Mercury 90 WOT Problems

[grumm90]

mercury 90 inline 6 ('82) ran really great for first 4 runs of the season. 5th run out I started having problems not able to get the boat above 3000 RPM. (overnight) The problem seems to be getting increasingly worse since then, now the thing seems limited to about 2000 rpm with surges up to 3000 RPM. When it surges it nearly throws you out of your seat. replaced all fuel lines and filter. Installed carb kit (carbs were spotless however). Idles and starts great. cyl pressure is between 125 and 130 in all cylinders. Seems to rev freely with no load. any ideas?

 

[Laddies]

Re: Mercury 90 WOT Problems

Sounds like your losing spark on 3 cyl. ck it for spark and let us know

 

[grumm90]

Re: Mercury 90 WOT Problems

Spark is good. Jumps half inch gap. (cranking speed) But yes, thats exactly what it seems like. Like 3 of the cylinders are only firing on occasion. (the surges)

 

[Laddies]

Re: Mercury 90 WOT Problems

A bad stator or switch box either one could cause the problem, maybe with an induction tack or timing lite you can find it while running in the lake, if you can get it to act up and you are losing 3 cyl. you can swap stator wires and box to see what it is, good luck thats the hardesty thing in the world to find a part timr electrical problem--Bob

 

[grumm90]

Re: Mercury 90 WOT Problems

So, I find out which three are cutting out, or in this case the three that are only occasionally firing.. (should be an easy task). Then I swap the switchboxes around(same part number)and test again. If the same three are at fault then it's the stator, if not the switchbox?

 

[grumm90]

Re: Mercury 90 WOT Problems

I guess first step is to get the boat out with a induction light and find out what's firing and whats not. They all fire at idle on the muffs. I would rather not drop the hammer while on the muffs. hehe. The problem is definatley RPM related, as it seems to run quite fine at displacement speeds. Sounds like stator, however couldn't the increase from low to high RPM cause same symtoms on bad caps? If not, then it seems time to order a stator. If its not the problem, can't hurt to have a spare stator. By reading these posts, anyone with an older merc should have one.