What is the problem here??? switch boxes or reeds?

[deep_blue]

I just rebuilt an older Merc outboard 90Hp straight six two stroke. After taking it out in the water during the break in period I noticed that cyl. 3 wasn’t firing correctly (piston's crown was clean while all others had a little carbon). I think it was firing at first and then spark died and all the clean gas moving around in the cylinder just cleaned it off (because I could clean other pistons with just alcohol and q-tips). The engine idles ok, however sometimes I can pull the plug wire off spark plug 3 and I cant hear a difference, and other times I can. And when in water I open the throttle, for the first few minuets it really gets up and moves the boat quite nicely. but after that it drops in performance (quickly). I have switched coils around and I can’t say for sure, but I don’t think that’s the issue. Doing an impedance test that I got from the manual the switch box did fail (I think its 110 Mohm to 140 Mohm) I got a 100 Mohm on one of the three test combinations all of the others I think tested 120 Mohm. I flipped the switch boxes from 1-2 to 2-1 and that seemed to make a difference. I took it out on the water and it ran harder for longer but still died out over time. Now the reason i'm not saying that its the switch boxes for sure is because during the time that cyl 3 didn’t fire, I took off the carbs and poked at the reeds, put them back on and within minuets the piston had a little carbon on it. Please help, it’s giving me a huge headache going back and forth from my shop to the dock. I just want to go fast!!!

Thank you all for any help

[deep_blue]

oh and i would also like to point out that the carbs are rebuilt and cleaned and a new fuel pump was installed. i tested the engine at night, shining my super intense LED flash light into the carbs i can see the vapor trail coming off of the jets (and all three carbs have a very nice equal amount of gas flow). i cotton swabed the bad cylinder for gas and yes there is fuel in there, its just unburned! and all cylinders have almost equal compression at 130 psi. with new spark plugs every time i test it.
i think that narrows it down a bit.

[Dan Swanson]

Hey deep_blue,
It sounds like we have the same problem. I have an '86 75Hp Merc inline 4cyl. I've done it all with the carbs and replaced coils, wires and plugs and yes, the compression is strong an even with all 4 cylinders. I've recently acquired a Seloc manual on the motor and tests on the switch box are my next step. My motor has a "Type V Thunderbolt Ignition System".
The motor is sluggish to start and then up in mid to high throttle it surges like all 4 cylinders finally kick in and then after I back down on throttle it falls back into sluggish again.
Everytime I check the plugs, #4 looks like its been washed and not igniting compared to the other 3 which all have alil carbon on them.
I'd be very interested in what you find out. I will be testing my switch box and can let you know what I find.
You can email me direct at: dpsprima@yahoo.com
Dan S.