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Subject: Carburetor Problems

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Tom S.    Posted 05-25-2018 at 17:37:13 [URL] [DELETE]        [Reply] [Email]  
  • Carburetor Problems
  • I have a 49 8N, and I have two carbs I've been switching back and forth because neither seems to work right. For the past month or so I've been using a M/S knockoff, which was working fine until I neglected to turn off the gas when I shut the tractor down one night and, when I went to start it, wound up with a crankcase full of gas. (I did a class A oil change afterwards.)

    Since then, the tractor won't start when it's cold using the M/S carb - instead it floods out almost immediately and I end up with maybe a half cup of gas trapped in the connector to the air filter. (And I do shut off the gas now religiously - lesson learned!) However, when I switch to the old Zenith carb, the tractor starts fine but runs poorly - not much torque, stalls out easily, and moving the throttle doesn't vary the engine speed a lot (I suspect the idle is set too high).

    Yesterday I got the tractor going using the Zenith carb, ran it for awhile, and switched out to the M/S. It started right away and ran beautifully - nice low idle speed, power when I was climbing a slope (I think the governor was doing its job, which it doesn't seem to do with the Zenith). I worked it for half an hour or so, shut it down, and when I tried it again a couple of hours later it started right up and ran well. But this morning, with the M/S carb still attached, it flooded out and I got lots of gas in the air intake pipe again. I switched back to the Zenith. It started and, again, ran okay but not great.

    So, my questions: 1.) Should I have that much gas in the air pipe after only cranking it for less than a minute? (I get that with the M/S but not the Zenith). 2.) Why does the M/S work well when the tractor is running, but gives me headaches about starting? 3.) What might be up with the Zenith? It's a pretty old carb, whereas the M/S is almost new?

    Sorry for all the text. Thanks.

    Tim Daley(MI)    Posted 05-26-2018 at 06:51:24 [URL] [DELETE]        [Reply] [Email]  
  • Re: Carburetor Problems
  • HiYa Tom-
    Did you just remove the 'new' carbs from their boxes and install them on the tractor? Never assume anything out of the box is good to go. Always double check settings and parts internal and external -goes for carbs, distributors, governors, et al. even if new out of the box. Many of the 'new' parts made and sold today are crapola too. Always try to use the original parts -they were designed to be rebuilt over and over. ZENITH carbs were used on Model A's and early V8's ( as well as HOLLEY too ) but FORD eventually went with Marvel-Schebler because they had the best models and held up the best. I avoid using the ZENITH carbs, prefer the M/S ones over all, but will work on them if a customer has one on his tractor. Have you verified the floats are properly set? Are the venturis brass or plastic? Is the float needle valve rubber tipped or solid brass? I avoid any plastic and/or rubber parts. Todays 10% ethanol gas has a tendency to eat away at hose materials especially when left idle for long periods. You can also check the fuel shut valve for leaking too, Do this: When you shut off Fuel Sediment Bulb Valve, go to the fuel line brass elbow carb inlet and loosen the fitting and gently slip line off the elbow. Observe if any fuel is dripping down. There should be NO fuel dripping out with valve closed. If so, the valve is worn -there's a seal in it (see picture) that gets chewed up and thus creates leaking. The M/S carb settings listed below are from an old list Bruce(VA) posted years ago.



    When adjusting the carb, turn main jet out 1-1/2 turns and leave it. Then adjust side pointing idle mixture screw to fastest idle, not smoothest. This procedure has to be done when the engine is really warmed up and often takes 2 or 3 tries. When you turn a screw in or out, watch how may turns you go, best to only go 1 turn at a time, then WAIT for the engine RPMS to catch up. The proper method is to make adjustments while under load and at certain rpms, but early 8Ns have no Proofmeter to show R's so best guess scenario takes over. Both Ford and Marvel/Schebler say to set both the side-pointing idlemix {Idle Mixture Adjustment Needle} and the down-pointing main jet {Main Power Mixture adjustment Needle} to 1-turn as a starting point. I set the down-pointing main jet {Main Power Mixture adjustment Needle} to 1-1/2 turns and don't fool with it anymore.

    Then adjust the side-pointing idlemix {Idle Air Mixture Adjustment Needle} for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed (throttle) set-screw for very slow 400-rpms idle. Do that idle mix {Idle Air Mixture Adjustment Needle} adjust for maximum idle at least 3-times. Do it like this:

    1. Adjust idle mix jet {Idle Mixture Adjustment Needle} until RPM increases

    2. Adjust idle-mix set screw {throttle idle-speed set-screw} until the engine nearly stops (as slow as you can get it unless you have a tach that tells you 400 rpm)

    3. Grasp the rod between the carb & governor & twist/shake it slightly; the slop in these rods often stops the adjustment cam from moving.

    Repeat steps 1 - 3 three times with about 10-15 minutes between tries.

    Remember the side-pointing idlemix {Idle Mixture Adjustment Needle} is out for lean, in for rich.

    I don't let my N's run dry. The M/S carb has a cast iron bowl & it will rust when it gets dry. Running aluminum carbs dry is ok.

    The Main Power Adjust needle is CW for reducing fuel (leaner) and if fully functional will shut off all carb fuel (even idle) when screwed in fully CW to it's seat. This leans it to point that engine dies. You say that this results in backfire. Is that correct? Just before all the way in (so lean it just barely runs, I don't see that you should observe a rich-condition-black-smoke at this point. Do you see that at this point? The Idle Mixture Adjust needle is just the opposite, that is CCW leans the idle mixture. That is because the Idle Mixture Adjust needle controls AIR, unlike the Main Power, which controls fuel. More AIR (same fuel)=leaner idle mixture. CCW=more air=leaner.
    Main Power needle is generally not very sensitive except near fully closed (full CW), whereas the Idle Mixture needle is more sensitive.

    * If you get stumped and so desire, you can send me your carbs and I will attempt to see what is the problem with them and rebuild if needed. My email is open.

    -Tim Daley(MI)

    Tom S.    Posted 05-27-2018 at 08:20:41 [URL] [DELETE]        [Reply] [Email]  
  • Re: Carburetor Problems
  • Tim: Thanks for the helpful response. Before I started my carb adventures earlier this week, I set both the Marvel and the Zenith using the directions you've posted above. When the Marvel started giving me problems, I took it apart hoping there was a very obvious problem that my untrained eye could detect - like a piece of gunk somewhere. There was a tiny piece of what looked like sealant at the bottom of the main mixture needle, which I removed, but it didn't help matters.

    Is there a good way to see if the floats are set right? When I took the carburetor apart they didn't look bent. I have an IT manual that says they should be 1/4 inch from the surface of the throttle body. Does that sound correct? (BTW, my fuel shut-off valve from the gas tank is working as it should)

    I very much appreciate your offer to take a look at the carburetors. I'm also thinking about buying a completely new, good quality one from a reputable vendor (not Amazon this time!). Do you have any recommendations if I decide to go that route?


    Bruce (VA)    Posted 05-26-2018 at 05:26:00 [URL] [DELETE]        [Reply] [Email]  
  • Re: Carburetor Problems
  • Your Marvel clone was junk out of the box. Use it as a paperweight.

    Your Zenith needs a rebuild.

    Rustyal    Posted 05-25-2018 at 21:07:11 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Carburetor Problems
  • Seems like your float valves need replaced.

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