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  1. #1
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    Chainsaw fuel system... HELP!!!

    Just bought a used McCulloch Chainsaw on the cheap knowing that the fuel system needed work. As with most used chainsaws, it had been sitting, and the fuel system had all aged and cracked. So, I bought new lines and filters yesterday, cleaned the saw up, put in a new primer bulb etc... and started running into a issue.

    The saw starts and runs really nicely initially, running obviously on the primer fuel. It then dies after a couple of seconds. Keep lightly pushing the primer bulb, and it will keep running. I took apart the carb, and everything looks really nice and clean, but what I don't know about two stroke carburetors fills books.

    My question is, "How does it work?"

    There was four total rubber lines in the saw, one obvious one going from the bar oil to the oiler system, two going into the fuel tank and one going from the bulb to the carb. For the sake of discussion, lets identify the two tank holes as "A" and "B" and the two Carb fittings as "1" and "2". The primer bulb lets go ahead and throw in here with fittings "I" and "O" for in and out.

    The hoses were a mess when I pulled it apart, and broken in several pieces. So, I don't have a guide to go from. I have ordered a new carb kit on general principles, so that is coming, but lets make sure I got the plumbing right to begin with.

    The two holes in the tank are just that... holes. The hoses pass through them to seal them. So, I have no idea how far to pass them through.

    The filter came out of the tank on a longer hose, so I assume that the filter needs to be in the bottom of the tank, and that it goes to the bulb.

    I have plumbed it like this... "A" goes to a long hose to the bottom of the tank and to the "I" fitting on the primer. From the "O" fitting on the primer I went to the back of the carb (Fitting "1"), to a fitting just below the idle circuit needles, and into what appears to be, once pulled apart, a diaphragm that controls a needle and seat valve. On the forward part of the carb there is a 90 degree fitting (Fitting "2") that didn't appear to go all the way into the tank bottom, and I assume is some kind of vent?

    Bottom line, it doesn't work, and the carb didn't look that bad. Did I mess up the plumbing?

    PS... How do these things pump fuel?

    Thanks,

    Wes

  2. #2
    Senior Member rrrr's Avatar
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    The fuel pump has a diaphragm that is operated by crankcase vacuum as the piston moves up and down.

    I found a link for you....it's easier than 'splanin how it works.

    http://www.gardengrapevine.com/ChainsawCarbRepair.html

    Maybe the diaphragm is holed or there is a vacuum leak in the crankcase.
    Last edited by rrrr; 03-15-2009 at 05:43 PM.

  3. #3
    Don't tax me bro! Outnumbered's Avatar
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    I have had to cut the "plastic cams (stops)" off of the mixture screws to get some of those little 2-stokes to run. The CARB rules mandate limited range on the mixture screws. If you take a razor and cut the plastic stop cams off you can tune them better and usually get them running better. Usually there is a high and low mixture screw. I think the black one is low and red is high on most of them.

    Sounds like you plumbed it right. I think the diaphram is your fuel pump like on the old Sea Doos.

    Hope this helps.

  4. #4
    WILL WORK FOR FUEL DirtyWhiteBoy's Avatar
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    I don't know about that brand, but how is the compression? (diaphragm pump?)I think the plumbing should be ok if it starts at all. I have not worked on a 2 stroke in ages. Hopefully the carb kit will help out.

    Good luck keep us updated, I have a old Pulan that I'm sure will need help pretty soon:o
    Last edited by DirtyWhiteBoy; 03-15-2009 at 05:43 PM.

  5. #5
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    Thing runs hard once it is running, and only for a couple of seconds. It even idles nicely once it is running.

    I have a sneaking suspicion I put the carb gasket on upside down and it isn't allowing vacuum into the carb. That is the only thing that makes sense.

    The vacuum is in the baseplate, right?

    Thanks guys.

    Wes

  6. #6
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    Quote Originally Posted by rrrr View Post
    The fuel pump has a diaphragm that is operated by crankcase vacuum as the piston moves up and down.

    I found a link for you....it's easier than 'splanin how it works.

    http://www.gardengrapevine.com/ChainsawCarbRepair.html

    Maybe the diaphragm is holed or there is a vacuum leak in the crankcase.
    Thanks for the link. It that link though, lies my biggest question... Which is the fuel inlet, and which is the vent?

    The one that I thought was the fuel inlet is smooth, with no barbs. It is under the pump diaphragm, and I think it went to the primer bulb. The other side is a black fitting with barbs.

    Does make sense that the "pressure" side of this equation would have a barb, while the vacuum side wouldn't, but once you push the bulb, it is pressured too...

    Bottom line, which is the inlet, the brass or the black fitting? Look in the upper right picture, the lower left shows the brass fitting, upper right shows the black one.


  7. #7
    Senior Member rrrr's Avatar
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    After thinking a bit......

    Plumb the fuel line from the tank to the brass fitting and from the nylon fitting to the primer. The outlet of the primer should go back to the tank return line.

    The gas has to feed the fuel pump first, and the other side is the return. If the saw didn't have a primer it would go directly to the tank return.....so plumb the primer inline.

    I useta race karts with McCulloch 125CC engines but that was 35 years ago....the mind is weak LOL.

  8. #8
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    Quote Originally Posted by rrrr View Post
    After thinking a bit......

    Plumb the fuel line from the tank to the brass fitting and from the nylon fitting to the primer. The outlet of the primer should go back to the tank return line.

    The gas has to feed the fuel pump first, and the other side is the return. If the saw didn't have a primer it would go directly to the tank return.....so plumb the primer inline.

    I useta race karts with McCulloch 125CC engines but that was 35 years ago....the mind is weak LOL.
    That was it! Saw runs well now. I had the primer on the wrong side. I didn't realize it was on the return side. I plumbed the pickup to the black fitting, the brass to the inlet of the primer and the bypass from the outlet of the primer back to the tank and it works well.

    That is, it works well with the choke on... won't run with it off. I am going to start messing with the mixture screws now.

    Thanks bro!

    Wes
    Last edited by Froggystyle; 03-15-2009 at 07:10 PM.

  9. #9
    Senior Member TITTIES AND BEER's Avatar
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    its almost summer you won't it so store it till next year
    " FILL YOUR HANDS YOU SON OF BITCH "

  10. #10
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    Well, that sucks.

    I just started playing with the mixture screws and now it won't run. The manual says to have them both baselined at 1 turn out, and that optimal will be between 1 and 1 1/8 turns out. When I put them in, they turned over two turns each...

    So, I tried putting it at one out, and it won't run, choke on or off. Then I added another turn to each, putting me where we were, and it still won't start or run...

    I think it is time for a rebuild kit and someone who knows what they are doing.