| Subject: |
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Carburetor Adjustment |
| Name: |
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JimmieD |
| Date Posted: |
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Nov 3, 08 - 8:59 AM |
| IP Address: |
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66.81.198.141 |
| Message: |
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Hey Norm, maybe this could be cut from Forum and go in your DTA Tech Section to help guys as needed? I had posted this on another website and then added later follow up info and thought it might help somebody here. Long, but that couldn't be helped.
Carb adjustment:
The following applies to most downdraft carburetors used on almost any American engine. Whether 1bbl, 2bbl or 4bbl the procedure is the same, of course modified for number of idle mixture screws.
Check all vacuum lines. Remove distributor cap and pull vacuum hose at carb. Suck on the end of the hose and make sure that distributor advance plate moves smoothly. A little spray lube here and there won't hurt a thing. If it's electronic ignition use a non-magnetic brass feeler gauge and check the air gap at reluctor/pickup. Reluctor is the toothed wheel. Make sure screws are tight and no wires are chafed. Some ignition pickups will break the tiny little fine wire at magnets and this can cause a complete failure, therefore be careful of it.
Trip the throttle one time all the way down on a stone cold engine. Check that this engages the choke butterfly to full closed position if manual choke. If electric choke turn ignition switch on but don't start engine, and check choke butterfly for closed. Switch off ignition.
Snug the carb hold-down bolts to manifold, but don't overtighten. GENTLY turn in the mixture adjustment screws at base of carb one by one, counting the number of turns to closed for each. Stop at first felt resistance, don't tighten down! A shot of spray lube before is a good idea as the old springs and screws will rust up some. Write down the number of turns on each screw to closed. Remove screws and don't lose springs. Slip on your safety goggles and spray a bunch of Berryman's Chemtool carb cleaner in the holes. Do this repeatedly over time, say 30 minutes at least to let it soak in and do it's work. Easy to use 2 cans before you're done.
While you're at it snip off the very tip of the spray tube on carb cleaner at a 45* angle. This allows you to spray cleaner into the bleed orifices [super small little holes] in the carb throat. Look close and you'll see them, at least one. Let all this carb cleaner soak in for a good hour while you fiddle with other stuff, then hit it all again. If carb is partially disassembled you can follow with a strong blast of compressed air in all these passages to blow out debris. Worst case requires a swabbing with a piece of fine steel wire [guitar string?] & carb cleaner to remove deposits.
Replace mixture screws, GENTLY turn down to just stopping, then back out to number of turns your wrote down. 1 1/2 to 2 turns out will start engine in case something goes wrong. Start the engine and warm up to normal ops. Check to make sure choke butterfly has opened fully at warm engine. Set curb idle to normal speed with the adjustment screw at throttle linkage. There's a flat metal cam there with several steps that screw presses against. After engine warms blip the throttle and the screw should drop to the lowest notch on cam, then adjust for curb idle speed.
Slowly, at 1/4 turn increments, back out one  mixture screw until highest engine speed occurs. Reset curb idle to normal. Back out the other mixture screw to highest engine speed, and reset curb idle to normal. Now turn in each screw one at a time until the very slightest decrease in engine speed [only about 20 rpm decrease] occurs. This mixture setting is 'Best Lean' or 'Lean Best' idle mixture. It gives the best performance and fuel economy. It's okay to set it back at the higher setting if you prefer, as this method is used to determine where mixture should be within maybe 50 rpms.
A slightly higher curb idle setting above mfgr. specs will often assist in smoother shifts with a manual trans. Once engine has warmed be sure choke butterfly has opened properly.
*
That's a 'Most Carbs' scenario but still applies to just about anything with venturis and jets.
Idea is to set from a baseline of curb idle and get the various metering circuits in a carb to sing together, on key, throughout operational range. While adjusting you keep returning to a curb idle correction so that settings remain in the 'baseline' specs. If not you can wander off and end up unbalanced.
Some carbs like Webers & DelOrto call for a more Rich base mixture: that is 'Rich Best Mixture' that you would set them for maximum flow & highest rpm with adjustment screws and not leaned back out as with other carbs.
Carbs like Eddies, AFB, most Holley, Carter 2bbl's and most Strombergs & Rochesters respond best by getting the fattest mixture settings then peeling a little off the top [20-30 rpm's] for a 'Lean Best Mixture' curb idle setting.
Once the curb idle settings are good you can play with the accelerator pump linkage to get your solid fuel enrichment shot timed precisely for how you drive the truck. Shortening linkage stroke by moving linkage closer to fulcrum is a quicker/sooner shot, farther away is a longer and more gentle shot but both delivering the same amount of fuel, it's a when thing.
Of course it's possible to find nothing works quite right which then calls for jets or metering rods or both to be changed for a richer or leaner potential, then back to setting the baseline mixture. Reading sparkplugs and exhaust color and vacuum gauge readings can give a big hint there. Some carbs have auxiliary shooters in throat that squirt raw fuel at WOT [Wide Open Throttle] and these can be swapped or drilled out for higher power or leaner mixture at WOT as needed.
Careful attention to all gasketing & insulations if of course important too, as well as insuring adequate flow through filter, adequate fuel pump pressure & volume, well insulated or properly isolated fuel lines, and adequate clean air supply through filter media or substance.
Finally, I have found overall in most applications, 'When in doubt, Lean it out'. Majority of non-race vehicles are baselined for economy with power.
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