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Not fast enough!

1986DIPLOMAT

1986DIPLOMAT

Member
Ok so I started with my 1986 17' diplomat with a 1986 evinrude 90hp outboard got about 34mph outa it best figured id do some upgrades, picked up a 1979 140 merc and installed it with a jack plate, has 5" setback and is jacked up about 4" prop shaft is about 2.5-3" away from the keel. Tried it out last night finally and could only get 37mph outa it running a 21p prop. Motor only sounds like its turning 3000rpm (i dont have a tach on it but it does not sound like its screaming like the tower of power usually does) Just wondering does anyone know if these older outboards have a different ratio gearcase than a newer merc? My buddy has an 86 115 merc and runs a 22p on his 17' glastron gets mid 50s with it... Im gonna drop it back to a 17p and try it i guess.
 
Gotta get a tach on it, they're cheap. At this point you're just pi$$ing into the wind with no info.

John
 
Diplomat Speed

Back in the day I had a 1984 Diplomat w/ a 1985 140HP Johnson. If I remember right I was running a 19" Spinelli with a CMC jackplate. When I raised the motor with the propshaft just below the pad and alot of trim the boat would run in the low 70's----But it would Really Chine-walk almost to out of control.I would guess you need less pitch--mine ran 50 w/ an older 85hp before the motor upgrade.Good Luck & enjoy!!!
 
Christo680 said:
Back in the day I had a 1984 Diplomat w/ a 1985 140HP Johnson. If I remember right I was running a 19" Spinelli with a CMC jackplate. When I raised the motor with the propshaft just below the pad and alot of trim the boat would run in the low 70's----But it would Really Chine-walk almost to out of control.I would guess you need less pitch--mine ran 50 w/ an older 85hp before the motor upgrade.Good Luck & enjoy!!!
Click to expand...

It just doesnt make sence to me I had the 90hp and only got 34mph is my boat filled with concrete?? lol I want to get into the mid 50s i dont think its much to ask outa the setup.
 
ckaluger said:
The motor cannot be running correctly!!
Click to expand...

Compression is 130-140 across the board just installed new timing trigger and resynced timing rebuild fuel pump and carbs runs and turns up great outa water. This prop im using was for a newer outboard i had to swap anode plate cuz it wouldnt fit with my factory one, i never had a prop that came with this motor so i had no idea where to start.
 
I believe those older Mercs were rated differently. They were rated 140 at the crank. Later... like 1986+ they were rated at the prop shaft.

So that 140 hp is more like 115 at the prop. That being said... you should still see 50 mph.

Are you using any trim?

Is the boat heavy with water under the floor?
 
illMATTic said:
I believe those older Mercs were rated differently. They were rated 140 at the crank. Later... like 1986+ they were rated at the prop shaft.

So that 140 hp is more like 115 at the prop. That being said... you should still see 50 mph.

Are you using any trim?

Is the boat heavy with water under the floor?
Click to expand...

Correct pre 83-84 were rated at crank but its not that much lost and plus the 140 is just a rebadged 150 with electronic ignition, they did that so they could introduce the v6 150 and people wouldnt confuse them. it might be around 130-135 at prop that being said..

My trim is currently in the works had it on second bar hole my 79 motor has remote trim pump setup which i have not got done, i have a newer style trim setup from an 86 (pump and hydraulics on the tilt brackets) which i might just swap it over to if this motor works out for me. But regardless should only be like 5mph or so more trimmed up to being down.

There is no water in the hull and it really doesnt feel like its a heavy boat moving it around, in fact it almost tries to bring the bow off the trailer now with the extra weight off the back w new motor, and it pulls right up outa the water and gets on plane no problem even though it wont really turn up. but its always a posability that it could be heavier than normal water soaked into fiberglass ect...

Ive got new gauges w tach and a 17p prop on the way hopefully that will get me closer testing wise, if i cant get mid 50s outa it at least gonna end up going a different route with the whole thing.. :brickwall:
 
I used to get 52 MPH out of a Johnson 90 with a 19 pitch SST . Granted my hull is a little narrower than yours and a little lighter but even fully loaded with kids and gear it would still top 48 MPH. It never got past 62 MPH with a fresh 140 Johnson or a 1500 TOP.

I think your prop sucks for one thing. Some of those factory aluminum props were completely worthless for everything.

There were more differences in the 1500/150's and 140's/115's other than badge-ing. The later motors had significantly lower compression and less standard timing spec'd than the earlier motors. Had a lot to do with the quality of gas in the 70's taking a nosedive.
 
You will also see more than 5mph going from 0 trim to properly timmed! I have seen 20mph before. 55 with no trim and 75 all the way up.
 
I agree, a lot more than 5 mph to be had by properly trimming.

I think that motor has about a 1.87 lower and assuming a somewhat conservative prop slip of only 10% you'd have to turn 7100 rpm to see 55. Certainly not gonna happen. Not trying to throw a wrench into your plan but just being a realist.

John
 
wired said:
I used to get 52 MPH out of a Johnson 90 with a 19 pitch SST . Granted my hull is a little narrower than yours and a little lighter but even fully loaded with kids and gear it would still top 48 MPH. It never got past 62 MPH with a fresh 140 Johnson or a 1500 TOP.

I think your prop sucks for one thing. Some of those factory aluminum props were completely worthless for everything.

There were more differences in the 1500/150's and 140's/115's other than badge-ing. The later motors had significantly lower compression and less standard timing spec'd than the earlier motors. Had a lot to do with the quality of gas in the 70's taking a nosedive.
Click to expand...

It was a michigan 21p i think it was a larger diameter that what mine ran stock too.. either way things arent right we will just have to wait and see once i get more stuff lol. didnt wanna go all out buying a nice prop until i knew where i wanted to be or if this motor is gonna work for me..

on another note i was running the 1986 evinrude 90 with 19p n only could get about 34mph outa it.. but it wouldnt trim up without cavitating so i think it had other issues too
 
atc250r said:
I agree, a lot more than 5 mph to be had by properly trimming.

I think that motor has about a 1.87 lower and assuming a somewhat conservative prop slip of only 10% you'd have to turn 7100 rpm to see 55. Certainly not gonna happen. Not trying to throw a wrench into your plan but just being a realist.

John
Click to expand...

So what are you saying? it will never get there?

I have a gearcase off a 1986 115 merc is that different ratio?
 
I dont think thats true at all . Ive gone 75 MPH with a 4 blade 21 pitch prop. Granted it was attached to a 260 at the time but it was only turning 7300 RPM with a 1.87 lower trimmed all the way down. My steering at the time was the God Awful factory checkmate steering and I couldnt trim it up any or it would start "Chine Walking" something fierce. Funny how changing my steering completely fixed my chine walk :).

Youve got a 2:1 lower on there unless someone switched it to a 2.3 inline 4 lower.

A 2:1 ratio lower with 10% slip at 6000 RPM and a half decent 21 pitch prop will go 54 MPH. I typically get 7% with my choppers or ET's running them at or a bit above the surface. With a 23 pitch laser 2 buried DEEP I get 5% slip at 6600 going 73 MPH on a 1.87 lower.

http://www.mercuryracing.com/propellers/propslipcalculator.php
 
wired said:
I dont think thats true at all . Ive gone 75 MPH with a 4 blade 21 pitch prop. Granted it was attached to a 260 at the time but it was only turning 7300 RPM with a 1.87 lower trimmed all the way down. My steering at the time was the God Awful factory checkmate steering and I couldnt trim it up any or it would start "Chine Walking" something fierce. Funny how changing my steering completely fixed my chine walk :).

Youve got a 2:1 lower on there unless someone switched it to a 2.3 inline 4 lower.

A 2:1 ratio lower with 10% slip at 6000 RPM and a half decent 21 pitch prop will go 54 MPH. I typically get 7% with my choppers or ET's running them at or a bit above the surface. With a 23 pitch laser 2 buried DEEP I get 5% slip at 6600 going 73 MPH on a 1.87 lower.

http://www.mercuryracing.com/propellers/propslipcalculator.php
Click to expand...

So do you think this motor should turn a 21p prop? Do you know what this inline motor should be turning for rpms? I was told around 5800??

Also if anyone can help here I got thinking I was only told this is a 21p prop.. there's no numbers on sides of prop like normal just inside hub has michigan 031044 stamped.. wondering what it actually is..
 
Get a different prop and a tach that works. You should be pushing that motor 5800-6000 . Bad aluminum props will run you in all sorts of circles. If it were mine I'd be running a Solas 19 pitch 4 blade stainless Titan 4. You'll go over 50 with a great holeshot IF your motor isnt having problems..
 
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