1950 MC surging
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    ctrider's Avatar
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    1950 MC surging

    Hi all,

    Glad I found you, we have a few green tractors in the yard, from a 1950 MC to a 1985 318, and a much newer 425. Anyway this 1950 MC will start very quickly but within probably less than 5 seconds it starts surging. Then only way it will run smoothly is if the choke is 1/2-3/4 closed.

    We had to pull the head but got that all back together, new ignition switch, points condenser and coil. The carb was rebuilt at the dealer. I've tried adjusting the governing and carburetor but none of these seem to change anything on it.

    I'm going to check for an intake leak tomorrow.

    Any other thoughts?

    Thanks!

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    Superglidesport's Avatar
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    It sounds like the carburetor is too lean or you may have a vacuum leak. You can use a propane torch modified with a length of flexible hose to dispense gas as you probe around the carb base and intake to find a vacuum leak.

    Remove the mixer nozzle from the propane torch and slip a 24" - 30" length of hose over the tube. Make sure to hold the tank upright while using the end of the hose as it dispenses propane gas to probe for leaks. Open the valve about 1/2 turn and that should do it. The engine RPM should change if the propane gas is introduced into the leak.

    If you don't find a vacuum leak your carb may be set too lean or you may have a blockage. Turn the idle screw in and lightly seat it. Back it out 2 - 2 1/2 turns and you should be in the ball park.
    Last edited by Superglidesport; 02-09-2017 at 12:51 PM.
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    I agree with Super. Usually when I see surging it is a restriction in the idle fuel circuit. When rebuilding carbs very particular care must be taken to ensure all tiny passages are clean and flowing. Engine drops below idle speed and the governor opens the throttle and you get flow from the main jet to surge but when rpm's get over your throttle position setting the governor releases and you are back on your starving idle circuit. Rinse and repeat... Older engines have idle mixture screws, sometimes under plugs. Many newer carbs rely on a fixed orifice. If this orifice is checked and not clogged then it can be reamed a touch larger using a torch tip file. Sometimes as these newer orifice type engines get to need a little more fuel to idle correctly after they wear for a few years. Just my experience, others who know more are sure to post up with more or better advice. Good luck.
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    ctrider's Avatar
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    Wow, thanks so much for the quick responses. I'm planning on starting with the vacuum leak to rule that out. I'm suspicious of the carb because neither of the 2 adjustments seem to do anything to change how it runs, but it was just rebuilt at the dealer, so that's what had me thinking maybe an intake gasket or some other leak.

    If we don't get hit too hard with this Noreaster, I'll work on it today and let you guys know what I found. Thanks again!

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    Superglidesport's Avatar
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    Quote Originally Posted by ctrider View Post
    Wow, thanks so much for the quick responses. I'm planning on starting with the vacuum leak to rule that out. I'm suspicious of the carb because neither of the 2 adjustments seem to do anything to change how it runs, but it was just rebuilt at the dealer, so that's what had me thinking maybe an intake gasket or some other leak.

    If we don't get hit too hard with this Noreaster, I'll work on it today and let you guys know what I found. Thanks again!
    If you have a significant vacuum leak a carb idle mixture adjustment won't make much of an improvement. Check for leaks first. If there are none it wouldn't be the first time a carb rebuild failed due to residual debris clogging internal passages.
    DRobinson, Drifterbike and Jeff B like this.
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    ctrider's Avatar
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    Yeah that's what I thought too. There really isn't much for an intake gasket but we checked it with propane anyway and decided to pull the carb and take it back to the dealer to see if there was any blockage in the passages.

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    ctrider's Avatar
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    Well I thought I'd update this. We got everything back together and it still surges. So we checked valve clearance again, checked for intake leaks with propane, checked the governor again and determined it had to be the carburation. So I took the carb off and disassembled it and found that they didn't take the discharge nozzle out and one of the passages is clogged for the idle circuit. We tried everything we could think of, but unfortunately the technician damaged the brass slot in it so the only way to get it out will be to drill it and use an easy out. I'm not real happy with the dealer. They never said anything about it, put some of the gaskets back in it and charged my father $450 for rebuilding it. He complained and reminded them that he told them he could get a new carb for $350 before they started. They knocked off $100. To me they should have called and said they couldn't get it apart instead of putting it back together and not saying anything.

    So it looks like we'll get in touch with Yesterday tractor tomorrow and get a new carb coming. I'll post up when we got it straightened out.
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    That sucks. Sorry to hear that.....
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    That's terrible. They had some nerve charging you that much for basically damaging your existing carb. Thank you for confirming the diagnosis on here, if anything it may help others when they have similar characteristics happening.
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    ctrider's Avatar
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    I plan to take the carb back to the dealer and show them. I'll let everyone know when we get it back in running order too. Its a Marvel Schebler TSX245 carb, single barrel, updraft type.
    BigJim55 and Evergreen like this.

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