1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ later

[pooh_b_21632]

Beginning of the summer I took my boat out and the original problem was an intermittent misfire. After talking to this person and that person and reading forums, and my manual we have replaced alot of parts. Took the boat out today and runs phenomenal compared to before but still has a slight intermittent misfire that irritates me. My last thought is that I should replace the coils. It has the prestolite ignition with the original now 21 year old coils with the original plug wires built in. I feel that could give me the intermittent misfire but want other opinions.


List of Replaced/Cleaned Parts/Items:

1. Cleaned and emptied fuel tank
2. New Fuel Lines
3. New Fuel/Water Separator
4. Cleaned/rebuilt both carbs (went throught them several times and I could now disassemble them with my eyes closed)
5. Rebuilt fuel pump
6. New NGK spark plugs
7. All four Pistons/Rings/Wristpins/Bearings are new (they were missing chunks of metal from them original owner either ran it too lean for too long or over heated it)
8. Many new gaskets


Once I got the pistons replaced and we put it all back together (after cleaning the carbs again) it started good but idle was rugged so checked the timing it was off so we reset it, now at WOT it goes to 30 degrees BTDC where the manual says it is supposed to be at. We had the carbs at 1 and 1/2 turns out but it was too rich and I was having trouble getting it to go into gear without dying. Now carbs set at 1 and 1/4 turns out runs great starts great goes into gear without dying. only problem is intermittently without a specific pattern or cause the engine will buck or jump real quick then return to normal sometimes it does it a few times sometimes only once. We circled the river many times it only did it maybe 5 or 6 times but its irritating and I want to fix the problem before I have another expensive or time consuming repair. The coils were arcing once before you could see it in the dark but I put dielectric grease around the coil where the sparkplug wire comes out and in the spark plug boot of each one and the arching stopped. I think I need to Just replace the four coils and plug wires. Anyone else have a similar issue? or any other ideas or thoughts?

 

[droach77]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

when you rebuilt your carbs did you pop out the welch plugs and check for debris in there. I have a set of carbs that i had apart 3 times soaked and sprayed and all that good stuff and then i decided to pop the welch plugs and they were full of crap. There all little tiny passages under them and its hard to the grit out of them sometimes without pulling the plugs out.



dave

 

[pooh_b_21632]

Re: 1990 Force 120~Intermittent misfire back to my original problem hundreds of $$

Re: 1990 Force 120~Intermittent misfire back to my original problem hundreds of $$

Actually Dave I had not pulled the welch plug, I sprayed seafoam in each one let them sit for a few minutes. then cleaned them with 3M throttle plate and Carb cleaner until the cleaner ran clean (no more black tinge to the cleaner) through each and every opening, I used quite a few cans too. I did not pull the welch plug because it looked like if you pulled it you had to replace it (Like a freeze plug) and I did not have a new one.

 

[droach77]

Re: 1990 Force 120~Intermittent misfire back to my original problem hundreds of $$

Re: 1990 Force 120~Intermittent misfire back to my original problem hundreds of $$

They are cheap and available for your carbs.....$1.50 a piece and clear fingernail polish to reseal them, you would be surprised whats hiding in there.


dave

 

[jason32038]

Re: 1990 Force 120~Intermittent misfire back to my original problem hundreds of $$

Re: 1990 Force 120~Intermittent misfire back to my original problem hundreds of $$

The only time ive ever heard a backfire was when I had my idle mixture screws set to 1 turn out to attempt to adjust but it was too hard to start so I set them back to 1 1/2. Note that it was an explosive backfire and I had the LU in a tote full of water and the water shot everywhere. You should clean carbs again and replace welsh plugs. If your engine is still dying at 1 1/2 turns out after cleaning your carbs again then you need to speed up your idle by turning the screw on the towershaft clockwise. Playing with the idle mixture screws on the carbs does not adjust idle speed and you could damage your engine.

 

[bentle]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

When you had your carbs off did you check your reeds? Also another thing that can cause your miss is a plugged or faulty fuel return system. 20 year old coils may be your problem but check the above first if you haven't already done that and forgot to list it.

 

[pooh_b_21632]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

Reeds look like new, idle and lean/rich have been adjusted to 1 and 1/4 turns out runs great at that setting and shifts in and out of gear smoothly. Idle is set to 900 factory manual specification. like I said it runs fantastic until it gets the misfire and its not a continuous misfire its more occasional like every now and then. Anyone got a good site to order the welch plugs from? many of the fuel lines and return lines are new there are only a few that are not new but they looked ok, I guess I will get on replacing those too.

 

[droach77]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

You can get the welch plugs on any site that has parts including this one. Where on the eastern shore are you , you might have a merc dealer or service center near you that might have them or that can get them for you. Shorts marine is in milsboro DE they were a big force seller when they were popular on the bayliner and maxum boats.


dave

 

[Jiggz]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

Personally, I would start "low" by changing all the spark plug wires first. It's easy to find which wire is "leaking" by touching the one wire at a time while the engine is idling. If you get a tingling sensation the wire is bad. But since the wires are cheap, might as well change them all. Test and if the problem persists, try to get a better idea when does the engine misfires, like at what RPM. Does it misfires at idle-neutral? Or only when in gear at what RPM. The arcing from the coils is a good indicator of failing coil insulation and can actually cause the intermittent misfiring but start with the wires first and then move on from there. You can almost tell if the source of misfiring is fuel related if it only happens at high-rpm. If there is misfiring both at idle on muffs and at high RPM on the water, it's most likely electrically related. And then instead of buying all sets of coils, start with one coil first and switch it around one at a time until you isolate which coil if faulty.

 

[pooh_b_21632]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

Jiggz,

My plug wires are connected to my coils that is why I think I need to change all four coils otherwise I cannot change the plug wires. It happens at all different times sometimes at idle sometimes while running slow or fast, in gear or out of gear, does not matter. That means its electrical then right?

 

[bentle]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

If you can? Then hook up in line spark tester and then you will be able to see if and when there is a short. A good timing light works also. The miss could be both electrical or fuel related. Another thing to check is check all wires going to the power box on port side. Make sure all are tight and in good condition. Gently move each wire while motor is running to see if there may be a short at the but connections. Also inspect wires coming from stater and trigger. I had a wire break on a trigger once right were it comes out of the housing and causing a miss. In my case I had to replace the trigger. Also if your wires run behind the mounting plate your coils are attached to it might not hurt to check there as well. Also a good sign of a faulty fuel return system would be excessive smoke at startup and at idle. It also can cause a miss at any rpm range.

 

[Jiggz]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

If that's the case, follow Bentle's recommendation either with a see through spark tester to see if there is a "missing spark" when you heard the misfire. You will have to closely watch the tester while waiting for the misfire. A timing light is easier to watch although the spark tester is more accurate. If you do not see the spark when you hear the misfire, that particular coil could be faulty. So to make your troubleshooting even better get a brand new coil (just one) and switch it around a coil you think is not giving the spark when you hear the misfire. I know it's probably easier if you can just get all 4 brand new coils but why if you can just get away with one or hopefully no more than two. If after you replaced a suspected coil and the spark is still missing on the same coil, it's time to check the trigger.

 

[jmulz824]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

Cheap spark plug wire tester, while running spray the wires with a fine mist of water, watch for any arcs.

Just a spray bottle and some water normally works well.

 

[pooh_b_21632]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

I received my new coils Sierra part # 18-5186 but I cant find a part number for the wires, anyone know a part number for me?

 

[pnwboat]

Re: 1990 Force 120~Intermittent misfire back to my original probleml hundreds of $$ l

Item number 32 in the parts diagram below.

http://www.crowleymarine.com/mercury-outboard/parts/1470_40.cfm

Or item E29 in the diagram below.

http://www.boats.net/parts/search/Merc/Force/1991/H0908P91C/IGNITION%20COMPONENTS/parts.html