Changed lower unit...Now it won't go

I've pulled my hair out and spent almost as much as I paid for the boat trying to figure this out. I have a 78 Terry 15ft bassboat with Mercury 850. I have a 4 blade 17 pitch prop. It ran great for 4 years after I bought it, GPS read 39-40mph with two 200 pound guys it it. Reverse went out and the mechanic said it would be cheaper to put on a used lower unit than replace the gears. So....he put on the used lower unit and when I put it on the water it it would get up to around 1800 rpm and bog out. had the cables replaced, the carbs rebuilt and still 1800 rpm is all I will get. It will bog around all day at 1800 but will not go higher. The mechanic who rebuilt the carbs (not the one who replaced the LU) hooked it to a dyno and said it was putting out something like 78hp at the prop shaft. He suggested that the new LU may have a differnt gear ratio than the old one and I need a new prop with 15 or 13 pitch. Before I spend another $115 on a boondoggle, can anyone give me some advice? It is killing me as I love the boat and it ran soooo good before this ordeal started. Thanks and this is a great resource.

Re: Changed lower unit...Now it won't go

To get from 1800 up to 5000 by changing from a 17" prop, you would need a prop with a pitch of 1". Now that, I'd ilke to see. Gear ratios...don't come in that big of a range...within 1.8:1 to 2.0:1 - to rob you of 65% of your RPMs would probably take an overdrive gearcase. No, I take that back...I don't believe it's possible.
the carb guy's story sounds a little...fishy...
What rpm did he take that dyno measurement at?

The ratio you have now is easy enough...put it in forward, rotate the crankcase (clockwise) all the way round, count how many times it rotates for one revolution of the propeller. Look up your engine's stock gear ratio.

I'm thinking, back to basics:
Compression, spark, fuel, timing.

Keep us posted...

Re: Changed lower unit...Now it won't go

Motorrad, The 850 Mercs had a unque gear ratio, compared to the other 4 and 6 cylinder Mercs. It was 2.3::1 (I think), whereas the rest of the LU were 2::1. Also, if you over prop those motors it was not unusual for them to refuse to rev up. If I were you, I would borrow a used 15" pitch prop from a dealer to test. If it performs well, you can buy one.

Re: Changed lower unit...Now it won't go

curious...not disputing Chris, just wondering...isn't increasing the gear reduction ratio analogous to reducing the prop pitch...I mean, wouldn't that unload the engine, vs. loading it up?

interesting scenario...

[edit...added 8/28...duh!!!...learn something new ever day...]

Re: Changed lower unit...Now it won't go

Yes j it is.

I had an 85 4 cy Merc back in the early '80's on a 16' BB and ran a 13x19 for 39 mph. Had no tach so I don't know what the rpm's were. Engine was running real good; singing pretty.

My current 3 cyl 90 hp is a 2.3:1 gearbox. The lower unit is not of the same design as the 4 cyl so I don't know if they dinged with the ratio or not.

I just went to the BAM prop slip calculator to see if I could determine what my gear ratio was but without knowing my rpm's it could have been either. I could have been running anywhere between 5000 and 6000 and it does make a difference.

I think Merc does run the higher ratio gearbox on the midsize range of engines. The 50-60ish hp range runs a 10" prop and high gear ratio. Then the next range (3 cyl 75-90) runs a 13" prop with a high ratio for pushing big loads, and the bigger engines go back to the 2:1 or less as they get higher in hp. The older 6 cyl's ran a 2:1 as I recall because I was pushing a 19" prop over 40 mph on a heavy 17' boat with my 115.

Best I can do.

Mark

Re: Changed lower unit...Now it won't go

Jtexas and Motorrad, Just to be clear, when you changed the LU, the gear ratio went from 2.3::1 to 2::1. The have the same effective overall pitch your prop must have less than 17" of pitch. A 15" pitch prop is the next closest.

Re: Changed lower unit...Now it won't go

Thanks for all the responses. I will borrow a 15" pitch prop and check it out this weekend and let you all know if that worked. thanks again for the help.

Scott

Re: Changed lower unit...Now it won't go

Everyone....Switching to a 13 pitch prop solved the problem. I took it out on the river today with a $50 used prop and she hopped right up on plane and ran up to 5200rpm. Thanks to all for the advice!

Scott