mcculloch backpack blower problem

Discussion in 'Mechanic and Repair' started by jestal, Jul 29, 2007.

  1. jestal

    jestal LawnSite Member
    Posts: 7

    Volunteered to help my neighbor with his McCulloch backpack blower and now it has me stumped.

    It is a 30 cc version.

    After a lot of pulling and choking, etc..... it will run briefly then flame out. Not to run again at all. All the characteristics of fuel starvation/carb problem but when it refuses to run I give it a shot of gas and/or ether and it STILL refuses to run making me think it is spark. The spark seems solid, however, when I remove the plug, ground it and spin the motor over. Even after choking the carb and pulling repeatedly the plug still looks dry when I remove it making me think it is fuel. But it won't run with the shot of ether so that says spark.

    Carb has been apart several times and is perfectly clean and nothing apparent wrong with it. Diphragms look good and are soft and in good shape. Primer pump works fine. Fuel lines were suspect but have been replaced with no change.

    Muffler is clean and not plugged. Makes no difference to the problem if the muffler is off or on the engine.

    Pulse port from crank case to carb is open and clean.

    Adjusted gap from coil/ignition to flywheel with no change.

    Engine has good compression and the piston and ring seems fine looking thru the ports. I haven't pulled the jug off yet but that may be next.

    I'm not new to diagnosing and fixing engines but this particular one has me scratching my head at the moment. It is almost as if the spark is working fine under no load but drops out when there is compression/fuel present. I haven't resorted to buying a coil/ignition yet but I wondered if there is any one with any history of one failing like this or any other ideas....... I'm open to them.
     
  2. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    Some plugs will not fire under compresson. It's cheap - change it out. Then that can be eliminated.
     
  3. jestal

    jestal LawnSite Member
    Posts: 7

    Yea, should have mentioned that was the first thing I tried. It had some sort of chinese chenge fenge (sp?) branded plug in it. Two Champions later it acts the same.... That wasn't it.

    BTW...other things changed are draining the tank and replacing the fuel/oil with known good stuff. I pulled the flywheel off to make sure the key is in place and not sheared or something. Timing seems close to my preconceived notions at about .024 before TDC. Close enough to run, anyway. All the kill switch wiring and such is elminated at this point on the bench so nothing is shorting the coil as far as systems on the blower.

    This thing is still a mystery to me. It isn't so much the idea of repairing it at this point as it is the challenge of figuring out what the heck is going on. :hammerhead:
     
  4. Breezmeister

    Breezmeister LawnSite Bronze Member
    Male, from South Jersey
    Posts: 1,571

    Never worked on a Mc, so.. did you check for air leaks on the gaskets to the carb and insulator ? When you had the flywheel off, how was the seal on the crank, and there is one on the recoil side as well, was there any fuel or oil residue ? Bad seals will effect the pules port. And lastly, I had a tanaka TBC 2501 come in last month, after about an hour of working on it, I called
    Tech Support and walked though what I did, even a new carb, and what we came up with was that the skirts on the piston had worn just enough to effect the pulse port so the carb would not work......
     
    ericg likes this.
  5. jestal

    jestal LawnSite Member
    Posts: 7

    Checked for leaks at the carb and insulator by dissassembling and inspecting gasket. Whoever put the thing together loves loctite..... I felt lucky to get all the screws out even with a hand impact. Gaskets all look good with even, continuous contact patch. The pulse port seems to "pulse" fine as it will blow grease back out of the port if I pack the grease in the port with the piston at TDC and pull it over.

    I swapped a Zama carb from my Homelite leaf blower onto the MC (it came with a Walbro carb) and it still will not run even though the Homelite works fine before and after.

    I went ahead and pulled the jug off. I have had the ring stick in the piston groove on some of my Homelite stuff due to poor oil/overheating (which seems cronic with my Homelite equipment until I went with "good" oil) so I thought it might be that. Piston looks fine. Ring looks good. Free in groove and not worn perceiveably. Base gasket and end cap gaskets look fine with no sign of leak at all based on lack of leak track, deposits outside of engine or anything like that and the gaskets have a nice, firm continouus contact pattern. The mystery deepens.

    This motor has a one ended crank with a seal only on the flywheel side. Seems to be fine. No oil or deposits past it on the flywheel and the seal looks good and is completely intact.

    Hard to understand the piston skirt worn enough to cause the pulse port to not work unless the piston was rocking so much it was unseating the ring..??..then the compression would be the issue. In any case, the piston skirts on this Mc look fine anyway. Slight coat of varnish on the skirt and no sign of wear or skuffing. Still has the machining marks in the piston skirt so it cannot be worn much. Piston fit feels tight and it did have good compression when it was together. No sign of detonation or overheating of the piston crown.

    I am still open to any ideas and haven't discounted anything yet.....
     
  6. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    I don't know why you can't get it to pop with starting fluid. That makes me think that it a coil issue. It's harder to jump a spark under compression.
     
  7. Breezmeister

    Breezmeister LawnSite Bronze Member
    Male, from South Jersey
    Posts: 1,571

     
    ericg likes this.
  8. jestal

    jestal LawnSite Member
    Posts: 7

    Finally broke down and bought a new coil/ignition module for the McCulloch. $42 bucks so I wanted to be sure. That fixed it. The old coil/module would spark fine when the plug was out of the engine and grounded. Must have been breaking down under combustion loads and/or heat. Starts and runs like new with the new coil. Thanks for the ideas and help.
     
  9. SLR

    SLR LawnSite Gold Member
    Posts: 3,218

    "Psst..Jestal"..all McCulloch was innocently trying to do was patent a good solid paperweight to to paying public,stop papers flying around on windy afternoons..however some have gotten resourcefully wise,and discovered it makes one of the best fishing spot makers! ;)..abruptly tell your neighbour to 'stop' shopping in the tackle box section!
     

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