1998 Force 120- Throttling Up and Down on its own

[Jdiraff]

Wondering if anyone can help with this ?
I recently bought a capri 1800 with a 1998 Force 120. The engine, sat for a while, so I replaced the carbs,fuel pump, fuel lines, filter, plugs & wires, water pump, and thermostat and replaced the tank with fresh fuel and treated engine with seafoam.

While running on muffs , engine starts right up , idles fine, but when I throttle it up to say 2,000 RPM and hold , the engine stays at 2,000 for a bit but then throttles up to 3, 4,000 rpms on its own without me touching the throttle control . It seems to go back down on its own , but I usually throttle the control back before it does to avoid possible red lining

Anyone ever heard of this or have a suggestion of what may be causing this ?

Thanks !

 

[Georgesalmon]

Re: 1998 Force 120- Throttling Up and Down on its own

Uhhh, put it in the water on a boat and run it with back pressure and a prop on it. I'm sure it will run fine.

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Thanks...not being an expert on this kind of stuff, I thought about that, but didn't realize the back pressure would correct that problem.
Now all I need is for the temp outside to get above 35 so I can drop the boat and I should be good to go.
Thanks again...

 

[pnwboat]

Re: 1998 Force 120- Throttling Up and Down on its own

Did you check the float level? Idle mixture? Speed should not change on it's own.

Here is a video of a Force 90 running at 2000 RPM's for a charging system test on muffs. Very little variation is speed. This is what you would expect.

1990 US Marine Power Force 90 - RadioShack Rectifier (3 of 3) - YouTube

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Thanks
I was told that those settings were preset in the factory since I installed brand new replacement carbs ?

 

[jerryjerry05]

Re: 1998 Force 120- Throttling Up and Down on its own

New carbs still need to be set. Turn air screw(both carbs) in to the bottom(gently) and then out 1 turn.
Then adust from there.
Motor warm.In gear. Running. Turn top air screw in till it begins to buck,stall,stutter. Then back out 1/2 turn.
Do this on both carbs.Twice.

 

[pnwboat]

Re: 1998 Force 120- Throttling Up and Down on its own

Like Jerry said, they still need to be checked.

Those carburetors have been out of production for many years, so if they are brand new, they've probably been sitting on a shelf for a very long time. Long enough that I would not trust that the factory settings are still set correctly.

 

[Frank Acampora]

Re: 1998 Force 120- Throttling Up and Down on its own

Be certain that the rubber grommet holding the enrichment solenoid to the manifold is not missing. Excess vibration will cause the solenoid to flutter richening the mix and slowing the engine. When it clears itself, it will speed up. Crimp the feed from the carb to the enrichment valveclosed and see if the problem occurs. If it does not occur, then replace the enrichment valve.

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Thanks everyone.....
Certainly going to check the idle on the carbs

Frank, can you further clarify what the enrichment solenoid is , what it looks like and exactly where it is ?



Jeff

 

[Frank Acampora]

Re: 1998 Force 120- Throttling Up and Down on its own

Enrichment solenoid takes the place of a Choke. It is located --I think, been a while since I worked a late 120 Force-- It is located on the bottom of the top manifold between the carbs on the port (starter) side of the manifold. It will have two wires going to it and two small hoses: one from the bottom of the carb and one that splits and goes to the flanges of both carbs. When you press in the key, the solenoid opens allowing fuel to be pumped from the bowl of the carb into the intake manifold enriching the mixture for a cold start. It is usually brass colored with a black button (manual override) either on top or bottom, depending upon its orientation. It MAY have a red lever that is manually turned.

 

[pnwboat]

Re: 1998 Force 120- Throttling Up and Down on its own

This is the fuel enrichment solenoid. It is mounted just below the top carburetor. It takes the place of the choke plate that you normally see in a carburetor. Notice the rubber grommet in the mounting hole. There is a small fuel line that runs from the bottom of the top carburetor fuel bowl to the solenoid. Another small fuel line that has a "T fitting and runs to a small fitting at the base of each carb.

1990 1999 Force Mercury Outboard Fuel Solenoid Primer 819503 | eBay

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Got it , Thanks . I just looked.
Mine has a black override button on top. I actually used clear hose when replacing the carbs so I could observe the flow of fuel into the carbs when I push the key at start.

There is a grommet there, however it might actually be a bit worn.When I installed the new carbs, I noticed that the screw wouldn't tighten completely and the Solenoid was in fact loose and would definitely move around from the vibration of the engine.

If I crimp the hose and the problem stops, do I need to replace the entire solenoid or just the grommet so I can tighten it appropriately to the manifold. You had mentioned replace the enrichment valve and I was wasn't sure i if that's the same thing as the Solenoid.

 

[jerryjerry05]

Re: 1998 Force 120- Throttling Up and Down on its own

Those 2 screws aren't idle screws. they're for air/fuel mix. Idle is adjusted by the towershaft on the stbd. side of the motor.

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Right...that flat stainless screw head which is impossible to turn on my OB.
I left that alone since the engine seems to be idling ok, at around 900-1000 RPMs in Neutral on muffs ( May be a little high ?)

So Ill first try crimping the Solenoid valve lines to see if that corrects the problem.
If it does, I will replace the valve and see that it fastened securely at the grommet to minimize vibrations.

Second, If the above doesn't correct the problem, Ill try resetting the fuel/air mixture on the carbs as described below. Maybe I should do that anyway ?

Have I missed anything ?.... Thanks everyone I really am appreciative of all the advice on this

Jeff

 

[Frank Acampora]

Re: 1998 Force 120- Throttling Up and Down on its own

While it should be able to move, the solenoid should not be loose or flop around. There is an aluminum bushing inside the rubber grommet so you can not tighten it too far. The vibration problem will actually be caused if the solenoid is mounted solid to the engine block--that is the reason for the rubber grommet.

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Thanks
In that case, Ill still crimp and test the lines, , but it looks like I am back to the adjusting the air settings on the carbs

So I need to gently tighten ( turn in) both screws in all the way, then turn them back one full turn.
Run the engine for a bit, put in gear, turn screws in slowing until the the engine bucks, then back out 1/2 .

Far from an expert, but I am relatively mechanical . Any advice or other pointers so I don't screw this one up ?

 

[Frank Acampora]

Re: 1998 Force 120- Throttling Up and Down on its own

Where the carbs are set in the driveway on muffs is not necessarily where they will want to be set on the water. After setting them at home take the boat out. Now, do a full throttle hole shot from a full stop. Quickly push the throttle from idle to wide open. If the engine "sags" and sounds like it will die but then picks up and runs well, the low speed needles are set too lean. Turn them out 1/8 of a turn until the engine picks up smoothly on the hole shot.

If the engine "burbles" or coughs and runs rough then clears itself out and picks up, the needles are set too rich. Again, adjust 1/8 turn at a time until the engine picks up smoothly.

In any case, NEVER set the needles leaner than 3/4 turn out from lightly seated no matter what the idle or performance is. This is too lean and usually results in melted pistons at or near full throttle.

Again, Usually 1 turn out will be close to correct on most engines.

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Got it , thanks

 

[jerryjerry05]

Re: 1998 Force 120- Throttling Up and Down on its own

In the mid 90 Force sent out a bulletin stating the air/fuel screws should be set at 1 turn out and leave there.
It was supposed to go back to the motors made in the 80's and 90's.The later Mercury motors???? But setting them the way we suggested will save you a piston or 2.

 

[Jdiraff]

Re: 1998 Force 120- Throttling Up and Down on its own

Re: 1998 Force 120- Throttling Up and Down on