VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Carburetor and rough idle

I have never bushed the throttle shaft on a W1 carburetor either........never found the need for it.

May still have a vacuum leak at the carb to manifold gasket. Next up on a rainy day is to pull the pan and clean out the assumed accumulation of gunk, so will order a new gasket along with the oil pan gasket.

Again, appreciate all the input and guidance!

Steve

"I don't ever remember having to put bushings in a W-1 (cast iron throttle body). Machining a new throttle shaft always put the unit back in spec."

Jon,

Thanks for that insight. I've questioned why putting bushings in an iron W1 reduces shaft looseness even when using the same shaft after putting in the bushings. My theory is that the bushing ID is smaller than the original hole size in the iron. I've found that bushings marketed for Quadrajets are a fit in W1 carbs after enlarging the holes with a drill followed by a reamer. A side benefit is that bronze may be a better bearing material than cast iron. The few Weber carburetors I've seen go even farther, using sealed ball bearings to support the throttle shafts.

On W1 rebuilds I've found the accelerator pump/metering rod bell crank shaft to often be a sloppy fit in its hole in the float bowl tower and I've made bushings to restore a proper fit there also. That's more involved because the hole does not go all the way through but it's still an interesting challenge that should yield more consistent metering rod positioning and accelerator pump shot volume. A perk of having a home machine shop is being able to do projects like that for only the cost of the parts or scraps of metal

"No need to raise the cost just to be able to adjust the idle mixture."

Chipper,

Since I'm doing this as a hobbyist for fun it's well worth doing the work to improve idle quality. It may be the "placebo effect" but after tightening up the throttle shaft fits, carburetors seem to be much more responsive to idle mixture screw adjustments.

Ray W

I also have bushed the bell crank hole on numerous W-1 carburetors. Also straightened a bunch of float bowl covers.

Having enough equipment to make shafts, bushings, etc. is valuable if you rebuild many carburetors. Also being willing to try a multitude of tweaks in an attempt to understand and rectify problems.

Ray - depending on the model of carburetor, Carter, Holley, Stromberg, and Zenith have all used ball bearings to support the throttle shaft. Just a matter of cost. Generally, the American companies used this procedure on carburetors designed for heavy-duty applications.

Not saying Autolite and Rochester haven't, but memory doesn't find any for these two brands.

And the bushings for the intermediate shaft going through the "tower" are NOT fun, but have done many of them. One of the reasons we recommend leaving the original 1932~1936 carbs to the folks doing numbers matching restorations, and replacing these carbs for the folks that are building "drivers".

Jon.

Ray - concerning more idle adjustment:

If one "fast forwards" to the year 1968. What happened significantly in the auto industry in 1968? Easy - federal smog emissions implemented.

One of the initial carburetor modifications to take place in 1968~1970 was the change in taper on the idle mixture control screw. Earlier screws had a large angle and short taper. The newer screws have a smaller angle, with a longer taper. The longer taper means a given rotational degree change in the setting of the screw will result in a smaller incremental change of the effective idle discharge area (the area of the idle discharge port less the dynamic area of that portion of the taper currently in the port). This enabled the mechanic to make a finer idle adjustment for smog emission considerations.

So why is this important?

It really isn't on the brass bowl updraft carbs, as they use an inverse idle circuit (the idle mixture control screw meters air, not fuel).

But on a W-1 and/or other carbs with the conventional idle circuit, if one does ones homework, one can generally find a post 1968 screw with the same thread, but the longer taper.

Personally, I don't feel it worth the effort, but if one enjoys "tinkering", this is an area that can offer results.

Jon.

"One of the reasons we recommend leaving the original 1932~1936 carbs to the folks doing numbers matching restorations, and replacing these carbs for the folks that are building "drivers"."

Jon,

You've mentioned that before and I took your advice to heart. My '36 Chevy engine likes a '39-'40 Chevy carb with a '37 main jet and metering rod. I've put the original '36 carb aside with the original rear axle (replaced with a '37 car rear axle with '50-'54 3.55 Powerglide gears), original tie rod ends (replaced with Moog ES-29 R & L), original air filter element (replaced with identical sized NAPA paper element), original lower radiator hose rusty metal elbow (replaced with identical stainless steel plumbing item) etc, etc.

Thanks Jon!

Ray W