carb idle question

Billmc52

Billmc52

Member
I was adjusting the idle mixture screw according to model-a.org instructions to turn screww clockwise until engine stalls and then out until it stalls then set between the two. My adjustment screw turned all the way in until it seated with no change in engine. It took about 1.5 turns out when it began to stall and I set it at about 3/4 turns out. My question, what is the reason there is no change in engine with screw turned all the way in, and what is solution? Scored screw and replacement? Or problem in carb at point where screw seats? Thanks for any info.

Bill
 
Make sure you follow all the steps in the correct order.

a) Is the engine completely warmed up to running temperature?
b) Is the GAV turned completely to the right?
c) Is the spark fully retarded?
d) Did you adjust the idle speed on the throttle arm until it was just about ready to die?

If all the above are true, then it should stall at some point before the idle screw is completely turned in either direction.
 
The idle adjustment bleeds air into the mix. When the adjustment is closed, no air is being bled in BY THAT ROUTE. If the motor continues to run ok, it seems to me that it must be due to sufficient air leaking in from another route, like maybe around a worn throttle shaft, a pretty common scenario.

Steve
 
steve s said:
The idle adjustment bleeds air into the mix. When the adjustment is closed, no air is being bled in BY THAT ROUTE. If the motor continues to run ok, it seems to me that it must be due to sufficient air leaking in from another route, like maybe around a worn throttle shaft, a pretty common scenario.

Steve
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Thanks, Steve. What's the procedure for checking for an air leak at that location?
 
Billmc52 said:
Thanks, Steve. What's the procedure for checking for an air leak at that location?
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Grab hold of the end of the throttle shaft (the piece the linkage attaches to) and try to wiggle it around in its bearings. There should be no play, but most old carbs will have some slop due to the millions of vibrations experienced.

If the car runs ok otherwise, it's probably not worth worrying about. If you decide to pursue a carb rebuild, make sure the guy will repair the shaft, and not just swap in a bunch of inferior repro jets for you. After much frustration, I finally went with one of the brand new Zenith-1 carbs being reproduced to original specs. It worked really well except for vapor lock issues, which I have now pretty much solved by going to the non-original Zenith 13922, which also is much more resistant to stalling on hard stops, but it is noisy and really looks even more non-original than even the Tillotsons or Allstates.

There are other places that might be leaking air: wiper line, manifold gasket, carb gasket. Some folks recommend spraying WD-40 around potential leak sites with engine running to detect leaks; if there's a leak, spray should be sucked in and change how it's idling. I've also heard of using propane from an unlit propane torch bottle--not as messy.

Again, unless you're terminally obsessive, which a lot of us are, I advise to do nothing if it's running ok.

Steve
 
steve s: You mention the new carb is "noisy". I find on my A, Zenith 2, it's VERY noisy sucking air, even if I put a spark arrestor on it, it's still "noisy", is this "normal"?
Paul in CT
 
1931 flamingo said:
steve s: You mention the new carb is "noisy". I find on my A, Zenith 2, it's VERY noisy sucking air, even if I put a spark arrestor on it, it's still "noisy", is this "normal"?
Paul in CT
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Paul,
Noise is too subjective for me to be much help. Best to have an experienced Model A guy give it a listen and tell you whether it's normal or not. It is normal to be able to hear it sucking air.

In my youth, that sound was highly prized on summer nights when out with buddies in the family car. The first order of business when out of sight of the house was to pop the hood and spin off the air cleaner--I can still hear those 4-barrel carbs kicking in up and down 8-Mile Rd. or Woodward Ave when out cruising in Dad's big Nash. Just for fun, here we are the year before I got my Ford.

thebignash-enhanced.jpg


Back to Model A's, I once tried an Air-Maze with paper filter inserted; to my surprise, it was so much noisier that I removed it. That was even before I knew the business about not using filters on unbalanced carbs.

Steve
 
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I also found that air maze type filters made my carburetors much noiser, I no longer use filters of any kind because I am convinced that besides effecting power, they cause more problems than they could ever solve. As for adjusting the idle air mix, there are different length screws on the market. The early brass air adjust screws that I have used are longer. Chances are better than even that the needle end of the air adjust screw is too short and is not reaching the seat in the upper carb casting.
 
Purdy Swoft said:
I also found that air maze type filters made my carburetors much noiser, I no longer use filters of any kind because I am convinced that besides effecting power, they cause more problems than they could ever solve. As for adjusting the idle air mix, there are different length screws on the market. The early brass air adjust screws that I have used are longer. Chances are better than even that the needle end of the air adjust screw is too short and is not reaching the seat in the upper carb casting.
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I did not know that. Do you have any idea what the different lengths may be and who may sell them? I see Snyder's have two types, brass for 28-29 and cad for 30-31. I thought that had to do more with material fpr the correct year rather that length.
 
I don't have one out to measure. I got mine from Brattons a few years ago The part number is 14380 this is the large 3/8 head brass needle adjust screw that was used mid 1928 thru mid 1930 I believe that this is the longest. they also list another brass idle air adj. screw with a 5/16 head that was used mid 30 to end, I haven't used this one. there is also a cadmium plated small screw used march 30 till end , I used this one and it is shorter These screws are on page 63 of the 2011 brattons catalog or call 800-255-1929 to order or ask Walt about exact specs. You could remove the cup and spring from your air adjust screw for a test . with the spring and cup removed would allow your screw to farther penetrate the housing .
 
Ok, thanks for that info. This screw is cad, I'll check it's length and try it without the cup and spring, then check with Bratton's and see what the brass screw measures.
 
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