20 HP Mariner 4 stroke not as fast as old 20 HP Mercury 2 stroke

aarrjimlad · #1 July 7th, 2009, 08:45 AM

My new engine is no where near as fast as the old one, the old Mercury 2 stroke pushed my fishing boat along at 18 knots, the max I now get out of my new engine is 13 knots, but the engine only revs to 3500, which is 2000 or so less than it should do. The prop is a 10p, I have checked the transom height AV plate is level with bottom of boat and leg is aligned at 90 degrees to the water line, as per manufacturers specification. I have also checked the throttle to ensure correct range of movement.
Your help would be greatly appreciated.

CanWoodsman · #2 July 7th, 2009, 11:31 AM

There is defiantly a problem with that low RPM. A 10" pitch doesn't seem to high. As it's a new motor if it's under warranty have the dealer check out the problem. Sounds like a fuel flow problem of some sort.

Rick

aarrjimlad · #3 July 15th, 2009, 08:59 AM

Thanks CanWoodsMan, returned to the dealer and was unable to identify any fuel related problems, have since tried 2 additional props, a 8p and 5p, both of which gave higher rpm, but also a slight reduction in speed, seemed like constant speed/variable noise. But............. then fitted Davis Doel Fins and what a difference, can now reach 16 knots (it is a heavy boat) have yet to try with good weather/sea conditions, a rare thing here in the UK.