Mercury 850 thunderbolt 85 hp ser 4539621

[Bob1944]

I think this is a 76 or 77 Thunderbolt.

to start it and keep it running the choke throttle needs to be more than half open. After a good warm up it can be shifted as long as that choke throttle is still open that far. Otherwise it stalls. Once I get it going I can open it up and put the choke lever down but then after maybe four or five minutes it quits. I have a dickens of a time getting it started again.

Do you think the fuel pump bladder needs to be replaced or is there someplace else I should look?

bob

 

[Chris1956]

Re: Mercury 850 thunderbolt 85 hp ser 4539621

When you say "Choke Lever" and "Choke Throttle" you mean cold start lever, right?

It sounds like the idle speed is set incorrectly, the carb idle mixture is set incorrectly, the link and sink is incorrect or the carbs are dirty, or some combination.

When was the last time the boat was used? Was is possible that the old gas was left in the carbs to varnish up? Do you think someone messed with the idle speed or timing adjustments?

If you have no answers, you probably need to start with getting a Service manual, cleaning the carbs, setting the timing and adjusting the idle mixture.

 

[Bob1944]

Re: Mercury 850 thunderbolt 85 hp ser 4539621

Super, now I know what to call that lever. Cold start lever. Great. According to the seller he had it tuned a year ago. But it was last year's fuel in the tank so I think the same with the carb. Based on your answer I think I will take it to a marina and have them check the things you suggest. Thanks.

bob