How can I prevent this - Force 70 Low Compression

atengnr

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Hello. I picked up 2 parts motors, 1995 70 hp forces over the weekend. One of the motors was purchased new by seller, who admits he used quicksilver oil, mixed correctly for its life. Motor did well for years, but eventually lost power. I pulled head just to peek at it, and top cylinder seems to have a damaged top ring land (actually looks like ring broke piston in a small area. What causes this? The second motor I believe also suffered a similar fate. It seems that Ive seen a fair amount of this on forum pictures.
 

atengnr

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The Force power

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Hello. I picked up 2 parts motors, 1995 70 hp forces over the weekend. One of the motors was purchased new by seller, who admits he used quicksilver oil, mixed correctly for its life. Motor did well for years, but eventually lost power. I pulled head just to peek at it, and top cylinder seems to have a damaged top ring land (actually looks like ring broke piston in a small area. What causes this? The second motor I believe also suffered a similar fate. It seems that Ive seen a fair amount of this on forum pictures.

I think JerryJerry has explained this in your other post (Force 70hp general question)
 

racerone

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Running the wrong prop for the boat / load.-----Running without a tachometer.------Running wide open throttle at the wrong RPM.-------Running wide open throttle before motor is warm.----Etc, ETC.------Folks want to have fun and do not know the fine details of operating a boat motor.
 

atengnr

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When did design change occur to correct this ring and port issue?
 

The Force power

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I believe on the '95 units Mercury has done the update. regardless of the update done or not; I think racerone has pretty much sum it up in post #6, he really knows his stuff, so you can take his word for it. (lord knows I do)
 

DunbarLtd

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Running the wrong prop for the boat / load.-----Running without a tachometer.------Running wide open throttle at the wrong RPM.-------Running wide open throttle before motor is warm.----Etc, ETC.------Folks want to have fun and do not know the fine details of operating a boat motor.

Would you be willing to elaborate a little because I think I may be running under what is suggested on my 85hp 3 cyl. I have a tach that is accurate and i have been getting 4.2k @ wot. Using a 17 pitch prop. So looking up the specs i believe i should run around 4.5k-5.5k range. If I am just under the range i assume im doing damage?

I dont want to hijack this thread to help with my issues Im just curious if you could elaborate a little to help better understand this issue. Not everyone is an expert with these engines.
 

racerone

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You should be running over 5000 RP.-------If not you are " lugging " the motor !----You have the wrong prop.----Or there is something wrong with your motor.---Or something wrong with your boat.-----The decals say 85 HP but a dynamometer or test prop will tell you what it is putting out today !
 

jerryjerry05

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The 70 has one carb.
The top cyl. isn't getting the lubrication the other cyl. are.

The 85 is actually a 90hp. they rated it at the prop.
WOT should be 53-5500
What's the speed?

The boat and load and if the motors operated right can affect the rpms.
You using the trim?
Get a tiny tach make sure the one you have is reading right.

Load, 2-3 chunky people, dog, cooler full of beer and the regular equipment.
Might be too much?
Size boat?
Water logged foam? (Cut up a 21 and the foam had about 700# of water soaked in the foam.
Turns out the manufacturer made this boat without a way for the water to drain.? )
 

flyingscott

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Force were never rated at the prop. There is a dyno test of a 120 force in B@WB from the 90s showing a 120 force making 107 prop shaft HP. And for those of you who don't think that is true here is another story. In 1996 I bought a brand new starcraft ss 170 fiberglass with a Mercury 125 2+2 motor. My friend bought the Bayliner twin to it with a 120 force he was so happy he paid less than me for the same power. His boat was significantly slower and had less power more than a 10 hp difference. He was about 8 MPH slower than and way slower hole shot. Force motors were never truly propshaft rated and Mercury made them worse. They are not bad motors but they were cheaper for a reason. Sorry to burst your bubble farce.
 
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jerryjerry05

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When they changed from the 85 to a 90 they explained they were rated at the prop and
then they would rate them at the motor raising it 5hp. I saw the bulletin.
They also said the 125 wasn't a 125 but closer to a 120 ??????

17ft Bayliner with a 120??
 

flyingscott

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There is more than a 5 hp difference between a powerhead rating and a prop rating. The force motors were never as powerful as the major brands offerings.
 

The Force power

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So.. what was the difference between the 90 & 85 HP if the fuel-jets are the same?? where did the 5 hp come from?
 

racerone

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Well----Maybe the marketing folks pushed for a 90 HP decal.-----Easy way to boost sales perhaps.
 

The Force power

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Well----Maybe the marketing folks pushed for a 90 HP decal.-----Easy way to boost sales perhaps.

lol

I think there was a bit more to it, there were a lot of carb. changes back then
I'll keep digging in the archives here
 

flyingscott

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Well farce the 70 hp Johnson/Evinrude motors had a lot of different carb changes and they were never more than a 70 hp. The Mercury 4 cylinder 2 carb 85 hp was re-rated to a 75 hp prop rating. You have a Force motor a relatively simple easy design to work on but low on power compared to more modern designs. The original design of that motor was from the 60s and never really updated. There is no magic bullet on a piece of paper that will make your motor more powerful. Run it and be happy.
 
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