs changed on last outing - Any Ideas? 6 Hours, 48 Minutes ago Karma: 2 Karma+ Karma-
Greg, I've learned not to think I was 25 years younger than I am. But I had fun.

Hey, Crestliner, how's the wake on that hoat? If she throws decent, might make for good time tubing for the kids! How does she handle heavy chop? Gotta tell ya, looks awesome. Here's to good times on the water. I grew up with my dad on the lake, and now I'm gonna show my kids a great time too.

Will


Will - she does make a nice wake - my kids love being towed (cannot talk em into skiing yet...)

When I first took out the boat, she handled chop terribly (and the bow was so high my wife could not drive. With the foil on it handles great, planes quick, runs flat - but cost me 1 or 2 mph.

Enjoy you boat and your family!
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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113623

Jim - Nice boat! Mine is about 850 with 3 6 gal tanks of fuel, plus people...

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113626

Also, don't discount the tack may be sticky.
If you didn't hear a difference.. suspect the tach. although you said you gained speed as well?
Hate to start messing with the engine if nothing is wrong with it.

For what it is worth, I replaced every part that touched fuel. Mine was in terrible shape and was stored outdoors for 13 years.

If it is the engine, you may want to consider going through the entire fuel system * if it has not been done already. Hate to toast an engine that looks that good.

*Rebuild all carbs with factory kits, replace all fuel lines, and fuel pump.

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Re:RPMs changed on last outing - Any Ideas? 3 years 1 month ago #113643

Also, don't discount the tack may be sticky.
If you didn't hear a difference.. suspect the tach. although you said you gained speed as well?
Hate to start messing with the engine if nothing is wrong with it.

For what it is worth, I replaced every part that touched fuel. Mine was in terrible shape and was stored outdoors for 13 years.

If it is the engine, you may want to consider going through the entire fuel system * if it has not been done already. Hate to toast an engine that looks that good.

*Rebuild all carbs with factory kits, replace all fuel lines, and fuel pump.


Hi Jim;

I have gone through the entire fuel system, so I am pretty confident that it might have been some stale fuel.

All tanks have been cleaned (no problems), and I re-built the pins and disks in the pick-up housings (o-rings were dried out from 20 years of non-use)

New fuel line, primer bulb, and connectors.

New fuel filter and gasket.

Fuel pump pressure tested in use and is to spec across RPM range.

Carbs were rebuilt by local Evinrude shop, said they were clean but changes floats and inlet needles anyway.

I will keep using Sea Foam at 6 oz per 6 gal tank for good measure.

Hopefully it will last another 43 years.... :)

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