1994 Mercury 40 HP, 4cyl missing, dying

First indication. It was running perfect and I was running it wide open and it just died. Started hard and idled poor after that. It misses now, and will only hit 2000 RPM in gear.
Background.
Engine overhauled 2 seasons ago from burned pistons, so I checked compression first, and it's OK. Both Carbs had new needle valves and cleaned after engine rebuild. The lower carb was dumping fuel again so I replaced the needle and float today, which solved that problem. I checked the coils, wires, spark, changed plugs with an old set that came out good, changed the fuel filter, the carbs open together, and I rebuilt the fuel pump for the second time today. It has new fuel and vacuum lines which I checked again today. None of this changed a thing.
I didn't bring a timing light, and all the work was done 30 miles from home at the dock, so I can't answer a timing question yet, but I wouldn't think that was the original reason for stalling.

Thanks!

Re: 1994 Mercury 40 HP, 4cyl missing, dying

it doesnt sound like timing, before you go into that, when it cut out did it like spit/backfire or did it just shut off. Does it hesiate? Timing seems odd to me, you have to find what cylinder is misfiring of if its cylinder(s). What were the compression readings? 2 Stroke require all the cylinders to be within 10% of each other to be good. You said your old plugs were good, describe good, Light tan brown? that would be good. Could be a failed powerpack although i would look last onto that.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

It does spit/backfire intermittently at idle and above now, which made me think timing or a stuck valve, but the compression on all cylinders was within a couple pounds of 100. What I meant by good on the plugs was I changed them only because I thought it was about time, but it was running fine with them. So I saved them for an emergency.
One thing I forgot was that the #3 plug looked dry when I pulled it today, but that was before the carb work. Running it after the needle I pulled it again and it appeared wet like the others.
Thank you.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

Forgot your question about hesitation. Yes, it seems to hesitate just above 1000 RPM in neutral then come back a little stronger and smoother at around 2000.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

Did you have the problem right after putting gas in the tank? Crappy fuel are all signs of your current problems, compression sounds good to me like you said.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

No. It has a 28 gal onboard tank, so it was running on the same fuel as the prior week. I checked that the tank vent was clear and ran a little fuel into a jar just for a visual on water, and couldn't see any sign of it.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

what i would do is start the engine, and let it idle, one by one i would remove a spark plug wire from a plug. See if the motor behavior changes, if it stays same then youll find what cylinder is the problem, if it changes then the cylinder is firing. i would still do a timing test just for the hell of it. i would do a pressure/vaccum check of the engine as well.

Also a "Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative carb. Spitting/backfiring is associated with this.

You could have a carb that is leaning out the mixture too besides ignition problems.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

I really appreciate your help! I'll invest in a new set of plugs, double check the fuel for water, check the timing, and study up on how to check the vaccum.
If the only fuel mixture comes from the idle mixture screw, that was done by the book also today. Could be that the upper carb was causing more problems than the lower leaking one, but I could only come up with 1 needle and float today and I assumed it was good since it wasn't leaking like the lower.
Do you think it possible that it could be an intermittent trigger,stator, or maybe a coil breaking down? I figured a coil might cause this, and had to replace 1 several years ago, but they test OK with the engine shut down.
I don't know enough about the purpose of the trigger and stator to make a cause/effect determination.

Re: 1994 Mercury 40 HP, 4cyl missing, dying

yea it defenilty could be, but its eaiser to troubleshoot fuel then spark in my opinion. So go go thru the fuel system and check the engine for pressure+vaccum. If after all this is good, then its time to go thru ignition. Hopefully you wont have to go that far tho, any questions feel free to ask, goodluck

Re: 1994 Mercury 40 HP, 4cyl missing, dying

MercBravoIII. Just an FYI.
I went back today. The fuel was good.
The #1 and 2 cylinders weren't firing. I used your suggestion to actually pull the plug wires one at a time to narrow it down.
I had someone watch my spark tester originally and apparently he didn't know what he was looking for.

The problem I found was the #1 and 2 coil ground wire had rubbed the extra rubber insulation through, against one of the trigger wires that lies next to it on the switchbox.

I'm going to try to look on the bright side this time, and figure I needed to rebuild the lower carb anyway before it caused me more engine damage. I probably would have waited until seasons end had I not had this problem.

Thank you very much for helping to eliminate some possibility's in my mind before going back out. I'm sure you saved me more time, work, and maybe expense, because you took your time to help.