RPM/MPH loss
 

Hi out there.
I seemed to have lost about 400 rpm and about 6-8 mph(gps)this season.
Last season 4400 rpm and just shy of 40 mph. 24' Cuddy
1999 5.0l Merc, 2bbl carb,19" SS rapture, Alpha Gen 2.
New plugs ,wires , cap etc. Carb rebuilt 2 years ago .
Timing was way off .Adjusted back to 10 BTDC. Checked vaccum at a very steady 19#'s. No improvement in WOT preformance.
Any help and suggestions?
Did I leave any thing out as far as info needed
Thanks

Is there any marine growth on the hull running surface, prop, and/or outdrive at this time? Just a thought.

Is the prop in good shape ?

No growth and the prop is like new.This started since 1st launch in May.Also no water in bilge .Waterline is in same place on hull.
Rechecked timing ,mixture on carb and steady vaccum at 18". All in specs.
Just at a loss as to what is causing this.

Thanks

It took me awhile to figure this out on mine last year. It turned out my new plugs were fouled by the Mercury fogging oil. Didn't get my rpm's back until I replaced the plugs.

This year, it took 4 full new tanks to get up to speed as well.

Would a missing vent plug on the prop have any effect?

Fixx
 

that would just cause more slippage on the hole shot..did you clean the rock fuel filter in the carb inlet..most people dont know that theirs 2 filters in some of the boats out their..also check your fuel pick up tube,,if it has a screen at the bottom of the tube it may be sludged up ..