Mercury 9.8 110 WOT issue

[River Rattler]

motor runs very well at idle and up to approximately half throttle then begins to exhibit some skip and throttle through that to wide open sometimes if you play with throttle but still weak in my opinion. Most of the time motor either surges like hitting a rev limiter or Boggs bad like one cyl dies. Spark strong on both cyl. Compression over 90psi. Does this sound like a timing, points, coils, or fuel issue? Any help would be greatly appreciated. Oh and motor previously ran like a top and began to seem boggy on too end after running a while then got worse so I have no top end now

 

[racerone]

What year ?----Serial # is ?

 

[Scott Danforth]

Need a year and serial number

Compression is a bit low for that motor

Do you have spark on both cylinders?

 

[River Rattler]

Apologies on year and s# I will post that tomorrow when I have a chance to get it off the motor. Yes spark on both cylinders at the shop strong blue will jump an inch on spark tester with slow pull of rope. The compression readings were from one pull as if I was cranking motor, motor cranks well with a fairly gentle pull. Next chance I get I’m going to take to the lake and pull plug wires while attempting to run wot and see if motor dies or changes to see if the problem is isolated to one cyl. Both plugs (permagap) however appear very dark with no noticeable difference between cylinders. I recently replaced them with NGK plugs to no avail.
all wiring looks to be oem and in fair condition, no signs of broken wire or bad insulation.
Motor runs like a champ in the barrel and revs well in neutral.
also noted that problem seems to worsen as motor gets good and warm and with heavier loaded boat.

 

[racerone]

They all rev well in neutral.-----That means nothing.------Could be a problem with crankcase compression.------Reed valves / crank sealing.----Water pump impeller replaced ?

 

[River Rattler]

Water pump hasn’t been replaced this year, though tell tale stream is strong. I have ordered a new impeller though. Is there a good way to check for crankcase compression or bad reed valves? I know there is no cylinder leak down, but what should the cylinder compression be after one good haul on the rope? Or should I pull it multiple times to get a good reading? I should be able to get the s# posted later today

 

[racerone]

You pull till the gauge stops going up in pressure !

 

[River Rattler]

Gotcha, serial is 3734178 which I believe corresponds to a 1973 model.
compression maxed out at 125 top cyl and 130 bottom cyl.