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Chainsaw fuel system... HELP!!!

Discussion in 'RD's Lounge' started by Froggystyle, Mar 16, 2009.

  1. Froggystyle

    Froggystyle Well-Known Member

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    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. rrrr

    rrrr Well-Known Member

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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. :D

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

    Maybe the diaphragm is holed or there is a vacuum leak in the crankcase.
     
    Last edited: Mar 16, 2009
  3. Outnumbered

    Outnumbered Don't tax me bro!

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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. DirtyWhiteBoy

    DirtyWhiteBoy WILL WORK FOR FUEL

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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:eek:
     
    Last edited: Mar 16, 2009
  5. Froggystyle

    Froggystyle Well-Known Member

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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. Froggystyle

    Froggystyle Well-Known Member

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    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.

    [​IMG]
     
  7. rrrr

    rrrr Well-Known Member

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    After thinking a bit......:hmm

    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. :D
     
  8. Froggystyle

    Froggystyle Well-Known Member

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    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: Mar 16, 2009
  9. TITTIES AND BEER

    TITTIES AND BEER Well-Known Member

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    its almost summer you won't it so store it till next year:D:beer
     
  10. Froggystyle

    Froggystyle Well-Known Member

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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.
     
  11. Froggystyle

    Froggystyle Well-Known Member

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    Actually, I need it badly. I am doing a little four wheeling for a contract I am working on, and need to open up the course a bit. Gonna use it quite a bit here in the next month or so, which is why I bought it.
     
  12. Roaddogg 4040

    Roaddogg 4040 Well-Known Member

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    I guess my advise in your other thread was spot on... Sorry that you had such a tough time. I love NEW stuff...

    Steve
     
  13. Froggystyle

    Froggystyle Well-Known Member

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    Absolutely good advice... but I paid $40 for this one and already slung it for $100 once it is running.

    I have used nothing but good saws up until now, and I talked myself into thinking this one was OK for the price, right up until I started dicking with it.

    Gonna look for a new saw more than likely. But, who knows... it wouldn't surprise me in the least to see someone on here looking to sell theirs.
     
  14. daniberry

    daniberry Banned

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    Before fueling, clean the fuel cap and the area around it to ensure that no dirt falls into the tank. Always position the machine so that the cap points upward. In order to reduce the risk of fuel coming in direct contact with skin and inhaling fuel vapor, remove the fuel cap carefully so as to allow any pressure build up in the tank to release slowly.
     
  15. Old Texan

    Old Texan Well-Known Member

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    According to dates, I'd be surprised if after 8 years, advise is still required.:skull
     
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  16. 2Driver

    2Driver Well-Known Member

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    Its a McCulloch. He could very well still be trying to start it.
     
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  17. Old Texan

    Old Texan Well-Known Member

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    I hear ya.....:smackhead:lmao
     
  18. rivermobster

    rivermobster Club Banned

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    Oh shit, who did the drive by????
     
  19. No Butt No Putt

    No Butt No Putt Well-Known Member

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    Take that McCulloch chainsaw over to your trash can and throw it in there and forget about it. You'll never make it run right. I used to fix chainsaws and these bastards never run right.

    Then go buy a Stihl and you'll never have another problem again.
     
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  20. JDKRXW

    JDKRXW Well-Known Member

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    Fixed it for you :)
     
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  21. buck35

    buck35 Well-Known Member

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    LOL! I bought a stihl weedeater many years ago . Ran great for a season, then wouldn't run for shit, took it to get fixed ... a hundred and a quarter later its moderately better. :grumble: Went and bought a honda and never looked back.:thumbsup
     
  22. No Butt No Putt

    No Butt No Putt Well-Known Member

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    Stihl!!!
     

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