Force by Mercury 120 will not idle

[Katmandew]

Having a problem with 1998 Force by Mercury 120 outboard. Will not idle. Have to take out of gear and put throttle down to start it. It's also fouling plugs. I recently tried Lucas Oil in with my standard 50:1 mix. That's when it stopped holding idle. Had been dieing if I didnt hammer down into gear fairly fast after starting but it did idle previously. I've changed lines and fuel pump. Can blow gas in tube and it comes out of the carb. Carbs look clean looking into them. Talked to local mechanic who suggested I do compression test on cylinders. PSI from each cylinder below.

115
115
127
125

Mechanic told me if these numbers were within 10% the cylinders were okay. Also read on here it was 10 psi, not 10% and my engine should get 120 psi in each cylinder. The next step the mechanic suggested was to increase idle speed since it's not holding 800-900 rpm without throttling up. Also wondering if the Lucas oil could be choking it out. Boat runs at normal speed on the water I just have to quickly put in gear and throttle Down immediately after starting and putting back in neutral (have to take out of gear and throttle down a bit to get it to start). Hoping someone on here has seen this before and can give me some guidance. Thanks in advance.

 

[Jiggz]

First question is why are you using Lucas oil on a 2 cycle motor? Since the plugs are fouling did you replace them or at least clean them? Is the fuel air mixture screw set to at least 1~ 1 1/4 turn out from lightly seated on each carb? Idle rpm should be at least 800~1000 in idle and 750~800 in gear idle. Are you using BUHW or UL18V plugs?

 

[Katmandew]

Friend suggested Lucas oil as a cleaner. Had a slight problem holding idle as I shifted into gear before the Lucas oil. Had to shift fast. Most likely because idle needs to be increased. Hindsight being 20/20 I realize I shouldn't have done used it. Using buhw plugs changed them twice. On my 2nd new set. I have no idea how to adjust the carb. I'll look into that.

 

[pnwboat]

You need to either dump the tank of gas w/the Lucas oil in it, or run it until it's almost empty and re-fill with a fresh tank of gas/2-stroke oil mixture before you do anything. Trying to adjust the motor with the incorrect oil/gas mixture will not work.

 

[Jiggz]

Before making any adjustments you need to follow PNW's suggestion of using fresh tank of fuel oil mix. Start adjustment with the idle screw first which is located at the bottom of the control tower. If this does not fix the problem you need to clean the carbs especially the jets. If problem continue to persists, you might want to check the reeds on the top two cylinders.

 

[Katmandew]

Already did this.

You need to either dump the tank of gas w/the Lucas oil in it, or run it until it's almost empty and re-fill with a fresh tank of gas/2-stroke oil mixture before you do anything. Trying to adjust the motor with the incorrect oil/gas mixture will not work.

 

[Katmandew]

Thx for the help. Do I pull the carbs off to clean the jets?

Before making any adjustments you need to follow PNW's suggestion of using fresh tank of fuel oil mix. Start adjustment with the idle screw first which is located at the bottom of the control tower. If this does not fix the problem you need to clean the carbs especially the jets. If problem continue to persists, you might want to check the reeds on the top two cylinders.

 

[Jiggz]

No need to. You can first try spraying them with carb cleaner to see if that will fix the problem. Although, you can drop the bowls also to give you more access.

 

[Frank Acampora]

Work from simplest to most complex! First change the plugs--you already did that. Then raise the idle speed UNLESS you know for certain the carbs are junked up. DO NOT play with carb idle circuit or low speed adjusting screws--that is a sure fire recipe for damaging the engine

When you raise the idle speed by adjusting the idle stop screw, since you have Mercury controls you MUST readjust the control cables. This is done where the cables are anchored inside the cowling.

 

[Katmandew]

No Title

Carbs were cleaned recently so unless the Lucas oil clogged somewhere I don't see anything to clean. Looks good all the way back.

No need to. You can first try spraying them with carb cleaner to see if that will fix the problem. Although, you can drop the bowls also to give you more access.

 

[T-sweat]

Work from simplest to most complex! First change the plugs--you already did that. Then raise the idle speed UNLESS you know for certain the carbs are junked up. DO NOT play with carb idle circuit or low speed adjusting screws--that is a sure fire recipe for damaging the engine

When you raise the idle speed by adjusting the idle stop screw, since you have Mercury controls you MUST readjust the control cables. This is done where the cables are anchored inside the cowling.

Do u mean tie them up higher? Or what do u mean by readjusting the control cables

 

[Nordin]

Read the rules for this site and start your own thread. This one is 7 years old and Frank A is not with us any more.

 

[racerone]

Remove cylinder head and inspect cylinder walls.

 

[jerryjerry05]

Start your own thread, and explain the problem???

 

[T-sweat]

oh! Oops sorry .. it’s kinda beautiful that his legacy lives on bc I learned a bunch from this thread, but I’ll start a new one. Thank you all for the work you do!