1988 Force 125 Model 1251A8B gear ratio

gt03 · Feb 1, 2005

I purchased a 1988 Maxum bowrider 20' with this motor (s/n 1075)on it. The last owner had a Michigan SS 19 pitch prop installed (no other props came with the boat) and I have no idea what the original prop pitch was. I purchased a SELOC manual from iboats but still could not find the gear ratio listed. I need this number to calculate the correct prop pitch. My WOT is only at about 4200-4300 rpm and I can't seem to get out of the hole quick enough to get a skier out of the water. Thanks in advance for your help.

Realgun · Feb 1, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

I think a 19 is the correct prop for your boat and H.P.. If your running 4200-4300 you about 200-300 on the low end. I would seriouly look at the plugs and carbs to be sure they are up to snuff. Replace plugs and rebuild carbs. First however do a compression check. This will tell you if you have a low cylinder. Second I would test for a good spark at all plugs. Good Luck and enjoy the boat!

Realgun · Feb 1, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

OH and BTW WELCOME TO iBOATS!!!!

gt03 · Feb 1, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

Thanks for the welcome! I did replace the plugs last summer. The ones I removed looked normal (no carbon deposits). I'm using NGK BUHX with a fixed gap. I also cleaned the fuel screen. Neither one made any difference. I didn't do a compression check gotta wait until spring for that unless a REALLY COLD check would work LOL. The operating range printed on the clamp bracket is 4500 - 5500 rpm. That's why I'm doubting the prop size.

roscoe · Feb 2, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

Thats a lot of boat for a 125 Force to have to push. "IF" all is right with the engine, you will likely need to go to a 15 or 16 pitch prop to get a decent holeshot. Where are you located in Wis? Is there a prop shop or dealer near by that will let you try few different props before making a purchase?

gt03 · Feb 3, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

We live in Sheboygan, WI. I would agree that a 15 or 16 might be the correct pitch. I'm unfortunately an engineer an wanted to run the calculation but without the gear ratio I'm stuck. And about the motor size, Maxum did that one... It's really amazing all the info one can find on the net about nearly everything EXCEPT boat/motor specifications, etc. I guess a simple question isn't so simple...

chuckz · Feb 3, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

As an engineer, I'm sorry but I have to embarass you. Set the flywheel, in neutral, to TDC. Put the engine in gear and turn the prop. Count how many turns of the prop it takes to return to TDC. Calculate the gear ratio. If you end up at TDC after a partial turn of the prop, you can keep turning the prop until after a full turn of the prop the flywheel ends up back at TDC. Just keep track of the flywheel revs also, then calculate the gear ratio. I did mine and came out to within .01 of the published gear ratio. I think that I found the ratio in my Seloc manual after I calculated it. The lower unit gear ratios are on page 3-32 of the Seloc Manual. Your ratio is 1.73:1

gt03 · Feb 3, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

You didn't embarrass me too bad about the engineer comment. I didn't say what kind of engineer I was. However, I am embarrassed about missing the page in the manual! Thank you for enlightening me! The calculation shows 17P for skiing, and 19P for speed, but the calculated speed is about 20 mph off... Oh well, what else are you going to do in Wisconsin in February??

Scaaty · Feb 3, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

Plenty of motor for that boat. Have the same 125 on my 19.5 (21 OAL)Bayliner. 17 pitch runs at 5200 WOT....19 too much for a 20, your'e lugging it.

roscoe · Feb 3, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

Yep, page 489 in the Clymers manual , 1.73:1 Sorry I didn't look it up for you earlier, but I didn't have my manual with me. Nice city Sheboygan, always liked it when I visited it back in the 70's and 80's. My great-great-grandfathers brother settled there. Recently hooked up with the family there after seeing one of them on tv.

chuckz · Feb 3, 2005

Re: 1988 Force 125 Model 1251A8B gear ratio

I found the gear ratios quite by accident. After I had calculated mine I was looking for something else. You would think they would be in the LU chapter. The variable you don't know is slippage. I used 15% and my calculated speed was very close to my GPS speed. Last question, since you didn't say what kind of engineer, does this mean you drive a train?

1988 Force 125 Model 1251A8B gear ratio

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