1980s mercury acceleration issue

[Cfrosty78]

I’ve got an early 80s 18hp mercury on a Jon boat that runs great and will full throttle when out of water with clamps on, but when in water I’m only able to throttle to a certain point before it dies. New spark plugs, water pump, carb seals, etc. trying to figure out what needs to be done to get full power in water.

 

[Droid]

Look down carb throat and see if plastic venturi is broken off

 

[racerone]

It will rev up on just 1 cylinder when running on a hose.----Post compression values.----And does spark jump a gap of 3/8" on both leads ?

 

[Cfrosty78]

It will rev up on just 1 cylinder when running on a hose.----Post compression values.----And does spark jump a gap of 3/8" on both leads ?

Showing 120 on both cylinders

 

[Cfrosty78]

Look down carb throat and see if plastic venturi is broken off

From what I can tell it’s all good. If your talking about the black thing with the two holes in it.

 

[Cfrosty78]

It will rev up on just 1 cylinder when running on a hose.----Post compression values.----And does spark jump a gap of 3/8" on both leads ?

also I’m new to this type of work. How do I check if it jumps a gap of 3/8”? Is that with a gap checker?

 

[Droid]

From what I can tell it’s all good. If your talking about the black thing with the two holes in it.

Disregard, the venturi I was thinking of is on larger engines. When they break, fuel only meters at idle.