Wrong Prop, bad tach or both ?

[greasemonkeyjmj]

I have a 24 foot grumman pontoon with a 48hp Evinrude, newly rebuilt due to score pistons and cylinders. I am running a 3 blade 13 3/4x15 prop. Average speed with 2 people according to GPS is 15 MPH. the boat didnt have a tach and I wanted one especially with the rebuild job so i installed one that was new but has never been installed, it has been sitting under a carpet in the damp weather. at wide open throttle having verified that both carbs are open fully, the tach only shows 2800 RPMs. I am thinking the speed is close to what It should be, but The 2800 RPMs is confusing me.

 

[SkiDad]

Your tach could be right. But your prop is not. You should be running an 9 pitch or around there.

 

[greasemonkeyjmj]

Thanks for the reply, I bought the boat at the end of last summer and this prop was on it already. I was figuring it was too much prop for it, but With there being no prop shop around, I will have to buy online. I cant afford to buy several props just to try and zone in on the correct pitch.

 

[steelespike]

I suspect the tach may be off low. Typical slip on a pontoon is about 20% or so.Yours figures at only 8%.
Many times odd slip numbers can be traced to the tach.If you operate the selector switch on the back it may work better.
If it's a universal tach it may be set wrong.
A dealer may have props and you may be able to try some props with the understanding your buying and a sizable deposit.
If your motor has a 2.42 gear ratio I think a 9 may be too low.
But your better off erring on the low side because you just need to watch the rpm. If too high you shouldn't use it.
Your slip is probably unusually high do to the high pitch and low rpm.
If your rpm was really 2800 you couldn't make up the rpm with a prop.
I suspect your rpm really is around 4000.a 9" pitch could put you at about 5200.

 

[greasemonkeyjmj]

Thanks Steelespike, Some of my problems are that there is no dealers anywhere close to me to be able to try out different size/pitch props, that is one reason I am trying to get as close as I can without too many trial and errors.Next problem is that my engine is an 87 model but someone actually put a 73 to 76 year model lower unit on this engine, they had to redesign the shift setup to make it shift in the correct direction, so I am not sure what gear ratio it is. I went ahead and ordered a new rectifier, I figured being that old it wouldnt hurt to swap it. and I also bought a new tach, since I knew this one had lots of moisture sitting in it when I got it.

 

[steelespike]

The 71-74 motors use a smaller prop 13 X 11.
75 on use 14 X 11.

 

[greasemonkeyjmj]

LOL, that is part of my problem, I am not sure what year my lower is. My engine schematic shows it should have a spot for the anode to fit into the lower unit and the series of round port holes above the cavitation plate. The lower on my engine has no spot for the anode, and has 4 vertical slots above the cavitation plate. I do know that I bought the factory parts to put my shift linkage back to stock, and when i did, it make my remote shifter work backwards, I had to put their homemade linkage back on

 

[greasemonkeyjmj]

No Title

 

[steelespike]

Dis regard what I said about 73 74 etc I was looking at Hydraulic units.
The blowup in my aftermarket manual the manual shifters prior to 89
the starboard side has a series of slits below the antiventplate (cavitation plate) and 4 vertical slots above the antiventplate.
Later lowers 89 on the port side has a squarish opening with a insert screen ? and 6 round holes above the antiventplate.
Just thinking generically I wonder if your remote shifter can be reversed.Either internally or by fliping it end for end?

 

[greasemonkeyjmj]

Hi again, Steelespike, as long as I can keep using the homemade shift linkage on the side of the engine, I am not to worried about changing it all around, my main reason for wanting to find out what housing I had was to be able to get order a prop and get the correct one for my case, which I assume is going to be considered the large gearcase, it is a 13 spline shaft, it measures best I can tell from the cavitation plate to prop shaft center is 7 1/4 inches, and across the hub area is 4 1/8 inches. would you say I could go up to a 14 inch prop, or would you say stay with something smaller, Right now I am running 13 3/4 inch diameter by 15 pitch

 

[steelespike]

Based on your present prop 13 1/4 " diameter I think it is safe to say you have a large gear case.
So yes a 14" diameter should fit.
The small gear case uses nothing bigger than 12 1/2" diameter.
If you look up props for 48 hp you'll note they use the small prop.
I'm thinking perhaps you have a commercial lower.That may account for the odd ball shift setup.
I'm guessing it's likely you have a 2.42 gear ratio. that and the large gear case is an advantage for a pontoon motor.
I think if you look up the commercial 50 you'll have the right prop.

 

[greasemonkeyjmj]

Good morning Steelespike, I had went ahead and ordered a Hydrus pontoon and workboat prop for the large gear case, I went a 13 7/8 x 9, I figured that would be a good starting point, and I will experiment from there.

 

[steelespike]

A good start I'm guessing it may be a little light but better light than too much.
just don't over rev too much.

 

[greasemonkeyjmj]

Yeah I am going to keep and eye on the revs, the 2800 rpm was an unloaded boat, just my wife an I, no cooler, grills, fishing equipment or any other things to weigh it down, so I was figuring after I add all of those things, that it would change again

 

[steelespike]

The 2800, the tach isn't accurate.
Are yoiu going to replace the tach or see if you can get it to work right?

 

[greasemonkeyjmj]

yes sir I have a new tach that will be here today, and will install it, I also went ahead and replaced the Rectifier to rule it out.

 

[greasemonkeyjmj]

No Title

ok got the new prop in today, and I am not sure if I like the way its made or not, on the rear skirt area of the prop where it fits up to the gear case, the skirt sticks out further than my gearcase does. the edge of the skirt area on the old prop was made so that it fit into the gearcase. my gearcase is 4 1/8 across, and so is the inside diameter of the new prop, but the outside diameter of the new prop is 4 3/8.

 

[greasemonkeyjmj]

ok guys, here is a not so good update installed the new tach and new rectifier, ran the boat with the 15 pitch prop and it still turned the same 2800 rpm's. so i ruled out my old tach and/or rectifier being faulty. put on the 9 pitch prop, gas on it and could tell it was spinning up faster than before, but it still only turns up 3500 rpm with one person on board. I am running out of idea, anyone have any new ones for me ?

 

[steelespike]

We need your gps speed at 3500 rpm.
Based on your test it appears to me something may be up with the motor.
Don't asssume that just because it was rebuilt everything is right.
See if it responds to a little choke.
Test that both plugs are firing.
There are folks that didn't realize they were down a cylinder but for the lack of power.
Disconnect and ground one plug wire at a time see if the motor will start and run.
If it runs on each then ignition is good.
Be sure the link and sync are set right,timing set right.
Check that the carbs open all the away.
If all else fails try it with the cover off.
A compression test won't hurt.
When you do the compression test note the condition of the plugs.
color? wet? dry?

 

[greasemonkeyjmj]

gps showed my speed at 13 mph, i rebuilt the engine, but I am not going to say i couldnt make a mistake, it dies out with a choke bump,both plugs are firing, will blue spark across 7/16 gap, will run on each cylinder one at a time, did link and sync by omc manual directions, used joe reeves wide open timing method, carb butterflies open all the way, compression is 147 on both cylinders, plugs are medium brown color mostly dry. a couple of thought, i had a bad carb after the rebuild, i bought a pair that were supposedly from a 1985 or 1986 50 hp, maybe possible they were not and the jets are too small ? when i went through the carbs i couldnt get the main jets broke loose so i soaked them with the bowls and made sure they were perfectly open, any idea on what number they should be ? also when i did the wide open timing check with the engine not running, but with the plugs out and timing advanced fully, it was on around 18 degrees or so, i brought it back down to 15, do you think maybe something didnt do correct and it is actually it too far down now ?