New 125hp no more powerful than my old 85hp???

[jbwillis76]

Hi all,

Just swapped from an '86 85hp Force to a '89 125hp Force on my '87 VIP fish/ski 16" (small/light boat).

Performance with 85hp:

5500 rpms , 38mph (GPS) , 13x17 Stainless Cupped Michigan


Performance with 125hp:

4900 rpms , 38-40mph (speedo-always matched the gps above +/- 1mph) , 13x19 Stainless Cupped Michigan

Something just isn't right. The 125 doesn't seem to have the power it should. Heck my 85 would turn that 19 pitch prop 4500-4800 if I remember correctly. The 125 is slow to plane, feels like it's slipping or surging but sounds like it's running like a top. Plugs all looked good. A little carboned up but it is firing on all 4. Link and sinc is good and timing set to 28* BTDC cranking. Compression is 140 on all 4. Will die and be hard to start after an extended wake zone.

Funny thing is this is the best cranking motor I've ever had. In fact you can let the boat sit for days then come back, hit the switch (not cold crank rpm) and it will fire up after about 1/2 second. Bulb always stays hard.

I'm tempted to just put my 13x17 stainless on it but I feel I would just have exactly what I had with the 85 and that isn't the reason I swapped motors. I do understand the 125 has taller gears but it should still turn this 19 at 5500rpm no problem on this small boat.

Maybe I should swap plugs and do a decarb but I don't believe it would help that much. Any suggestions??

Thanks as always, James

*EDIT* Just did a decarb for the heck of it and when I would spray it in the top carb the motor would stumble and sputter as it should. When I sprayed in it the bottom carb the motor didn't "cough" very much at all. I know it's getting spark, wonder if one of those plugs were already fouled making the performace the way it is?

Would still like some opinions on my motor and prop choice however.

 

[Frank Acampora]

Re: New 125hp no more powerful than my old 85hp???

In some design respects, the three cylinder engine is better than the four. It has to do with port timing and overlap. Don't forget, the 125 weighs a bit more than the 85 too. And, on a smaller boat, added weight affects the way the boat planes and top speed more than it does on a larger hull.

However, with a 40 HP gain, I still would expect to see a little more than a 2 MPH increase, even though small boats do not respond to a horsepower increase the way you expect. Example: My small 14 foot flat bottom hull did 55 with a Force 90 on the back. With a 125 hanging there, speed only increased to 60 MPH.

Do you still have the 85? If you do and IF it is a 2 to 1 ratio lower unit, swap lower units. You will probably see a 500 RPM gain with the 2 to 1 and the Michigan 19 pitch prop. You should also see a 3-5 MPH gain.

Note that the 125 is a small bore engine: 3.3125 diameter. This has engineering, torque and horsepower consequences. If you are not turning the engine at or near the top of rated RPM of 5500, you won't develop all the 125 ponies. Personally, I believe the 3.375 diameter, big bore 120 would outperform it. I have seen some 125s and heard of others that just are not capable of swinging big pitch props--for example: a 125 on an 18 footer needs a 15 pitch prop, simply won't turn a 17 pitch.

Mind you, I don't think the 125 is a bad engine, I just think the big bore Force 120 or Chrysler 140 gives more bang for the buck.

Now, I did an intake modification (on a modified 140) that increased top RPM by 500, but did negatively affect idle performance. It involves routing the oil recirculation hoses from the bypass covers to the intake manifolds in front of the reeds. I have yet to try it on a stock 125and have also not tried it on later model engines where the oil recirculation system is different.

 

[jbwillis76]

Re: New 125hp no more powerful than my old 85hp???

I still have the 85 but put it on the bigger bayliner boat and lord knows it's going to need the 2:1 gears to help it out.

I'm not above swapping lowers but would you think the 13x17 prop would do just about the same thing on the 125?

Thanks for your help Frank, James

 

[Frank Acampora]

Re: New 125hp no more powerful than my old 85hp???

No! It has been my experience on a number of Chrysler and Force engines that the 2 to 1 lower unit increases performance.

It takes a certain amount of horsepower just to turn the prop through the water. Turning a higher pitch prop a bit slower has less parasitic drag so more horsepower is available to move the boat rather than wasted in spinning the prop through the water.

 

[jbwillis76]

Re: New 125hp no more powerful than my old 85hp???

Thanks Frank. James

 

[jbwillis76]

Re: New 125hp no more powerful than my old 85hp???

Frank,

Because I'm hard-headed and lazy I swapped props instead of lowers simply because it was the easiest thing to do tonight.

Took it out and something just doesn't add up. I got 5200rpms at 42mph (speedo), 17P prop. I'm sorry, but I just can't be convinced that adding 40hp to the same boat, using the same prop (17P) off the 85hp and it won't do better than that. The 85hp would turn it 5500-5600. I FULLY understand the gearing difference in the two lowers but dang it's 40hp more and running in it's high rpm range. Heck, this motor was originally propped with a 17P on a 19 1/2' Bayliner Capri that's no telling how much heavier. Surely this doesn't sound right to you does it Frank? I will say if you tell me that's all it will do I will believe you and give up.

Decarbed before I took it out this last time and installed new plugs before heading out. I still swear it sometimes slips/surges while taking off and right before reaching WOT. Got home and did a compression check and here are the new readings which are now worse.

1. 137
2. 137
3. 130
4. 130

Plugs 1 &3 were heavily carboned, while 2&4 looked just about right, maybe a little too clean. Would have rather they all looked carboned up especially since decarbing. Extended idling and it will die going into gear, as well as recranking I have to high rev it for about 5-10sec. to clear it out before it will idle right.

Frank, something isn't quite right. Any other suggestions. James

 

[Frank Acampora]

Re: New 125hp no more powerful than my old 85hp???

Speed and horsepower are NOT linearly related. As speed increases linearly, horsepower requirements increase by the square. Thus: It takes roughly 4 times the horsepower to double speed. So 30 % more power will not give 30% more speed--more like 10%, which is about what you are seeing.

Going from 90 horsepower on my 14 foot boat, to 125 HP only increased speed from 55 to 60, and I was satisfied.

 

[jbwillis76]

Re: New 125hp no more powerful than my old 85hp???

I understand Frank. Though I'm really not interested in speed that much. My concern is why the motor won't turn a 17P prop on this light boat. I don't care if it only goes 30mph tops, as long as it's turning at 5500. Reading through numerous post I see where peoples 125 will max out (5500rpm) with a 17P on a much larger boat. Speed aside, this motor should turn it to full RPM and it isn't, something isn't right.

James

 

[sho3boater]

Re: New 125hp no more powerful than my old 85hp???

I'd start with a typical tune up, cleaning the carbs and checking the rest of the fuel system. Then check timing and spark on each cylinder. If it has two carbs, they can run the top cylinder for each leaner and the bottom might rinse the plugs more being richer. I'd reset the idle jets too. To read plugs cut it from wot and shut it down quickly as safely possible.

Are you sure you didn't end up with the LU lower in the water? I raised my 85hp so the cav plate is just above the pad (1/2" spacer plus some sealant) and it runs 41-43gps on a 16 BR capri bayliner. Have a worked 19p SS MI Wheel prop, thinned and more cup to get rpm down to about 5200-5300. Its a fun little boat. Sometimes the prop will slip when trimmed up and turning, it runs best just under max trim. I figured if I dropped a 4 cylinder powerhead on I would be pushing 50mph easy, but who knows. With a Merc 115 on a larger 17 with no pad I ran 50mph, this hull should be faster without a doubt....but I don't know your hull. Still, I'd say at least 5mph gain, its not running right or something.

 

[jerryjerry05]

Re: New 125hp no more powerful than my old 85hp???

Maybe I missed it.Did you change powerheads or motors?The lower for a 85 is geared different than a 125.
At 5200rpm you still aren't getting full rpm or performance.
Make sure the timing is set right.J

 

[sho3boater]

Re: New 125hp no more powerful than my old 85hp???

He changed the motor but either way he is not getting the speed increase.