90/65jet suddenly dropping rpms at full throttle

I have a 1995 Mercury 90/65 jet outboard. When I go past 3/4 throttle RPMs will suddenly drop by about 1/3 (I have no tachometer). If I let off the throttle just a little, RPMs suddenly come back up to normal. I thought this was a fuel problem, so I replaced fuel line, primer bulb, fuel filter, and rebuilt fuel pump. Poured a can of Seafoam in gas tank and ran boat-seemed like I could go a little past 3/4 throttle without RPMs dropping but still could not go to full throttle. Tried some more Seafoam with a tank of high octane gasoline-now worse than ever-cannot go past about half throttle without RPMs dropping. Started thinking maybe ignition problem. Pulled plugs (which were less than 6 months old) they all looked fine but replaced them anyway. Boat ran perfect for about 15 minutes-full throttle the whole time. Turned around and headed back to ramp and there it went-sudden drop in RPMs. I lift up on throttle just a little and RPMs come back. As soon as I go to full throttle, RPMs drop again.

Re: 90/65jet suddenly dropping rpms at full throttle

I forget to mention that I checked compression on all 3 cylinders-125,124,125

Re: 90/65jet suddenly dropping rpms at full throttle

Have you CLEANED the carbs yet?

You have addressed the rest of the fuel system but dirty carbs/jets limiting fuel flow may be the problem.

Also do a spark test to ensure the ignition is not 'spark limited'. Failure to make a bright blue 7/16 spark can limit WOT RPM.

Re: 90/65jet suddenly dropping rpms at full throttle

Test the switch box as well.A friend had same prob.Fiddled with carbs for months,but still dropped off at 2000 revs.Mechanic(friend) gave used switch box to try.Problem gone.This was on a Johnson 70hp.Hope this helps.

Re: 90/65jet suddenly dropping rpms at full throttle

Thanks a bunch for the replies. I will be checking these things this weekend.

Re: 90/65jet suddenly dropping rpms at full throttle

Okay I have good news. I checked the resistance on the ignition coils 890, 800, 890. According to the service manual, 800-1000 is normal. I went ahead and replaced the #2 coil. Took boat out this morning, went about 6 miles upriver and turned around and came back-ran perfect.

Re: 90/65jet suddenly dropping rpms at full throttle

Thanks to everybody on this forum for all the good info!