Warm up lever on 1974 Mercury Thunderbolt 850

[toughinboots]

Hello,
I recently bought an older boat. this is my first boat of my own. The engine is a 1974 Mercury Thunderbolt 850. 85 Horse. Ok here is my question... When I put the warm up lever on the engine will start right up and idle nicely. As soon as I lower the warm up lever to engage in gear, the engine dies immediately. It takes several attemps to get the engine to remain started and kick in to gear. Then it runs fine with lots of power. Just curious what might be the problem with this. When I put ear muffs on the motor in the drive way it will fire right up and idle just fine without the warm up lever engaged. Thats what puzzles me.

Also this is a dumb question but how do I know for sure if I need to add oil to the gas? The previous owner said one time you don't have to and the next time he said I did. But the gas tanks he gave me we straight fuel so I am just curious. This is a 4 cylinder engine. If anyone has any insight for me please let me know! Appreciate any help I can get.

Justin

 

[Steven1962]

Re: Warm up lever on 1974 Mercury Thunderbolt 850

Do not run that motor on straight gas. 50:1 mix unless you want to replace it.

 

[toughinboots]

Re: Warm up lever on 1974 Mercury Thunderbolt 850

Ok thats what I thought. Just wanted to double check. Thanks!

 

[Chris1956]

Re: Warm up lever on 1974 Mercury Thunderbolt 850

Tough, You have a very common problem. Your carbs are either misadjusted or dirty. I suppose there is a small possibility the idle is just set too low. Idle speed should be 850-1000RPM. if it is less than 950RPM, you can turn it up a bit. That motor should have a idle stop screw on the dissy upper right. Don't mess with the throttle stop or max spark screws...

If the RPM is OK, an the motor is warmed up and idleing, open the top carb idle mixture screw 1/8 turn (CCW) and see if theat keeps it from dieing when you put it into gear. Repeat with the bottom carb, and repeat if necessary. if it cannot be adjusted, the carbs need to be torn down anc cleaned. Get a service manual to help you.

 

[DRIFTER_016]

Re: Warm up lever on 1974 Mercury Thunderbolt 850

How long are you letting it warm up before putting it in gear?
My 90 horse take a couple of minutes on fast idle before it is warmed up enough. I get it running on fast idle and adjust engie rpms to about 2000 and let it run for a minuite or so before taking it off fast idle and attempting to move the boat.

 

[toughinboots]

Re: Warm up lever on 1974 Mercury Thunderbolt 850

I have let it idle for a couple of minutes before and the problem still happens. I was going to tear into the motor tomorrow and see if I can figure it out. I was also considering rebuilding the carb as well as that seems to be a common problem too. Thanks for the help!