Engine revs when I hit 4K rpm

Bruce858

Recruit
Joined
Sep 20, 2018
Messages
1
Mercury 60hp four-stroke on a 20 foot Bennington Pontoon. When the tach hits 4k, the engine revs and I have shut the throttle down to get it to stop. Someone suggested it may be a disruption of water through my prop and I need a different size. Any suggestions?
 

alldodge

Moderator
Staff member
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Mar 8, 2009
Messages
40,581
Your prop is probably ventilating, which means the prop is coming out of the water a grabbing air. Look at the motor ventilation plate (just above the prop) and see if it is at the same height of the toon. If its higher then your motor needs to be lowered deeper.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Could be a cocktail of poor deck weight distribution, engine sitting way high on transom along badly trimmed, prop hub with hidden issues..Check all those ingredients...LOL!!

Happy Boating
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,296
welcome aboard.

motor height is critical on 'toons

make sure your AV plate is about 2" under water when your moving along. most likely you have to lower the motor an inch or two
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
What happens when you turn the wheel hard over at about 3500rpm? Another vote for the engine being mounted too high if it ventilates doing that as well.
 

Sprig

Chief Petty Officer
Joined
May 2, 2016
Messages
571
First I’d check the prop. If there are dings in it and/or bent chipped blades you are experiencing cavitation. I’m assuming this is New and hasn’t always happened so I think it’s probably the prop. If the engine is mounted too high you’d be experiencing ventilation and it would have been happening since you got the boat. Could also be a spun hub. Check your prop and the hub. When did this start happening?
 
Last edited:
Joined
Sep 24, 2018
Messages
2
If this is a new problem on a boat that has previously run fine then I would say it sounds like a case of a classic failed prop hub.
 
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