rpm's jumping

[basherboy1]

Hello all !! I'm new to this so I hope you can forgive my basic questions. I have a new to me 1972 coronet with 2 volvo aq130's. When I plain the boat one of the motors jumps rpm's (3000 to 4000 and back again) it almost sounds like its coming out of the water, but I know its not. The motor the is running fine has a new prop the problem motor has an older prop with a few chips that looks like its been around the block. Could this be the problem..??? also can anyone tell me why the prop's turn in diffent directions.. thanks for all your help... Bill

 

[navigator336]

Re: rpm's jumping

The props turn opposite to counter balance the engine torque. This makes the boat handle better especially when backing up.<br /><br />Does the engine actually change rpm's or is the tach just saying it is? Tach connections will cause them to be flaky.

 

[airman]

Re: rpm's jumping

Sounds like the bad propeller is cavitating. This is when a propeller manages to suck in some air or is able to vaporize the water it is receiving. Make sure there are no obstructions to the water flow for that prop. If not, I would suspect the prop.