1995 Force 120 would not turn off

[MBLarry]

I just completed rebuild of my 1995 Force 120.New pistons, rings etc. Also replaced carbs with a rebuilt set I purchased on line. Before installing the carbs I noticed the float valve would not drop down and was stuck in the seat. I cleaned it up again and it seems to be working properly. Took the boat out last weekend and had bad grinding when going into forward and reverse. Cables are adjusted properly so I assumed my idle was too high so today I put it on the muffs and ran the engine to see if I could turn the idle down a bit to see if that would correct the grinding issue. That seemed to help some. While the engine was running at idle, it began to speed up until it was at almost full throttle and I was unable to stop the motor by turning off the ignition key or the dead man switch. I quickly disconnected the fuel line and blocked both carbs with my hands to kill the engine. Anyone have a clue as to why the engine would speed up like that and not be able to turn it off with the key? Recheked all the engine wiring and the kill circuit wiring and all seems well. Is it possible that the carb float is stuck and dumping fuel into the engine? Why couldn't I kill the engine with they key? Now I'm afraid to start it again for fear of blowing a newly rebuilt engine. Any likely suspects? Also, if the idle speed is too high, can that cause the grinding when going into forward and reverse? I know to shift quickly but it still grinds. Unfortunately, I do not have a tach on the boat.

All help and advise is appreciated. Thanks in advance.

 

[Jiggz]

To stop a motor, by design a stop wire is shunt or grounded to ground. Do you have the Thunderbolt (switchbox) or CDM ignition system? if you have the switchbox the stop wire is blk/yel wire. Check for continuity from the switchbox to the main terminal block in the engine compartment and then all the way back to the ignition switch. At the ignition switch, in "OFF" position, the two "M" terminals should be closed. Meaning the blk/yel wire is connected to the blu or blu/blk wire which is connected to ground.

Next make sure the ignition switch blu or blu/blk wire connects to ground. Note this ground wire connects thru the terminal block in the engine compartment via a bussed terminal which connects to the starter solenoid grounded terminal.

The Stop Switch is nothing but a parallel switch to the two "M" terminals. When motor is in "On' and running the two 'M' terminals are open . . . if you press the Stop Switch you essentially closed the two "M" terminals and should stop the motor.

Another method to emergency stop a motor is to unplug the plug wires asap.

The likely cause the motor ran away is a lose tie bar at the carbs allowing the throttle to fully open. Double check all throttle linkages. I seriously doubt a sticky float will allow a motor to ran away. Maybe flooding the carbs for a richer mixture and probably stop the motor. Throttle controls the amount of air and fuel oil mix getting into the cylinders.

 

[MBLarry]

Thanks Jiggz. I'll double check the wiring again. I have a Thunderbolt switch box system. I turned the motor on and off numerous times with no problem the same day it decided to run away. I did adjust the tie bar between the carbs per the tuning instructions to verify both butterfly valves were parallel. I'll go over everything again since it must have been something I did during the carb adjustment.; Thanks for the advice.

 

[MBLarry]

Update... OK, I checked all the wiring and grounds. All is good. Checked the cab linkage and throttle linkage. All was good. Can anyone guess why the engine sped up to WOT with both carbs still in the idle position and shifter in Neutral? During the test run I did have a handheld tach connected to the purple tach lead and grounded to the engine block. That's the only thing I can think of that may have caused a voltage to be applied to ground which caused the inability to turn off the engine with the key. Any idea how the engine could increase RPMs with the carbs in idle position? I lost.....

 

[blackd]

Your motor was probably dieseling. Which means a hot spot in one or more cylinders was igniting fuel. Doesn't take a spark to continue firing, just fuel and hot spot for it to ignite. I've had it happen while running a cleaner through fuel system. It can be scary!

My occurrence was on the muffs also, so I think not getting complete cooling water may be an underlying factor.

After cleaning your shorts, restart your engine with the air boxes off and a towel handy to throw over the carb throats to stop the engine from breathing and to see if there is a consistent problem or was the condition unique, once only.

If it re-occurs on the muffs, put it into a barrel or take it out on the water to test with the same stop towel conditions handy.

If it continues in those conditions you need to find that air/fuel leak and/or why it is creating a hot spot on the clean pistons and cylinders.

Good luck!

 

[blackd]

BTW, on diesels the condition is caused by cylinder blow-by, and is more common in two-cycle diesels. You might also have some blow-by conditions in a newly rebuilt motor still not fully broken in.

 

[MBLarry]

Thanks Blackd. Hope your right about the dieseling. I only have about 1hr run time on the motor after rebuild. Any thoughts on how the RPM could suddenly increase to WOT with the carbs still in idle position? How is it getting that much fuel?

 

[blackd]

I can think of two possible sources: 1) your needle valve stuck open causing fuel to over flow into the carb throat.
2) Fuel pump diaphragm is leaking into the cylinder.

My best guess is the sticking needle valve. Each carb feeds 2 cylinders, 2 cylinders feeding the problem versus 1, that's why I lean there first.

BTW, if you do the test again, after pumping fuel into the carbs, disconnect the fuel line before starting just in case it is the fuel pump. At least that will limit access to fuel to what's in the carb bowls.

 

[Jiggz]

If all wiring is correct, do a test again but this time with starting. Make sure cooling water is connected. Depress the kill switch and try to start the engine. If the wiring is correct, there should be no indication of spark in the engine. It should only continue to crank with the starter without a single incident of firing. If that is the case then you can assume the wiring is correct.

Next, reset the kill switch. And then back off the idle screw (bottom of timing tower) all the way out so it doesn't rest on the engine block. Verify throttles are fully closed. Try starting again. It should not fire at all considering the throttles are fully close. If it starts, that is an indication there is a leak either in the carbs' mounts or within the cylinder plenum from the crank bearings, etc.

 

[MBLarry]

Thanks Blackd and Jiggz. I'll pull it outside again this weekend and try again. Like Blackd mentioned, I'm leaning toward the sticking needle valve in one of the carbs. These were supposedly professionally rebuilt but I did find a rebuild mistake with the float pivot which was not seated properly causing the float to droop down. Also, when I put my hand over the carbs to kill the engine, one of them dumped a load of fuel on my hand. I'm beginning to believe they didn't do a good job with the rebuild. I'm gonna give them a call anyway. I'll verify the wiring per Jiggz suggestion first then check out the carbs. Your input is greatly appreciated. Thanks you.