90 HP only firing on 5 cylinders (help)

[nightstalker]

I have a 1985 90 HP Merc. (6 cyl). It lost power last week and is only running at 4200 RPMS. She still runs smoothly but I think it's not firing on the top cylinder. I replaced the plugs, ran it for a while and found that the top plug had no "burn" on it. It looked as new as it did when I put it in. The other 5 plugs showed burn. I checked the plug wire with a light wire and it seemed OK. I had the coil checked and it was also OK. I couldn't see a spark when I grounded the plug wire but it was sunny and I'm not sure if I could see the spark. What do I check next? Do a compression test? Check for spark in the dark ? Any help would be appreciated.<br />Nightstalker

 

[WillyBWright]

Re: 90 HP only firing on 5 cylinders (help)

Check for spark. If it isn't there, you need a switchbox. It could be the one that feeds that coil, or believe it or not it could be the one that is sparking all three of its coils. Try the one that feeds the dead cylinder first. If that doesn't bring the spark back, put the one you took off in place of the one you didn't.

 

[nightstalker]

Re: 90 HP only firing on 5 cylinders (help)

Thanks for the tip. Where is the switchbox and what does it look like?<br />Stalker

 

[jimmbo]

Re: 90 HP only firing on 5 cylinders (help)

I vote for a bad switchbox too.

 

[achris]

Re: 90 HP only firing on 5 cylinders (help)

Starboard side of your engine, two of them. Look like this (number 18)<br /><br />

 

[nightstalker]

Re: 90 HP only firing on 5 cylinders (help)

Thanks for the help guys. I'll give it a looksee tonight and let you know what I find out.<br />Stalker