Merc. 850 fuel problem

[stormmn]

Hi all...

I have a 1973 Merc. 850 that I just put on my boat to replace the old 850
(1974) that lost compression on #4 cylinder, note, the old 850 although running on 3 cylinder had no fuel issues, as part of the maintenance the tank was cleaned, fuel lines on the boat, prime bulb, and fuel seperator were replaced at the start of 2010.

The replacement 850 runs fine at all RPM'S as long as I keep it primed with the prime bulb. I replaced the fuel pump with the one off of the origional motor and still had the same issue, so put a rebuild kit in the other fuel pump and still have the same problem, engine will run out of fuel if I do not keep it primed with the bulb.

I will bypass the line to the fuel pump and the coupler and see if that resolves the problem as that is all that is left that is not new.

P.S. 140 to 145 compression on all cylinders.

 

[chum1]

Re: Merc. 850 fuel problem

I will bypass the line to the fuel pump and the coupler and see if that resolves the problem as that is all that is left that is not new.

let us know if that worked, sounds like its sucking air somewhere
( sorry for the sliding scale highlite as my computer geekness skills are still in the learning curve mode)

 

[dwco5051]

Re: Merc. 850 fuel problem

If that has the round fuel pump cover with the single screw in the center check it close for a crack. I have seen a couple of these that have been overtightened to try and stop the leaking by the "O" ring that have been cracked. This is the cover with the hose barb attached. The two I remember were on a '76 and I think the other was a "75.

 

[Chris1956]

Re: Merc. 850 fuel problem

The '73 850 should could have the dual fuel pumps mounted on the transfer port covers. Also, those old fuel lines have the brasss threaded ends, and wear out. I would look at the condition of the fuel pumps and the fuel lines.

Are you using the Merc Quick disconnect fuel line? I would replace the female fitting, if it is not real tight.

 

[stormmn]

Re: Merc. 850 fuel problem

this engine has only the one fuel pump with the three bolts and brass threaded ends, and no "round" filter.
I replaced the lines with new lines and straight couplers to eliminate the Merc. style lines, same problem.

Removed the intake line from the fuel pump and ran it to my portable fuel tank, same problem.

While toying with the issue I noticed that if I hit the choke when it started to die it would flair in RPM's and run for another 30 sec. or so before it would try to die out again, as long as I would keep hitting the choke it would keep running.

Since the RPM from idle to max. as controlled by the lever on the the control only advances the spark and not carbs. I'm now thinking it may be the carbs.

one question, how much fuel is the fuel pump suppose to put out at cranking RPM, both the pumps that I have only put out about a table spoon in 15 cranks....

thanks

 

[dwco5051]

Re: Merc. 850 fuel problem

Cranking speed will not show how well the fuel pump is working. A pressure gage teed into the top carb feed line should read at least 2psi when running full throttle. Testing the flow rate at cranking speed will only show it is working not how well it is working.

 

[Faztbullet]

Re: Merc. 850 fuel problem

Make sure you installed the correct fuel pump kit as the A1 is different than the A5 as gaskets for pulse chambers are cut differently.....