Re: 1975 Mercury 500, why wont the leg go back in?
1) Make sure the crank and driveshaft are engaged by rotating the flywheel. You are close enough at this point that they should already be, but it doesn't hurt to check. Note: make sure key is OFF and plug wires PULLED.
4) Make sure the water tube has properly engaged the pump. You can see this with a flashlight as you're pushing the lower unit up into place.
3) Upper shift shaft not engaging properly with lower shift shaft. Since you have a "problem child", it's time to disconnect the shift cable at the motor and 'jiggle' the motor's shifter arm back and forth to align the upper shift rod with the lower unit's shift shaft. If you can shift to Neutral and Reverse you're already there.
4) Not as common but seen it happen, the rubber gasket which seals the large exhaust tube can become damaged or distorted and if the exhaust tube doesn't slide cleanly into the gasket you're not gonna get it all the way up.
Salt buildup will distort the gasket. You may need to pull it out and scrape out the deposits. I put a thin layer of waterproof marine grease on both inside and outside of the gasket, and on the exhaust tube, too. Needless to say if the tube has nasty deposits on it you're not gonna be able to slide the lower unit home.
On rare occasion the exhaust tube is slightly out of line and then it won't slide into place. Reposition the tube and try again.
You're probably gonna have to pull the lower unit back down to inspect these things, check each carefully and I'll bet you find your problem area.
HTH & G'luck............ed
