Replaced Shift Cable on 1972 Merc 500 Thunderbolt with Pump- Now Boat Wont Plane

I replaced the shift cable on my Merc 500. It is a pump unit and the old cable split open. It's on a 19 foot Smokercraft Sled. Both the boat and motor are vintage 1972. When I went to test it in a nearby river the shift lever seems to work fine and the unit will go into forward, neutral and reverse but when I give it throttle in forward it will not get into plane. The motor will rev up to about 3/4 and then will bog down a little. The most I can get out of it for speed is about 8-9 miles an hour. Before the old cable broke the motor had no problem taking the boat 20+ miles per hour and would handle pretty much any river in Oregon. I've searched the Mercury forum but can't find a discussion that helps me know how to diagnose and fix the problem. Any help would be appreciated.

Re: Replaced Shift Cable on 1972 Merc 500 Thunderbolt with Pump- Now Boat Wont Plane

I am not sure what a "Pump unit" is. "Shift cable split open" is also a strange term. You may want to clarify. Are you talking about hydraulic steering?

You do have cables on correctly? Shift cable always moves first when moving throttle handle from neutral to forward, right?

Re: Replaced Shift Cable on 1972 Merc 500 Thunderbolt with Pump- Now Boat Wont Plane

Chris1956 said:

I am not sure what a "Pump unit" is. "Shift cable split open" is also a strange term. You may want to clarify. Are you talking about hydraulic steering?

You do have cables on correctly? Shift cable always moves first when moving throttle handle from neutral to forward, right?

Click to expand...

Pump unit is the jet unit. The motor is a jet and not a prop. There are two cables from the hand controls to the motor:the throttle cable and the shift cable. The shift cable got a split in the outer sheathing and split open. I replaced the damaged shift cable. Yes, the shift cable moves first when going into forward. The control plate on the jet unit seems to be working alright. Fully open in forward and fully closed in reverse.

Re: Replaced Shift Cable on 1972 Merc 500 Thunderbolt with Pump- Now Boat Wont Plane

Chris1956 said:

I am not sure what a "Pump unit" is. "Shift cable split open" is also a strange term. You may want to clarify. Are you talking about hydraulic steering?

You do have cables on correctly? Shift cable always moves first when moving throttle handle from neutral to forward, right?

Click to expand...

Pump unit is the jet unit. The motor is a jet and not a prop. There are two cables from the hand controls to the motor:the throttle cable and the shift cable. The shift cable got a split in the outer sheathing and split open. I replaced the damaged shift cable. Yes, the shift cable moves first when going into forward. The control plate on the jet unit seems to be working alright. Fully open in forward and fully closed in reverse. Is it possible the timing got out of whack when the shift cable was replaced?

Re: Replaced Shift Cable on 1972 Merc 500 Thunderbolt with Pump- Now Boat Wont Plane

The timing should not have changed when you replaced the shift cable. Do both carbs open all the way when you push the throttle wide open (motor not running)? If so, take a compression test and spark test and post back. If those tests are good, the next thing to do is a link and synch to assure the timing is correct.