1998 Force 120hp. runs rough mid RPM

[crashpro]

I have a 1998 Force (says powered by Mercury Marine) that is on a Nitro 170-LX that I just purchased. It starts easily, idles great, and smooth at WOT, however when accelerating from idle (either full throttle application or mid-way), it misses terribly and shakes the whole boat. Once up on plane and RPM increases above 3800, it smooths right out. Even misses while running at a constant 2000-3800 rpm without adding any more throttle. Compression is 123-125 on all cylinders, has new plugs, fresh fuel, and has been decarboned. Previous owner told me that the fuel that was in it was 3 years old, but had been stabilized. I siphoned it out before adding a fresh tank of fuel before I even ran it. Also, how hard should the water "pee" out at idle. Mine spits at idle and is a smooth flow as throttle increases, but is not forceful. My brother's 90hp evinrude flows so hard that it kind of stings your hand a little. Last thing: where do I find a manual for this motor?

 

[droach77]

Re: 1998 Force 120hp. runs rough mid RPM

If it were me i would get two carb gasket kits pull the carbs and clean them and link and sync, If the gas in the tank in the tank was 3 years old then problably gas evaperated out of the carbs possibly leaving varnish and blocking the mid range cicuit passages, same goes for the water pump impeller. You would be surprised what a little bit of junk in carbs will do to an outboard motor and change the way it runs. good luck with it


dave

 

[crashpro]

Re: 1998 Force 120hp. runs rough mid RPM

Sounds logical. Do you know of a good outlet for parts and do I look under "Force" or "Mercury"?

 

[droach77]

Re: 1998 Force 120hp. runs rough mid RPM

At the top of the page on the left under iboats.com click on marine store. Then to the right of the middle of the page look for outboard motor parts click that scroll down and every thing you need is right there, you will see force category. Some internet stores you can find stuff under mercury if you do not see force, they have force in a subsection.


dave

 

[crashpro]

Re: 1998 Force 120hp. runs rough mid RPM

Thanks for all of your help, Dave. I'll give it a try and let you know here in a few days how it works. I found a Tracker Marine parts store about an hour from home near a Bass Pro Shops store in Cincinnati. I'll go there tomorrow and get the gaskets. I will, however, bookmark the section you mentioned for future reference.

 

[jason32038]

Re: 1998 Force 120hp. runs rough mid RPM

Stabilizer doesnt last very long..atleast not for 3 years. If the fuel in the tank was 3 years old then that means that the fuel in your carbs was the same age. Most likely the carbs are gummed up and need cleaning.

 

[crashpro]

Re: 1998 Force 120hp. runs rough mid RPM

I bought a gallon can of that stuff that you put the carbs down into it in a plastic basket and it cleans them inside and out. I used the same stuff on a carb from my generator and it was awesome. If it doesn't work, I'll tear them down and rebuild them. There is rain in the forecast for the next two days here, but as soon as I find some results, I'll post it here.

 

[crashpro]

Re: 1998 Force 120hp. runs rough mid RPM

Well, I bought the can of parts and carb cleaner and soaked each carb for 30 minutes. They both came out looking like brand new. I followed Dave's suggestions and set up the carbs as he recommended. Took it out today and it ran like a new engine. No hesitation, lots more power, and best of all, no smoke. I'm very happy and thanks to all of you for your help.