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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4717
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a 76 mercury 115hp thunderbolt. I took it out for

Customer Question

Hi, I have a 76 mercury 115hp thunderbolt. I took it out for the first time and didn't have any problems. The next time I took it out, I could get about half throttle, right before the boat started to plane out, and it would throttle way up and slow down( could still drive around half throttle). We had thought the lower unit is going out. I had got another lower unit and installed it and same problem. What do you think is my problem, any info would be greatly appreciated.
Submitted: 1 year ago.
Category: Boat
Expert:  Jeff G. replied 1 year ago.
From your description the hub in the propeller is slipping. There is a rubber shock absorbing hub that will slip particularly if you have hit anything, even if weeks ago. You can take the propeller in to a prop shop, or if there isn't one near you check at your local boat dealer, they may be able to replace the hub or will know of a shop you can send the propeller in to.
Customer: replied 1 year ago.
Yes, that is what my first reaction was is that the propeller was slipping, but when I changed out the lower unit (propeller and all) that would have a different hub on it right? It seems like it would be coming from lower unit, propeller shaft, or propeller hub, but I changed out whole lower unit. Just don't see it being the problem with two different lower units.
Expert:  Jeff G. replied 1 year ago.
Try this.
Remove the spark plugs, Put a block of wood between the propeller and the cavitation plate so the prop can't move.
Put a 15/16" socket on the flywheel nut and turn it clockwise. A large breaker bar is best.
Have a helper watch the prop and propeller shaft. Shift to forward gear.
If the flywheel turns and the prop and propshaft doesn't then likely you have a stripped spline in the crankshaft.
This will eliminate any mechanical problem with the crankshaft.
Typically that engine doesn't surge like you describe unless you have a fuel related problem. The ignition being a distributor ignition usually doesn't surge, it works or it doesn't.
Try this and post back and we can go from there.

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