Re: Carburetor Problems

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Posted by Tim Daley(MI) on May 26, 2018 at 06:51:24 [URL] [DELETE] :

In Reply to: Carburetor Problems posted by Tom S. on May 25, 2018 at 17:37:13:

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)

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