Force 120 - No Thrust

[ut_tran]

Has anyone out there experience this problem? I have a 1990 force 120 that doesn't seem to produce much thrust- can't get it to any faster than 10mph. Carbs been rebuilt,timing adjusted , lower unit checked and shifts fine, compression is between 125 to 140 on all cylinders, new prop hub. Starts and idles fine. At full throttle, engine revs up and prop spins and churns water- but won't move the very fast. I have twin 120's, and only one engine is doing all the work. Does anyone any one have any suggestions??

 

[texasabil]

Re: Force 120 - No Thrust

Im sure someone will chime in!!! what rpm are you getting I had a spun prop one time I thought it was the lower unit!!!! I was getting 5500 rpm's and I thought my gears where striped replaced my prop and no more problems just a thought!!!

 

[ut_tran]

Re: Force 120 - No Thrust

Im getting roughly 5500 rpm. My thoughts were like yours, spun hub. Ordered and replaced with new hubs, same result. Weird thing is when at full throttle, I can see it churns water like my other engine, but much less thrust. Huge difference between engines. Also, its not the lower unit, because I swap lower units with same result.

 

[dwco5051]

Re: Force 120 - No Thrust

Just curious, are both props the same as to diameter, pitch, and rotation? You say you have swapped lower units and the same engine acts weird. Does that happen if you just switch props?

 

[texasabil]

Re: Force 120 - No Thrust

Thats is very strange!!

 

[antique fisherman]

Re: Force 120 - No Thrust

Just a thought. >>> if you have counter-rotating engines, the one with no thrust could have the wrong prop on it.

 

[87blf125]

Re: Force 120 - No Thrust

if i read this right you have 2 of the exact same props on the motors, i am not sure but i would think if you have counter rotating motors would you not have a prop that was the other direction than than the other. my thought is if the props sre the same but motors are turning dif directions you would have 1 prop spinning the wrong way?

 

[jerryjerry05]

Re: Force 120 - No Thrust

Forces don't have counter rotation.Is the exhaust snout still on that motor?
Is the linkage connected right.No broken plastic connectors on the throttle linkage.The trigger is it advancing all the way.
It does it on the same motor even when you switch lower units?
Fill out the profile. ANY other info you can give,list it.Jerry

 

[j_k_bisson]

Re: Force 120 - No Thrust

Had the same porlem with my 1991 Force 120. Your compression numbers say it all. Need to list the compression on each motor for each cylinder. It sounds like you have a scored cylinder. My readings on my one motor was 145, 140, 125, 140. Turned out that I had a ring that had gone south in the one cylinder. It had scored the cylinder wall. This was leading to blow by and low torque condition. Would run great, just no power. You might want to pop the head off the motor and check the cylinder walls for soring. For a 25-30 buck gasket it is nice to know what is truely going on. Also if it is a bad ring, running it much longer will end up costing you more money in the long run to fix.

Just my opinion.

 

[havasuboatman]

Re: Force 120 - No Thrust

put the motor in forward gear, engine off, KEY OFF, and try to turn the propeller backwards. It shouldn't turn without trying to rotate the engine.
If you are turning 5500 rpms while only going 10 mph, either the prop is severly damaged and cavitating, way under sized/pitched, or a left hand rotation propeller.

 

[healey8390]

Re: Force 120 - No Thrust

That must be a monser of a boat if one engine will only push it 10mph. Probably a bayliner 2509 if i had to guess. Im leaning toward havasuboatmans take and jk bisson. Stumper???

 

[CharlieB]

Re: Force 120 - No Thrust

put the motor in forward gear, engine off, KEY OFF, and try to turn the propeller backwards. It shouldn't turn without trying to rotate the engine.

Just to be sure, pull the plug wires but leave the spark plugs IN, you want the cyl pressure to provide resistance to the motor.

A stretched shift cable may not be holding the gearcase fully into gear and may allow 'slipping' once under load.

STEP on the prop blade and MAKE the engine turn, turn the prop CCW when shifted into Forward then repeat test in Reverse and stepping on the prop to turn CW.

 

[CharlieB]

Re: Force 120 - No Thrust

Is this problem only in Forward?

Is the prop thrust washer correct?

Too thick, or doubled will allow exhaust gases to cavitate the prop.