'75 Merc 850 issue once warmed up..

Ok, so I've read countless threads in regards to the merc 850's, but haven't found any with issues like i have..

I acquired my 1975 merc 850 about 6 weeks ago. From the beginning I've experienced the fun task of cold starts which seems to be normal with these.. now onto the actual problem.. I've also been having an issue where it'll run great once warmed up, throughout the rpm range.. I do have to back it off from 3/4-WOT slowly or it will die, but my biggest issue is, for example yesterday, on the boat for about 3hrs., 20min WOT, trolled at 700rpm for about an hour, then 1/2-3/4 throttle for another 45min., pull in to the launch, let her idle for a couple minutes and then shut it down.. 5 min. Later i have the trailer in, get in the boat and have to choke the crap out of it to start, then it won't stay running anything below about 2k rpm.

This has happened every time I've had it out, and also when I raise the motor I get about 3/4 quart of fuel dump from bottom carb into the case. I just put new NGK buhw plugs in it yesterday before our trip, and actually it cold-started without even touching the choke and idled right away, but then that problem when we got back in..

I'm thinking maybe a bad float? I've done a lot of engine work on 4-strokes, but this is my first 2-stroke..

One other thing, once the engine is shut down for 1-2min, the primer bulb takes about 1 1/2 - 2 pumps to tighten back up.. sorry this is so long, but trying to give all the details.. thanks in advance for any help here..

Re: '75 Merc 850 issue once warmed up..

Sorry, wanted to ask one more thing... Im really wanting to tear the carbs apart tonight after work to give them a good cleaning, but worried about destroying the gaskets, and don't know how long it will be till I could get rebuild kits... Is this a valid concern with these carbs like a truck carb?

Re: '75 Merc 850 issue once warmed up..

The carbs are pretty simple. The gaskets are pretty wimpy, so you will likely destroy them. Carb kits are avail lots of places, including on the web, Merc Dealers, here at iBoats, ebasicpower and maybe at a local Auto Parts store. Gasket sets are less than $10. You probably also want inlet needles and floats. Foats are usually sold seperately.

Re: '75 Merc 850 issue once warmed up..

Thanks Chris, I kinda figured the gaskets would give me a problem since I've had to tear down and clean or rebuild carbs on a lot of atv's and almost always had to replace gaskets.. Now I know I'm gonna get crucified for this (LOL) but are there any gasket sealants I can use in place of or to seal a ripped gasket?

Re: '75 Merc 850 issue once warmed up..

You can try some auto silicone. Give it a few minutes to dry and then assemble the parts. Don't tell the other folks on this forum or they will really come down on you. All the other sealants (permatex) I know of are disolved by gas

Re: '75 Merc 850 issue once warmed up..

I decided to not go that route Chris lol. I'm gonna wait to tear into the carbs until I can at least get the gaskets... Anyone have any other ideas or things I can check?

Re: '75 Merc 850 issue once warmed up..

Sorry, I hate bumping threads, but... *BUMP* Really looking for some advice here on what the possibilities are of what's causing this problem..

Re: '75 Merc 850 issue once warmed up..

The older Mercs really loved a LOT of fuel to cold start, and cranking speed is essential in creating the intake vacuum to pull in that fuel.

Check/clean starter commutator as well as all battery cable connections, BOTH ends, including the starter cable.

Re: '75 Merc 850 issue once warmed up..

CharlieB said:

The older Mercs really loved a LOT of fuel to cold start, and cranking speed is essential in creating the intake vacuum to pull in that fuel.

Check/clean starter commutator as well as all battery cable connections, BOTH ends, including the starter cable.

Click to expand...

Thanks Charlie, I'll give a look at those things this evening.. but the cold start is whats throwing me off since it actually fires right up and idles when its cold, but turns into a teenage drama-queen when i try to start it after its warmed up.

Re: '75 Merc 850 issue once warmed up..

While you have the carbs out, pick up some carb dip cleaner and soak them over night (available at just about any autopart store).

Just make sure you remove any plastic parts from the carb, including the little venture that sits at the end of the main jet (should be able to see it looking through the air intake).
If it breaks make sure you replace it.

Also make sure that your choke has the return spring on it, and that your pumping water
The top cylinder should be about the same temp as the rest of them, rest your hand on the top of the engine just above the head to get an idea.
If you feel a difference in temp, you may not be getting water to the top.

In general you should be able to hold your hand on the engine with out pain at full temp.

Re: '75 Merc 850 issue once warmed up..

Carb cleaner will not harm the plastic venturis, but if you break them, they may be NLA. Blow out the little drilled holes on the starboard side of the carb throats. They are real small...

Re: '75 Merc 850 issue once warmed up..

Carb cleanliness is only have the equation, float height is a critical factor, too high and they run overly rich and can warm flood in a heart-beat.