Seems to be some confusion here. Adjusting fuel mixture screws out to highest engine speed has an absolute limit, on each screw and in total. Air/fuel mixture will reach a point as described in that Best of Forum piece, where the engine speed will increase no higher by adjusting mixture screw out. Yes, one can continue to richen mixture by turning the screw out but that's not the idea. Just turn them out until RPMS will not increase any more, and only to there.

The instruction to turn them back in to slightly decrease engine speed helps on the first adjustment step, to be sure that you have not turned it out too rich, beyond highest rpm.

SO: With engine fully warmed up and throttle linkage on proper step [usually first step] of stepped cam on linkage. Check for excessive wear or play in the steps & linkage. Set curb idle to factory specs using idle adjustment screw on throttle linkage.

Then adjust one idle air/fuel mixture screw, at base of carb casting, to highest possible curb idle speed. You'll need a tachometer to get really precise but it's sometimes possible to adjust by ear, not anywhere near as good though. An old trick is to use a piece of fuel hose as a stethoscope into carb throat so that you can hear air speed if you don't have a tach, a higher note being a higher speed.

Once highest rpm is reached with that mixture screw reset curb idle with linkage screw. Now do exactly the same thing with the other mixture screw, opening to highest rpm. Then reset curb idle again with linkage screw.

Now both idles circuits are almost balanced, but do it once more: because one effects the other, and when you adjusted the second one it had an effect on the first one. So open up first one again to highest idle, then turn back in to drop speed about 20 rpm. Re-adjust throttle linkage to correct curb idle speed. Adjust second idle mixture screw to highest rpm, then turn it back in to drop engine speed by 20 rpm. Now readjust linkage screw to set curb idle to factory specs or wherever you want. Slightly higher than factory spec can help auto trans operations.

What all this does is first it balances the two circuits as if they are independent, and then it synchronizes them one to another. The idle mixture feeds the transition from idle to part throttle and even to WOT - Wide Open Throttle. You want the best lean mixture, so that no gas is wasted, but all necessary fuel is available to feed transitions to higher engine speed of acceleration or load carrying. In addition the 'Lean Best' mixture enables the engine to decelerate properly.

You may still have stumbling or bogging when suddenly stepping on throttle. That stumble/bog is usually caused by accelerator pump shooting too much, too fast, into air/fuel mixture, flooding engine & quenching spark. Some carbs have a selection of holes in accelerator pump linkage, others you bend the wire-rod linkage, some you can do both.

This is a lever & fulcrum relationship: the longer the lever, the slower [and less fuel in] the pump shot; shorter linkage equals a quicker & stronger pump shot. Most often a stumble is caused by too short of a linkage adjustment, that an over-rich pump shot is flooding carb. Only way to tell is to look at linkage adjustment & experiment.

Holley junk garbagecrap uses a 'Power Valve' which is an ingnoramus diaphragm fuel enrichment device higly prone to failure. Even the slightest backfire can blow them out, plus its effected by atmospheric pressure, some fuel types, deterioration from age etc. On Holley crapola you can only replace power valve, until it blows out again.

Also, with protective eyewear, look down carb throat and see if shooter or cluster jets are spraying a good strong shot of fuel into carb throat. All should spray equally on most carbs.

Backfiring through carb throat is caused by an air leak, either in intake manifolding/gaskets or at carb attachment & gaskets. To find a leak use a propane torch with gas turned on but without lighting flame. Play the gas mixture around edges of intake manifold and around carb base. When this causes a sudden change in engine speed you have found a leak, but keep going until all areas are tested. Obvious: fix leaks.

Engine bogging at rapid throttle increase can be either too rich mixture or too lean mixture.

Curb idle speed that is 'hunting' or rising & falling in an irregular rhythm may be caused by imbalance in mixture adjustment or air leaks in manifold, gaskets, or in carb casting where thottle shaft passes through, or a bad accelerator pump 'leather' plunger. A casting may be redrilled and a brass insert used, reamed to correct size of throttle shaft, to repair throttle shaft air leaks. Some foreign car carbs such as Hitachi use a brass plunger in the aluminum casting for accelerator pump. Here the only fix is to upset the metal to increase diameter of brass plunger, by carefully forcing grooves with knife blade around outer circumference to increse diameter by displacement of metal to correct diameter.

All of the above expects that the fuel inlet valve float height is first adjusted correctly. Again it's a lever & fulcrum relationship: Too long = too little; too short = too much. Bending tab on float lever changes the relationship and is set to a particular float height, also called 'float drop'. Top of carb is removed & inverted and distance of float drop is measured. Accelerator pump well should be fully covered with fuel, plus a couple of 16th's inch.

All of the above also assumes the carburetor & its air/fuel passages to be spotless clean with no restrictions. PLEASE CAREFULLY PROTECT EYES, SKIN AND REPSIRATORY SYSTEM, INCLUDING LOTS OF FRESH AIR WHEN USING CHEMICALS!

Berryman's Chemtool continues to be the very best carburetor cleaner available. All carburetors have tiny 'air bleed' holes in the casting in the throat & venturi of carb body, usually several. These sense relative differences in vacuum-to-atmospheric air pressures. When cleaning, use small pieces of wire like guitar string or piano wire, and gently swab out the air bleeds.

These are usually a 'straight through' drilling, that a wire should pass into casting until it comes to stop. Sometimes a good bit of spraying and gentle pushing is needed to remove stubborn deposits, working wire bach & forth. Not much chance of doing real damage with the wire if you're gentle because passageways are relatively long. However make sure tip of wire is polished a bit on a piece of emery paper so no jagged rough edges are acting as reamers.

In addition an air supply helps, that you can finally spray a heavy shot of carb cleaner in each & all assages & then blast it out with 100 lbs or more of air pressure.

All of the above assumes that sufficent fuel is supplied by fuel pump or its all rather an excercise in futility...

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Edited 1 time(s). Last edit at 04/29/2009 04:09AM by Phil.