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Author Topic:   Limiting Max RPM's @ WOT
Legobusier posted 06-04-2004 11:00 AM ET (US)   Profile for Legobusier   Send Email to Legobusier  
Hi all.

I have an 1987 Montauk with a 1998 Honda 90. When I throttle all the way up, the RPM's get up to 6200-6300 or so - above the max 6000 Honda has posted.

Is there an adjustment in the control housing or linkage that I can tweak to limit the high end here? I just want it to top out @ 6000. Top speed is not really a factor (it will do about 42 right now), I just don't want to hurt the engine by exceeding the max RPM.

I haven't poked around under the control housing as it's been siliconed down due to a broken clip and I don't want to pop it off if that's not where I need to be.


Moe posted 06-04-2004 11:35 AM ET (US)     Profile for Moe  Send Email to Moe     
The problem is that you have a prop with too little pitch. One of the symptoms of that problem is that the motor overrevs the stated operating range. The other symptoms are the motor is revving higher than necessary in all areas of operation, incurring higher than necessary wear and fuel consumption.

To address the overrev at WOT, you could simply mount a bar across the slot the throttle travels in, or fabricate something that protrudes down into that slot, i.e. do something that physically prevents the throttle from fully advancing. However, that is also going to reduce your acceleration at WOT at speeds below maximum rpm.

Playing with the cable and linkage isn't a good idea. It affects the motor's throttle position at idle/shifting speed as well as at WOT, and as above will also reduce your acceleration at WOT at speeds below maximum rpm.

Keep in mind that even with a carbureted engine, you probably have a rev limiter in the electronic ignition. It may shut off 1 or more cylinders, resulting in a feeling of "missing" or it may do something as subtle as retarding timing to 0 degrees before top dead center.

You'd be better off addressing the problem, and getting a prop with 2" more pitch.


Legobusier posted 06-04-2004 11:41 AM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Thanks Moe.

I figured I could do that, but was looking for a less expensive solution than a new stainless prop. ( I know I don't NEED stainless, but....).

What are the associated advantages/disadvantages of repropping (other than the cost of doing so)? More pitch would give me more power/speed, no?

Moe posted 06-04-2004 12:11 PM ET (US)     Profile for Moe  Send Email to Moe     
If you are overreving now, more pitch should give you more top speed at rpms within the recommended range, as well as more speed at any given rpm (assuming not much change in slip). Said the other way, for any given speed, it will give you lower engine speed, with hopefully, a quieter engine, and better fuel consumption. More pitch will also likely reduce your acceleration slightly.

The only reasons, besides cost, I'd keep the currently overreving prop, would be if you frequently have a heavy load in the boat, or pull up adult skiers slalom.

Plotman posted 06-04-2004 12:33 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Driving you boat around with the wrong prop on it is like driving your car around in the wrong gear.

Get a new prop. You are far, far better off with the right aluminum prop compared to the wrong stainless. In fact, there is absolutely zero benefit to having a stainless prop if it is wrong for the boat.

You pony up to buy stainless because you want to get that last 5% or so of performance. But they way you are set up, you are 20% off.

You should be able to find an aluminum prop of the right pitch for $50 or so off of eBay if you don't want to buy new.

Legobusier posted 06-04-2004 12:40 PM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Thanks guys.

I RARELY run WOT - in fact pretty much never, so I'm not prone to over-revving the engine. Given that, is there any "damage being done" by keeping my current prop and just watching the RPMs or should I put a new prop high on the priority list?

Moe posted 06-04-2004 01:30 PM ET (US)     Profile for Moe  Send Email to Moe     
It's a lot like driving around in a truck with 4.10 or 4:30:1 towing gears when all you need is 3.55 or 3.73:1. You're spinning the engine faster than it needs to be, and yes, that's increasing wear and fuel consumption somewhat.

I think you'll like the more relaxed pace the engine runs at if you reprop. If I didn't carry heavy loads or pull a slalom skier up, it would be high on my list to reprop. Besides, you can carry the old prop on board as a spare.


Perry posted 06-04-2004 01:35 PM ET (US)     Profile for Perry  Send Email to Perry     
Legobusier, I have the same motor on my Dauntless 160 and most the time I run a 13.75 X 15 pitch prop. It gives me 6200 rpm at WOT. My boat is heavier than yours and I like having a good hole shot so that is why I usually use this prop, I just pay attention to the tachometer and keep it below 6000 rpm.

If you adjust the throttle linkage, you will not only limit WOT rpm but will also limit overall performance of your motor. Not a good idea in my opinion.

jimh posted 06-04-2004 01:37 PM ET (US)     Profile for jimh  Send Email to jimh     
Usually the outboard engine manufacturer specifies a range of RPM that the engine should reach at wide open throttle. If you are above that range by a couple of hundred RPM, you could change that with a new propeller, but I don't think there is a mandate that you have to.

You might find yourself out one day with three extra people, a full fuel load, a strong head wind, and some head seas, and then you will be propped just right.

If you run the engine 200 RPM higher than you might otherwise have to run it to cruise around for a few hours, I don't think you are going to significantly cut engine life.

It is much like having a truck with a 3.73:1 rear axle. If you never hauled a trailer, it would be better to get the truck with the 3.23:1 gears. But then on those days when you are trying to pull a heavy load up the western slope of the Appalachian mountains, you'll be glad you have the lower gear ratio.

I think it is much better for the engine to be slightly under-propped, that is, to have a propeller whose pitch is perhaps a bit smaller than the optimum, than it is to have an engine that is over-propped. An over-propped engine is struggling all the time.

Legobusier posted 06-04-2004 05:42 PM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Good point on the loads Jim. I guess I should have noticed that this "excess rpm" happens when I'm alone. I don't believe I have a problem with the "crew" on board.


Bigshot posted 06-14-2004 11:39 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Only costs about $50 to have your current prop repitched or double cupped.

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