governor effect

[ALX357]

Could anyone give me some starter thoughts on this problem..
My Mercury 50hp 2-stroke seems to run fine and smooth, and i have had a 13-pitch prop on it with which the boat reached about 14.5ph (@GPS)
while the motor only reached about 3400 RPM max at WOT. The motor controls have been re-installed by a Merc dealer, and some related work on the motor done.... and they say everything is done right on the motor.
They suggested that my prop pitch was too steep, saying that i should be revving at about 5500 RPM at WOT for best power, and that i would get about 200 RPM additional for each 1-pitch drop on the prop. So now that i am using a 10-pitch prop, the revs have increased to right at 4000 RPM. But the speed at WOT is only about 13MPH. Medium loaded boat the same for both props etc.
10 pitch, 4000 RPM, 13 MPH
13 pitch, 3400 RPM, 14.5 MPH
OK, questions.....
1) would a higher grade of gasoline help any ? using 87 octcane now....
2) anyone else have this exact engine, and what are you getting for speed and RPM, at WOT, and with prop ?
3) could it be a mis-adjusted throttle linkage, or cable ?
4) what else ?

[Catigale]

You are still about 1500 rpm off your WOT so something isnt right. Sure sounds like your linkage isnt setup right or something..

[Moe]

The two-stroke Classic Merc 50 has a 1.83:1 gear ratio. That means it just right for a less than 1,000 lb loaded aluminum jon boat. It needs to turn less than an 8" pitch prop to achieve anywhere near an engine rpm as a 2.33:1 geared motor with a 26X (i.e. the 2.33:1 BigFoor with a 10" prop.... wihich is about right for a 26X).

You're still way overpropped, for your gear ratio, with a 10" pitch.

Top speed is less a function of prop pitch, than it is of horsepower, boat weight, current, and wind.

If you aren't doing runs in 180 degree opposite directions and averaging them, you can't compare props. Properly geared and propped, it is horsepower that determines top speed. You still aren't even close to your motor's max horsepower rpm range.

BigFoots, with their LOW (numerically higher) gearing are desinged to push heavy boats at lower speeds. Your motor is diesigned to push a very lightweight boat at higher speeds. You're going to have to have an extraordinarily low pitch prop to compensate.

--
Moe

--
Moe

[ALX357]

makes good sense, and factual.... Moe,
but when i re-propped downward, i lost a couple of miles per hour top speed, based on numerous observations with the GPS, going and coming, a noticable overall drop, even though i did not do an averaged back and forth run under strictly controlled conditions.
IF the motor gained RPM towards it powerband, which has any sort of parabolic curve, I was looking for some speed increase, not loss. As i drop the prop pitch even lower, i now suspect i will get more revs, but less speed, or no noticable gain. Otherwise the graphed curve would be more of a "S" shape, Power, RPM, vs. Speed, @WOT.
I realize that the curve may have a thresh-hold RPM for its power band, but are you sure that a lesser pitch prop's less thrust @ a given RPM will be compensated by the increase of RPM, and at its max power realize higher speed ?
I am angry at myself somewhat for buying this boat/motor together, and at the PO, after finding out that the Merc is not the same age as the boat, but made in the early-mid nineties. He is uncooperative in giving any consideration. It is possible he did not know either, as he bought the boat used, and I did not find out the motor's age until months later, when i was getting some parts for it.(Serial number) It looks fairly unused and has low apparent paint wear. I had already put extra money into ithe boat and had financing, so doing anything about the deal was too late by then. Although the boat is really fine, i wish i had waited and found a similar boat (Year 2000, or +) with a 4-stroke and more properly matched motor, maybe with a 60, or 75.
SO one buyer's caveat i did not watch for is to make sure of the motor's age and not assume it is the same as the boat. You would normally assume that if not mentioned specifically, the motor year would match the boat on a fairly recent boat. NO? i guess not.

[Moe]

Your prop slip at 13 mph is only about half a percent higher than that at 14.5 mph (37.2% vs 36.6%), so I don't think we can attribute the speed loss to a less efficient prop.

Even if horsepower was level and not rising through this rpm range, you still shouldn't have lost any boat speed, but rpm should've increased to 4,420 with a switch from a 13" to 10" prop. As you expected, your boat speed (and rpm) should've increased, since horsepower should be increasing in this rpm range. The loss of rpm and boat speed indicates that it's falling, and if the problem isn't with the linkage, as Catigale recommends checking, it's in the motor... carb jets gummed or clogged (although this usually results in missing), reed valves clogged, soft or broken, piston rings worn and/or cylinder walls scored... just some possibilities.

Higher octane fuel doesn't contain any more energy and won't make your boat go faster.

Most with 50HP here report at least 14 mph with ballast and at least about 17 mph unballasted, both normally loaded.

--
Moe

[ALX357]

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Moe, NOT that i am an expert on motors, but it runs smoothly for hours on end, without missing or stuttering. Idles normally, i think, and starts fine, if choked appopriately, or not choked, depending on its heat, etc.
Hard to think that it is gummed, or fuel obstructed. maybe the air cleaner, i will check next, but no, if i remember, there is none. ??
New fuel line, new filter, new tanks, new gas, ( i really didn't think the motor wanted high octane gas). Might be low compression, or mis-adjustment of the carbs. ?? I think the boat came from Florida before its PO, and that is about 490 feet lower than Nashville. Only a tenth of a mile. Don't think that could be it. Also remember it was serviced by a local Merc dealer, 'though they might have missed something.
Do you think another 1500 RPM or so would take it from 13 MPH to 18, another 5 MPH additional ? I am still trying to glom out your information on the previous post.
what with the age of the motor, as much as 12 years or so, it seems more and more likely that it is worn rings or reeds etc, just a loss of max power from wear.
IF so, maybe i should consider the replacement of the motor, unless i can just live with it working ok until it really craps out altogether.

[baldbaby2000]

I just had a similar experience with my new Tohatsu 40 hp carburated 2 stroke. I was only making about 4200 rpm WOT. The dealer sold me a different prop which gets me to 5000 rpm. The top speed dropped from about 13 mph to about 10 mph. My situation may be unsual in that I was running it at about 9000 feet in both cases. I can't remember the pitches of the props but I'll check them next time I'm on the boat. I'll ask the Tohatsu dealer about it.

[ALX357]

butterflies open flat at WOT.....

[ALX357]

Joined: 22 Jul 2004
Posts: 184
Location: Nashville TN / Louisville, KY -- 2000 MacGregor 26X, Mercury 50hp
Posted: May, Mon, 2005 Post subject:

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It would be valued information to hear from other MacX owners with Mercury Classic 50hp motors,
1) what's your prop pitch, ?
2) motor RPM @ WOT, ?
3) and what speed MPH do they get out of WOT ?

someone suggested i check my timing advance control....will do - next thing.....

[Casey]

My boat was originally bought in Colorado and propped with a 11.25 inch diameter with 10 pitch standard 3 bladed aluminum prop.

The motor is the newer serial number for the 1998 50hp 2 cycle model. It has always started well. I have been able to get 5500 rpm in South Dakota as well as on the Gulf Coast. Old gas nocks the rpm on wot to 5000 rmp. At 5500 rpm I am doing 19mph with two adults and one child with about 15 gallons of fuel. I was told by the mechanic that did a compression test that the engine was low on compression for the number of hours.

I have had two occasions were I could not get my RPM up above 3,000 rpm. One was a crack in the fuel recirculation line with a carbureator needle valve seat groved. The latest was a piece of trash in my number one carbureators metering jet.

[Casey]

ALX357

After rereading your posts, my engine ran fine idling and up to about 3,000 rpm. If your engine is like mine, it runs on different jets idling. In my research on my model, the only difference in the 50 hp and 60 hp engine is the size of the metering jets. If one of your metering jets are partially clogged, it cuts the high end power tremendously.

One nice thing about my engine. It is easy to take the carbureators off and a part for inspection. Also, check the valves on the recirculation line. It took me about 1 1/2 hrs to take all three off, apart, reassemble and start engine, but I had seen it done once before.

Do you have the electric choke? Do you have alot of excess fuel dripping into the engine pan?

[ALX357]

Casey,
i would be ecstatic if my X got up to 5000, or even 5500 RPM and 19 MPH. - low compression notwithstanding... that sounds great to me...
since i don't have mechanical experience with engines, i would be reluctant to pull apart my carbs to clean or check the jets. Is there a "pour-in" or "spray in" type of cleaner for carb jets ? I know it is not good to stick anything into the jets that could scratch the surface inside, but maybe a soft toothpick ? if this is not a good option, i am left with the tender mercies of a marine shop mechanic @ +/- $75.00 / hour.
But is would be a neat trick to re-jet the carbs and get the power of a 60-horse..... wonder if that is all it would take.

[Casey]

ALX357

I am not sure if just changing the jets is all the differences to get to a 60 hp. I have a couple of manuals and when I compare the 50 and 60 hp specifications, the jets are the only differences that I have found.

To dislodge the trash in my jet, I used the eye end of a long sewing needle. Probably not the best choice for a blind probe. A toothpick seems as if it might be too large. Perhaps someone else that has more carb experience can suggest a better probe for the metering jet.

What I would do if I thought one of the jets were partially obstructed and I did not want to remove the carbs:

1. plug transom drain with a stopper from the inside to prevent fuel from draining where you don't want it.

2. Place several paper towels in transom basin.

3. Remove engine cover.

4. Tilt engine slightly so that fuel will run forward. Make sure the vent is open on your fuel tank. (if it is closed and you have pressure, when you remove the carbureator drain plug you will have continuous fuel draining)

5. place several paper towels on the bottom of the engine compartment.

6. Remove the brass plug on the carbureator bowl on the top carb. (A socket , ratchet & extension worked better than a screwdriver for me. ) Hang onto carefully so as to not loose the gasket.

7. Allow fuel to drain. Give the fuel bulb a squeeze. Gas should come out easily. If no gas comes out, your needle valve seat is probably obstructed. Spray a carbureator cleaner inside. The metering jet faces the drain plug. If your eyesite is good enough and you can get an unobstructed lighted view, you may be able to see the numbers stamped on the metering jet. Take your probe and slide into your metering jet and out. Spray again. Squeeze the bulb again. Install the brass drain plug and gasket.

8. Change paper towels if necessary. Move to the next carbureator and repeat.

The other simple check is the fuel recirculation line (if your model has this line) (its purpose is to take unburned fuel from the bottom crankcase and pump it back up to the top crankcase). If you are interested and I can write the instructions.

If someone else has done this and sees that I left something out, feel free to interject.

[Casey]

ALX357

If you gave me your model and serial number, I could take a look in my manual and see if your engine is similiar to mine. I know that when ordering parts, I must have the model and serial number because of some changes that happenned mid year.