Tuning Two Zeniths (Just a Story)

[Brobrian]

This is my tale of tracing down carb issues.

On my 31' PU the original carb leaked and I had my doubts about whether I could get it in shape by myself. So, last fall when I knew I was going to put a new motor in, I decided I'd start out with a new carb and a new distributer. I purchased these from Snyders. Shortly after getting the truck going the new carb began to leak, like they do when the float valve is bad but not just there. Seemed like gas was everywhere. I figured I'd spent a lot of money on this thing so I'd send it back to Snyders to have them check it out. Meantime, I put the old leaky carb back on and away we went.
The report from Snyder's carb guy was that there was a lot of sediment in the bowl and he suspected that it was interfering with the float valve. I had previously spent a lot of time sloshing out the tank and doubted him on this at first.) He also suspected that I had a leak where the fuel line attaches. He was right on both accounts. He recommended new fuel line ferrules and an inline filter. I ordered new fuel line ferrules and another fuel line. I figured Old Leaky and the New Carb wouldn't necessarily have the same shaped seat for the ferrule, so each carb now has its own fuel line. When I swap them now, I detach the line from the sediment bulb instead of at the carb.
Last week I got my carb back from Snyders and after cleaning the sediment bulb, put it on. No leaks, but at the standard settings (1/4 open on the GAV and 1 1/2 turns off the seat on the idle air screw) i was getting a number of bad tuning symptoms. Namely, a rough rolling idle, and high speed backfire. Additionally, after decelerating for a stop the engine would hesitate or sometimes even die before establishing an idle.
I consulted Paul Moller's book on carbs and figured out that New Carb was running too lean and could have a clogged cap jet or compensator jet. Off it came again. I cleaned out the bowl (there were rust crumbs in there already) and blew out the jets and gave it another go. This time I opened the throttle until the motor started its high speed popping and then began opening the GAV until it quit. I had to open the GAV almost 1 1/2 turns. Then, at idle I leaned out the idle air screw until I got a near perfect idle. A spin around my test route revealed that all the symptoms were gone! Whoo Hoo! However, the top speed on the straight bit yielded a disappointing 48 mph. More surprising was that when I walked back in the garage to put away my tools, there sat the venturi! I hadn't put it in when I resembled the carb. Fortunately you can just take the bottom half of the carb off and slip the venturi in without removing the whole carb.
Venturi reinstalled, I restarted the motor,.. rough idle. But as I closed the GAV suddenly everything smoothed out again. After a quick recheck of the idle air screw I went back out for another test lap. A perfect run, and the top speed was back up to 55!
I have since tuned Old Leaky on the truck. Runs perfectly, but drips. I'll replace the float valve on that one soon, and hopefully give it a new name.

I've learned a lot. I feel I understand for myself what I've been told and what I read about the GAV and the idle air screw. I'm much less intimidated by carb adjustment. And now with the pickup running so well, I feel I can once again relax and enjoy driving the it while I tinker with the coupe.

[mccsix]

Way to stick with it!

[mshmodela]

Great job thanks for sharing your experiences with your Model As.

[mshmodela]

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