Merc 850 ?Fuel issues? when warm

[LV2BT]

Hi all,

Need help with a 1963 Merc 850 (85hp). This is an inline 6cyl with 2 fuel pumps and 3 carbs if its helps. Bought her used and it has been sitting for a few years so I was quite surprised when I filled her up with fresh fuel and she fired right up. Runs great with or without load for 10 - 15 minutes and then seems to starv for fuel as pumping the primer bulb continuously gets it going again. Rulled out the tank and line by using known good ones. Bought a pump rebuild kit and redid the top pump (stupid me thought there were 2 diagrphrams per kit) and no change. Haven't gotten to the bottom pump yet. Thought I would ask if this is a known issue before picking up more parts (2nd fuel pump kit and parts needed to do the fuel vacuum/pressure testing). Should mention that the diagrphams in both pumps "looked" fine (no visible pinholes or obvious tears) as did the check vavles. Not really keen on rebuilding the carbs since it seems to run 100% when cold or sitting for 20 -30minutes. Plenty of power, no issues at WOT until she warms up. Once dead, its prime, prime, prime, prime, prime, prime some more, prime, choke crank crank crank vrooom..... runs ok for 1-2 minutes then runs out of fuel (or so it seems)

Any ideas?

Oh.. motor also has a bad rectifier. not sure its relevant but though someone could confirm.

 

[Fuzzytbay]

Re: Merc 850 ?Fuel issues? when warm

Rebuild the bottom pump, those old girls like the gas...once you have done that, if you still have the problem, its on to the carbs. Don't fiddle with them untill the pumps done. This motor needs both pumps working in tandem to run correctly. Take a good look at the fuel lines too, make sure they are not leaking, its also possible they are crumbling inside and pluging up, if you are useing ethonal enriched fuels that is almost a garrenty.

 

[LV2BT]

Re: Merc 850 ?Fuel issues? when warm

Thanks Fuzzytbay, I agree, the 2nd pump is the logical place to start but I'm wondering why the system craps out after warming up? You would think that the pumps either work or not regardless of motor temp.

For the fuel, I run premium due to the lack of ethanol. Not sure what the previous owner ran though. As for the lines, I didn't see any signs of degredation in the fuel pumps when I dissassembled them which makes me think they might not be a factor. Having said this, I will likely replace them as they look original which makes them over 40yrs old.

I wonder if runing some carb cleaner in the next tank would help? My biggest issue is that test runs occur at my folks cabin which is a 2 hr haul overland + 20min boat ride (no road access is part of the appeal). Usually burn 5 gal just getting there and back. I would love to know that I have the problem licked before I get there. 'Suppose I could bring the carb kits along and do them onsite if needed.

 

[Fuzzytbay]

Re: Merc 850 ?Fuel issues? when warm

Ok if you put in carb kits you will also need to do a link and sync, which requires either a dial indicator, or two NLA mercury special tools, to complete. So before you go down that road, make sure everything in the fuel side, short of the carbs are good.
Here is a link to the procedure to follow, if you remove your carbs.
http://www.maxrules.com/timingindex.html
Now if you wish to get an idea of if the carbs are crapped up, you can, remove the high speed jet cover nut, (brass one on the bottom of the carbs) and look to see if you notice any corrosion/dirt/green stuff there. Second you can remove the filter screen cover (top nut on the carb, where the fuel lines attach) and check if they are dirty.
A note on the fuel pumps, did you test if the check valves are working? You do this by blowing into the pumps,against the flow of fuel direction, if there is any leakage, they too need to be replaced. Any weakness in the fuel system cause's these motors to not perform at least, and hole or melt pistons at worst.
As for a mechanic in a can, well if that worked, why have carb kits?
For a quick cure, now I have never done it, but I read it here on the forum, you could remove the high and low speed jets, and blow air through them, and see if it helps, but again, that to me is a last resort type method, since if there is dirt, then you need to take out the welsh plugs to properly clean the passages.