Topic: Mercury force 120 misfiring pics of spark plugs

This topic has 14 replies, 5 voices, and was last updated 5 months, 3 weeks ago by Beno.

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June 4, 2024 at 8:20 pm #288119
Beno
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I just put this 97 Chrysler blocked together and I normally don’t have any problems doing this but it’s misfiring like crazy and doesn’t even come close to full speed.

I pulled off all four spark plugs and took pictures and here they are anyone want to try to give me their best idea as to what’s wrong just by looking at the top of the plugs?

(Plug one and three of the worst two and

four are not too bad. I think this will rule out immediately fuel pump diaphragm cracking?)
June 4, 2024 at 10:07 pm #288124
Beno
Participant
Points of interest cylinders one and two share a carburetor as well as three and four share one.

Given that cylinders two and four both look to be better.. I find it hard to believe this could be a fuel issue.

Another interesting thing is that when the engine is turned off.. white wisps of vapor come out the carburetor/intake side. I never seen that before on 120 horsepower Mercury Force but then again I never really looked ?
June 5, 2024 at 8:51 am #288125
fleetwin
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US Member - 2 Years
Not familiar with how the Force powerheads go together. But, those damn surface gap plugs often look like crap even when they are firing ok. OK, so you just put the powerhead back together, did it ever run right after assembly? Does it idle OK and just lacks WOT power? Or, does it run like crap at all speeds?

I’m keying in on your comments about “misfiring like crazy”. Are you sure you have the plug wiring/switch box wiring/timing correct? I would recheck all the ignition wiring. Sorry, wish I had more knowledge about these, but don’t…
June 5, 2024 at 9:29 am #288127
Beno
Participant
Hey Fleetwin.. long time! I say put back together let me be more specific..

This is a very pretty seaRay F14 boat (#a) I got 120 horse powerhead (#A) imploded cylinder and needle bearings. I pulled off the stator flywheel trigger electronics coils all as a unit. Then I went and bought another similar jet boat (B) which had no spark. I checked the compression of B which was solid on all four cylinders, pulled the powerhead then trashed the boat (#B) but kept the power head.

So now i have power head block #B with electronics from #A in boat A

Honestly I didn’t think I was going to have a lick of trouble here. Since old powerhead #A had failed mechanically.. the thinking was that the electronics must be perfect. .. but i guess im wrong?

Today’s plan is to get the multitester and test the trigger ohms.. test the stator.. and then start testing the coils..

It’s interesting that I have never seen one of these Mercury forces without resistor plugs and i realized there are regular plugs in this one so I’m going to swap those off for resistors if I can find some.

.
June 5, 2024 at 2:08 pm #288128
Beno
Participant
Ok sorry for the trouble guys. As you know it was cylinder 1 and 3 black on the plugs. So I immediately traced cylinder one and three (and on this system they are brown and purple).

Course I immediately became suspicious because I know from experience that purple wires can look Brown after being on the engine for some time.

Upon further inspection .. even though I could see there was a brown to brown wire connection: looking further there was a brown to purple lower. So I took out my jackknife and scraped on the top brown and voila one micron thick layer of brown scraped off revealed purple. Ok HOW COULD IT HAVE BEEN RUNNING AT ALL?

Sea trials commenced. She ran pretty good…

Id say it runs at 85% power for a 120hp. I did not know what kind of gas might have been in the tank but i put in 10gals and 33oz of tcw3.. so if there was 5 gals in the tank. Im kinda covered.

So now i have to try to figure out why it occasionally misses at high speed. But i really need to put some time in on the river to get a feel for it. The new Hull numbers should be in tomorrow I think
June 5, 2024 at 4:46 pm #288130
fleetwin
Participant
US Member - 2 Years
OK, so the ignition wiring was mixed up?? I think I understand what you swapped out, but not sure if you still have the electronics for the other engine. Sure hope so. “misses at high speed”, seems like you are describing some sort of ignition miss.

Again, not familiar with the force engines. But, I do know that certain model OMCs will run like crap if you install “non inducter plugs”. So, you might want to start with a set of the correct new plugs.
June 5, 2024 at 5:01 pm #288131
Beno
Participant
The original electronics didn’t work at all.. I don’t remember exactly what part was bad. But there was absolutely no spark when I got that block.. They were even a different version of the electronics.

.. From 1989 through 1998 these Mercury Force jet boats went through more different stators. .. CDI boxes…..coils. big crazy capacitor boxes… its a little insane.

Yep im on the lookout in my warehouse for the resistor plugs .. I’m sure I have quite a few.

I took it on the river for about 20 minutes this afternoon and every so often I would stop and experiment changing the spark advance screw. And quarter turning in each direction the upper and lower carbs..

Shockingly little difference my adjustments made.. I would have thought that would have been big changes. Nope

Also I pulled the bowl off lower carburetor is absolutely pristine inside.

One tricky thing was happening that I still don’t understand is that when I went to take a left or right turn the engine would start to die and misfire .. now. That engine should have NO IDEA if it’s in a turn of not.. but did not happen just once it happened quite a few times. The moment I started the turn…. I was thinking maybe water in one of the bowls.. but these carbs pick up from the bottom… So that shouldn’t be true.

Any idea?
June 5, 2024 at 5:21 pm #288132
fleetwin
Participant
US Member - 2 Years
Perhaps one of the floats isn’t set quite right and one carb is flooding over slightly in a turn. Maybe try lashing the boat to the dock then turn the engine while the boat stays more or less level…
June 5, 2024 at 6:37 pm #288133
Beno
Participant
Good idea . Ill try that .

I just took it out for a straight 15 minutes it’s not run perfect but it’s … pretty good. Probably 85-90%

Im going to put more fuel in it and 50:1 mux tomorrow, and take it up to Saratoga lake tomorrow for some trials. Cooling system seems to work . I put the battery on trickle charge for the night. Just have to go get the life preservers and the anchor and the fire extinguisher tomorrow.
June 6, 2024 at 7:02 am #288145
crosbyman
Participant
Canada Member - 2 Years
when I went to take a left or right turn the engine would start to die and misfire .. now. That engine should have NO IDEA if it’s in a turn of not..

reminds me of my old the 4 cyl merc 500. the ones with the fat plug on the side….

the silly thing would die turning to one side …gee that was 40 + years ago…. don’t remember if it was left or right or wire color (probably red) but…

one of the wires in the plug’s bundle /harness had eventually broken from years of flexing. This was at a camp site 80 miles in the bush. Running in straight line was fine … trurning would kill the engine !

did some surgical work on the harness and patched up the wire and finished our week of walleye fishing with no problems and you don’t want problems on this lake !!

Albanel, Lac Fishing Map | Nautical Charts App (gpsnauticalcharts.com)

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