Here's my tech week contribution. It's not sexy or slick - but if it helps someone out, it will be worth it! This tech thread applies to folks who have checked out EVERTYTHING on their carb setup (linkages, timing, vacuum leaks) and still can't get the damned thing to run smooth. Recently, I rebuilt a Carter WCFB (vacuum secondary 4 bbl carb) and had one helluva time getting the bugger to run right - the motor took the gas okay, but the curb idle was rough and lopey - like the motor had a cam (it didn't). Here's why: Old carbs typically have a cast-iron throttle body. When these old TB's are allowed to sit, they corrode - causing pitting on the inner bores of the throttle body. The pitting allows more air to pass thru the throttle body with the plates closed - or nearly closed. This is important, as the idle circuit is calibrated at the factory to meter a specific amount of fuel relative to the position of the throttle plates at curb idle. On a 4 BBL setup - or a progressive multicarb setup - it is ABSOLUTELY IMPERATIVE the secondary throttle plates close TIGHT. The 'check' for tight closure (the one that works for me, at least) is to disassemble the carb, hold the throttle body up to a bright light, and inspect the mating surface between the edge of the throttle plate and the bore of the throttle body. On a new carb, the only light which will pass thru is at the edge of the throttle shaft and the throttle plate - there has to be some clearance there to allow free movement of the throttle plates. Typically on these older carbs - you'll see some light passing thru at other places; that tiny bit of light represents wild (unmetered) airflow, and can upset curb idle quality. There are a couple of fixes - the first is to loosen the throttle plate screws and reset the plates in the bores. Back the screws off a half turn and press the plates closed with your fingers. With the plates held in place, re-tighten the screws, and do the light check again. If you still see some daylight, then it may be time to fill the pits with some JB weld. A light coat swabbed in the bore will fill most problem areas - make sure not to plug any vacuum ports or transfer ports in the process! The key to getting the plates to seal with JB weld is to LIGHTLY sand the bores with emery cloth (less grit left over thataway) and then lap the plates into place by removing the throttle plate shaft and then slide the plates thru the bore several times in the closed position - effectively turning the plate into a no-buck broaching tool. The sharp edge of the throttle plate will cut away high spots in the JB weld, leaving a smooth (and much tighter) bore. In the case of the WCFB, this improved the idle, but it still ran rough. After fiddling with timing and idle mix - I noticed the mix screws were out too far.(2-1/2 turns) As mentioned earlier, the idle circuit is factory calibrated to provide a metered fuel flow rate with the throttle plates in a specific position. The WCFB had sat for some time, and the primary plates had rusted - while the rust had been removed, the edge of the plate was no longer smooth...This allowed additional air to pass thru the carb at curb idle, effectively 'leaning out' the circuit beyond the ability to adjust it with the idle mix screws. This is a bugger - you can't quite fix the edge of a throttle plate with JB weld! I spoke with a carb rebuilder, and he shared his 'cheat' for a lean idle with me. It's pretty simple: 1) Go to a hobby shop and buy a set of 61-80 wire gauge drills and a pin vise. They cost all of $10; there are guys on ebay selling the whole setup for less than that! 2) Disassemble the carb. 3) Remove the idle jets. in the case of the WCFB, the jets were long slender tubes (emulsion tubes) with the orifice at the end of the tube. 4) Find the wire gauge of the orifice using the butt end of the drill. The gauge size is the largest drill you can pass thru the orifice. 5) Once you've determined the gauge size, select the next larger drill and bore out the idle orifice with it. You'll only remove the tiniest amount of material - should only take a few twists of the pin vise to get thru the jet. 7) After boring out the jet, reverse the drill and use the butt-end of the drill to polish the new bore. Reason for this is drilling leaves rifling marks in a jet, and these marks can cause improper fuel metering. By running the butt-end of the drill thru the jet - you get rid of *most* of the rifle marks. 8) re-install jets. 9) re-assemble carb. I don't recommend going up more than one drill size at a time - you know you've got the right air/fuel ratio when the idle mix screws are at the 'factory' position. In the case of the WCFB, this is one turn out; I'm at 1-3/4ths turns and have a pretty good idle - so I'm leaving it a tad lean until I get a chance to test the curb idle in the summer. It's a dirt-simple mod - and helps offset wear in a throttle shaft, rust in the throttle bores, and pits on the edge of a throttle plate. The trade-off is a slight reduction in fuel economy around town, but I doubt y'all are concerned with that!
Nice write up Chuckspeed. Does this method apply to all carbs? Sounds pretty basic, until I try it on my tripple Strombergs.
Thanks! Yes - I wrote it specifically to apply to all carbs. We're trying to keep old iron on the street, and wear/corrosion mucks with carb calibration. The steps described above apply to multicarb setups as follows: a) When you're runnin three deuces, the center carb is the lead, and the outboard carbs are lag. The throtle plate fixes apply primarily to the outboard carbs, and the idle jet recalibration applies to the center carb. b) When you're runnin' two deuces on a flatty, the setup will be real sensitive to throttle plate position - so both carbs would require the same idle jet modification.
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