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1996 Force 120 issues

Z

Zlanetbl

New member
Hey friends, I'm new to the forum and needing some help if you'd be so kind!
Just picked up an older Nitro 17.5 foot with a 1996 Mercury Force 120. The engine runs fantastic at idle and seems to shift properly. Now here's my issue: When I put it in forward gear and begin accelerating, the motor barely reaches (approximately) 20% of its rpm capability. I put the motor at WOT and I'm barely moving 15mph at best.

To try to troubleshoot it, I beached the boat and removed the motor cover. Manually operated the throttle at the motor and it had no problem reaching a screaming high healthy RPM. So it's only in forward gear that this problem is occurring. What are your thoughts? Thanks!
 
Bad prop hub that's spinning. You can check this by making scratch marks on both the hub and prop and see if they move.

Jeff
 
fastjeff said:
Bad prop hub that's spinning. You can check this by making scratch marks on both the hub and prop and see if they move.

Jeff
Click to expand...

I feel if it was the hub or prop issue, I would still reach full RPM, but would fail to move very fast. But that's not the case. Motor in forward gear is staying at a low RPM even with WOT and the boat is moving in the water, but only slowly.
 
I'd expect fuel problems first. Get a can of Sea Foam aerosol...WW or auto parts store..SF contains mineral oil and you want that for doing the following with a 2 stroke. Remove the covers from the carb venturis and have someone operate the boat as you mentioned. Rapidly squirt SF onto the venturis of both carbs...back and forth in an attempt to get identical charges in all 4 cylinders. If the engine leaps ahead, you have bad fuel problems. You need to check anything internal that fuel touches from the tank to the intake manifold...where the carbs bolt on.

Agree not a prop slip problem.
 
Texasmark said:
I'd expect fuel problems first. Get a can of Sea Foam aerosol...WW or auto parts store..SF contains mineral oil and you want that for doing the following with a 2 stroke. Remove the covers from the carb venturis and have someone operate the boat as you mentioned. Rapidly squirt SF onto the venturis of both carbs...back and forth in an attempt to get identical charges in all 4 cylinders. If the engine leaps ahead, you have bad fuel problems. You need to check anything internal that fuel touches from the tank to the intake manifold...where the carbs bolt on.

Agree not a prop slip problem.
Click to expand...



Thank you, I'll definitely look into those things. I've also read somewhere It could be the shift cut-out switch being shorted, which will stop sparks on the 3 cylinders, placing the motor into SLOW mode. Do you know anything about that? How would I check that and where would I find it?
 
Know nothing about that. 120 engines have 4 cylinders last time I checked. Force is a basic engine design, no fancy stuff . I had an 85 hp 3 cylinder back in 1971 when it was a Chrysler product.
 
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