TOPIC: Thanks for help - still have low RPMs

Hi Folks,

First off… THANK YOU for all the help on this forum. My family just spent a week at the lake and had a great time. It would not have happened without the emails and phone calls from you all that helped me get this old motor up and running.

I am still struggling with low RPMs. Specs say this motor should do 5200-5600 at full throttle. The motor starts right up and runs really quite and smooth.

1964 500 50 HP
1964 Fabuglas 14 foot boat – 450lbs
Motor is mounted with plate less than 1 inch below bottom of boat.
Trim pin is set in the 3rd hole from boat. When I go to the 4th position, cavitation happens on turns

I have tried different props, thinking it was over propped and that did not allow full RPMs. The boat came with a 12 pitch 2 blade bronze. I tried higher and lower pitch props.

Prop 3 blade 10 Pitch aluminum
RPM 4400
Actual speed (GPS) 30

Prop 2 blade 11 Pitch bronze
RPM 4200
Actual speed (GPS) 32

Prop 2 blade 12 Pitch bronze
RPM 3500
Actual speed (GPS) 29

Prop 3 blade 13 Pitch aluminum
RPM 2800
Actual speed (GPS) 26

This is a small light boat with only myself and 14 year old daughter riding. With the 10 pitch prop, I should be having to limit the throttle to not over rev it.

The carbs seems to open all the way when at full throttle. The magneto turns and hit the throttle stop screw. To me it looks like the screw is set to limit the magneto rotation. I am hesitant to loosen the nut and change the screw setting.

Comments or suggestions?

Joe

check and make sure the carbs are getting full throttle look in and see if thr butterflies are open all the way.

Hi Dave,

Thanks for all your help with this. You have been great about taking my calls and posting advice.

The butterflies seem to go completely horizontal when the lever is pushed into full throttle. I cannot imagine them opening any more than that.

When out of the water on muffs, the engine will go to full, high RPMs. Without the water resistance on the prop, it is easy to spin up.

I added a tach a few weeks ago so I could check the RPMs. I assume it is set up correctly. It registers about 800 when in the water in forward gear.

It just seems like my boat, motor, prop and weight configuration should allow it to full RPMs on the water.

Joe

have you checked the timing belt to see if it is in time with the crank?
a boat like that should ru an 11 or 12in pitch prop.

Did you try putting the engine closer to the transom?

Hi Jim,

Yes, I started out with the trim pin in the holes closest to the transom and moved out each hole, one at a time. With it close to the transom, the boat rode very low (front down).

I just talked to a local friend about motors. He was having problems with a riding lawn mower engine. I told him of my challenge and he said some motors had a "vacuum advance". Not sure what this is. At one point the rubber tube that goes into the top of the magneto was off when I opened the cowl. I stuck it back on the top and never thought about it again.

Is that some kind of vacuum tub? Cold it be pluged? I just looked at my motor and the tube looks fine. It does not seem died out or cracked..??


Dave Bernard had some good suggestions for me to check out…make sure it is firing on all 4 cylinders…..run it without the cowl - possible exhaust leak…. while running, hand cover each carb looking for a seal leak in top or bottom of head….drag on boat bottom due to being on trailer for so long- bottom shape altered- hook vs rocker

Any other thoughts on this?

Joe

the hose is only a vent.

Personally, I think you have a tach issue, not a motor issue. 2800 RPM isn't even close to the power band on that motor and you shouldn't have gotten anywhere near 26MPH at those revs. Typically, you lose about 200-300 RPM for every inch you go up in pitch, or gain that much if you go down in pitch. You've got a 1400 RPM difference between an 11 and 13 inch prop - something isn't adding up.

How does the motor "sound"? When those old 4-cylinder Mercs are running well, they practically sing. If they're lugging, they get a bit of a whine in their song.

- Scott

Whats your HP on that motor? You stated that you got 30 something mph there. Are you going by the speedo or are you useing a GPS unit.I find that some of these older speedos can be off up tp 10 mph. If you are getting 30 something you are getting alot more than 2800 RPM. I kinda have to agree with Chinewalker, it is more likely a tach problem than motor. There are lots of tachs out there that are not right for the older Mercs.It is a common problem.Just how fast do you want to go with this boat and motor combo? Once you figure that out the members here should be able to help you get there alot faster without you haveing to pull your hair out. Have fun, Skip.

Again..thank you for your input on this. My tach may indeed not be reading properly. I think I can get access to a unit that will go on a single spark plug wire and I will compare readings.

Here is a short video I took last week at the lake.

1964 Mercury 500 50HP
11 pitch 2 blade bronze prop
Fabuglass Fury - 14 foot boat @ 450 lbs
2 adults in boat, 2 teenage girls on tube
GPS speed was 24 MPH, tach was reading 3200-3500 rpm
Throttle was at full @ 8pm position on controller
I was making S turns to give them more of a ride at 24 mph

You can hear the motor sounds. Is that the sound of 5200-5600 RPMs on this 50 HP motor? Maybe this is all there is on this motor and boat set up???




Any thoughts or comments are appreciated.

Joe

You might have better luck and speed pulling 2 people on a tube with a 3 bladed prop.I can hear qiute a bit of cavitation with that setup in the turns. People in a boat or even pulling a tube with 2 people on it is gonna slow you down abit. Tubes put alot of drag on these older hulls just due to the design of the hulls.They would pull a skier or two but they didn't run fast unless you got into a 6cly merc.Again it comes down to how fast do you want to go and just how good of shape is your motor in? Good luck with it. Skip.

Hi Joe, looks like the girls had fun, even at 24 mph. Your engine sounds good and sounds to be around 4000 rpm. With 2 adults and two girls on the tube I would say you are currently at the expected speed with a 50 hp. Your wake is very smooth (prop wash) which leads me to speculate that the engine may be too low in the water. You mention that you are 1 inch below the bottom of the boat. If possible raise the engine up an inch. A cut off hockey stick works good for that.

Seems Skip and I were posting at the same time. Good idea to try a 3 blade to help with the cavitation problem.

Joe
I cant listen as I dont have speakers. But with a 1960 16 foot Wagemaker Empire running a 1970 Merc 500 with a 13 pitch 3 blade Aluminum Mercury Brand I top out at 30 mph and 4800 to 5100 RPM (Tach is old and varies a bit). With a tube and that set up 1 teenage girl on tube and 2 people in the boat I believe I was at about 23 MPH.. and if I remember it was around 4000 RPM ( and not happy sounding) The prop is a bit much for the setup but is stuck on the shaft so rather than ruin a good prop I just run a little under a full Singing merc, and dont use it for more than running up and down the Hudson River. Just something to compare to.

Jesse

Hi Skip, Jesse and Dave,

Thank you for your input. I did put on a 3 blade 10 pitch prop on it the last day at the lake. I did not pull anyone with it. Just rode around a little. Cornering did seem smoother, but was not pulling a tube either. RPMs still seemed low for only 2 riders and a 10 pitch prop. I need to check my tach reading to confirm it is reading properly.

The 11 pitch 2 blade gave the best ride while we were there. The best hole shot for skiing and best top speed with the tube. I realize the drag of the tube is going to really hold things back. In this video, I was cutting things really sharp looking to whip them around some. So I did loose prop grip and it would spin up.

I do not want or need to go 40 MPH on this boat. Without pulling anything, I would like 35 MPH and 30 MPH pulling a skier. So I am only short 5 mph.

While riding in the boat at WOT it really does not sound like the motor is laboring at all. It sounds like there is another 1000 plus RPMs there.

Maybe this is the max output of this motor and boat set up?

My old Mercury prop chart says a Mercury 500 50 hp with my 14 foot boat @ 1100 LBS load (boat, motor, passengers, equip. etc) should spin a 13 pitch prop at 5200-5600 RPM traveling 34-40 MPH. The charts say a 10 pitch on a 20 foot boat should push a 2000 – 2600 LBS load about the same speed I am going in my boat that is 6 feet shorter and weighs over 1000 LBS less.

I happen to have a few old hockey sticks, so I will look at raising the motor up a tad. Looks like I can go up about ¾ inch without new holes.

Is this going to make the cavitation worse?

Joe

Joe, remember that prop charts are with a new engine running at optimin performance. In other words something we never see in the real world.If you can get that boat up to 35 you will being good.It took me 2 weeks of dialing in my 55 rude on my 15 ' Crestliner to gain 2 mph. Raise the motor the 3/4' and you shouldn't have to worry about cavitation.There is a point where a hull no matter what design it is will not go any faster with out making a big hp change.So just play with it for awhile and see what happens, Good luck, Skip.

Bones,
Do you know if your bilge is dry? Lots of folks are surprised to find water below the floorboards and often waterlogged foam. I found water and waterlogged foam under the floorboards in all three of my boats.

Dean

Hi Dean,

Yes, I did check the bilge water. The previous times I ran this boat, it was for an hour or two then I would put it back on the trailer. I would pull the plug and only a drop of water would come out. Well, while at the lake last week it was in the water for several days straight. I did not want to go back to the boat ramp with boat and trailer, but wanted to check for water in the bilge. The boat is so small, I lifted it out of the water on a jet ski lift !! No water to speak of.

Joe

There are a number of prop calculators on the net, I posted a snip of one using your numbers.

The charts that Merc printed were very optimistic. Sounds to me that your tach is OK, pick the prop that moved you 30 plus mph.

The engine sounds like my 1960 400 at about 5200 rpm. She is a 15 footer and is only doing about 28mph with the 3 blade 11 pitch prop. I think your tach is off.