Johnson Gear Ratios

[ssf4444]

So I have this 140 hp Johnson. Its been having some speed issues (i have had a couple other posts on it.) So for now its got a 21 pitch prop (engine is on an 86 diplomat, the engine is of the same model year i think.)

Itll run at 38mph @ 5500RPM. Does anyone know what the gear ratio of this engine should be. Because after doing some calculations (provided every piece of data is correct) its telling me that my gear ratio is 3:1. Is this possible. For some reason I assumed these engines were a 2:1.


Any help or insight is much appreciated.
Thanks!

 

[JT Patroni]

So I have this 140 hp Johnson. Its been having some speed issues (i have had a couple other posts on it.) So for now its got a 21 pitch prop (engine is on an 86 diplomat, the engine is of the same model year i think.)

Itll run at 38mph @ 5500RPM. Does anyone know what the gear ratio of this engine should be. Because after doing some calculations (provided every piece of data is correct) its telling me that my gear ratio is 3:1. Is this possible. For some reason I assumed these engines were a 2:1.


Any help or insight is much appreciated.
Thanks!

I may be wrong, but I think most of the V-4 and V-6 OMC are at 1.87 to 1 ratio. If you can provide the model number that is on the left side of the motor clamp (facing the back of the motor) it will be helpful. I would think you should be closer to 44 - 46 mph with that prop. Is it an aluminium stock prop or a stainless Raker or? Was the speed taken from the speedo or GPS?

 

[ssf4444]

Its an aluminum prop, 12 3/4 diameter, bought it used, someone had ported it for hole shot. Speed was done w/ GPS.

 

[Bill Gohr]

Well a year would help

1977 1.92:1

1978 n up 2:1 small gearcase

1985 n up Looper large gearcase 2.25:1

You have something severely wrong, even at 2.25:1 you should be in the mid to high 40's

 

[Bill Gohr]

just reread your last post, with a 12 3/4 diameter that would have to be a small gearcase so it would be a 2:1 unless its a 1977 motor

 

[ssf4444]

Yes, but it came with a 13x17 prop. I know it needed way more than a 17 form the start though but still, 13 in diameter is only a quart inch bigger but it can take it, im not sure what the size range runs...

However after speaking with a guy at a local marina, he said to check the how high the engine is mounted compared to the cavitation plate. The plate is an inch higher than the bottom of the boat, however what is weird is that its pretty much as low as it can go. I can only get it a half inch lower and thats if I modify a steel plate the engine is bolted to on the transom.

Also (the marina guy told me to check for this too) It will get some cavitation with on a sharp turn with the motor trimmed all the way down. (he said if that happens, the motor is def. mounted too high.) As stated above its only mounted an inch high....

Another guy suggested getting a fin for it? Do yo think this would draw more water into the prop and thus helping to resolve the problem of speed and cavitation?

 

[Jon Kansanniva]

Try another tach, it could be off. Also, try another prop the hub could be
slipping. As bill said that combo should be close to 50 on that boat unless
your dragging the anchor.