Re: '72 merc shift/throttle cable set-up
If you look at the fast idle lever you'll see a small indent in it. In the control box there is a plastic spring-loaded plunger that's supposed to push into this indent, and provide enough pressure to hold down the fast idle lever when the gearbox is shifted to fwd and the throttle is advanced.
If this plastic plunger is worn out or stuck, it won't do it's job and you'll get the fast idle lever going up as you've seen. This lever has to stay down or it messes things up.
Another reason for this problem is the indent in the throttle lever being worn out. You can tell if there's an extremely worn mark on the bottom side of the indent. To fix this you may be able to take a larger-sized drill bit and "re-form" the indent such that it will "latch onto" the plunger a bit better.
If it's a problem with the plunger, you'll have to take the control box apart and either free it up or replace it.
When pulling the box apart, note that the main control lever is held on by one screw in the backside of the box. Also note there that in the upper front corner of the box resides a roller for the neutral detent. This roller is held in place by a flat spring which fits in that corner of the box. Use care when taking the box apart that these pieces don't fly off into some corner of the boat where you can't find them! It'll be pretty obvious how the neutral detent works once you get the box apart.
The plastic plunger must be free to move in and out, if not it's probably bound up with salt. You'll need to remove it and scrape any deposits out of the hole until the plunger fits freely. Lube the plunger with a bit of waterproof marine grease. If the plunger is worn (I've seen them where the spring was worn thru the end of the plunger), replace it. Hopefully Merc is still selling this part, haven't had to do one for a long time so you'd have to check with the dlr.
Before reassembling the box, lube the shift and throttle yokes/gears with a good waterproof grease and you'll probably find your shifting/throttle action much smoother. BTW inspect them for wear and replace if you see a lot of 'flashing' or worn spots on the gear teeth.
Place the neutral detent spring and roller assy in place, with the roller positioned in the 'notch' in the control yoke. As you reassemble, you'll have to move the fast idle lever a bit to align the brass roller into the groove in the control yoke. You'll also have to push the detent spring in a bit with a screwdriver to get it to engage the other half of the control box.
Hopefully, that gets to the bottom of your fast-idle lever difficulties!!! HTH...........ed
