Carburetor Problems

Poor Idle Quality:

• Air leakage (check gaskets and vacuum fittings)
• Throttles not synchronised (re-balance carbs and reset linkage) on dual carbs
• Air valve sticking (clean and reassemble)
• Obstructed float chamber or diaphragm vent holes (ensure that air filter and gaskets are correctly fitted)
• Incorrect fuel level (clean or replace needle valves and reset float height)
• Metering needle wrong or incorrectly fitted (check needle number, and install to correct height)
• Diaphragm incorrectly located or damaged (holes on bottom of piston should be in line with and face throttle shaft – replace damaged diaphragm)
• Temperature compensator faulty (with engine and carb cold, compensator cone must be seated, but free to move off its seat)
• Vacuum leak around throttle shaft (replace seals and/or shafts)

Hesitation or “Flat Spot”:
In addition to the above, check:

• Piston damper inoperative (ensure damper is filled to proper level with correct oil – check operation by raising piston, whereupon resistance should be felt)
• Air valve spring missing or damaged
• Ignition timing incorrect
• Throttle linkage operation incorrect (readjust and check for correct operation)

Heavy Fuel Consumption:
In addition to all of the above, check for fuel leakage

• Lack of Engine Braking
• Faulty bypass valve (replace diaphragm or entire unit)
• Throttles sticking open
• Ignition retard inoperative (if separate retard unit fitted to distributor, check operation – if not, ensure that distributor returns from advanced position)

Lack of Engine Power:

• Damaged air valve diaphragm 
• Piston damper torn will give symptom of not being able to rev over 2000 or similar rpm
• Low fuel flow (check fuel pump output, needle valve and seat assemblies, and float height)

The first step in dealing with carburetors is ensuring that the engine and ignition system are in good order. This includes valve clearance, valve timing, compression, point gap, spark plug gap, ignition timing, and overall condition of engine and ignition components. Otherwise it is impossible to make correct settings and adjustments. Many alleged “carburetor problems” are really engine or ignition problems.

The place to start with your carbs is with information from your workshop manual or the Haynes Stromberg CD Carburetors manual (Moss #212-940). I strongly recommend the Haynes manual, as many factory workshop manuals give little detailed information. Before touching your carbs, study all information on them you can obtain, including this article.

Areas of concern are: mechanical condition (including cleanliness), fuel/air mixture, and air-flow balance (multi-carb only). As these overlap and affect each other, a “hit or miss” approach is self-defeating. Ask yourself what the carburetor needs, and how much work you are willing to do. Does it need minor adjustment, or is a full rebuild required? Determine what the symptoms of the problem are, determine the cause(s), and take appropriate remedial action. The following troubleshooting chart covers the most common problems.

Mechanical condition is relatively easy to determine. Are the carburetors clean? Do the linkages and other moving parts move freely? Are all gaskets and O-rings sealing properly? Does everything look and feel right? There should be no fuel or vacuum leaks. Inside the carburetor, check the large air piston diaphragm – it should be free of holes or tears. The bypass valve diaphragm (when fitted) should be flexible and free from holes or rips; these often tend become burned and hardened. Floats must not have cracks or holes, and should not contain gasoline. Floats should be set to the correct height (check your manual). Throttle shafts should not bind, nor should they “wobble”.

While your manual will give basic “bench” settings only, tuning is performed with the engine at idle. Refer to your manuals for the required engine speed. Mixture setting on the very early Z-S carbs is easy – adjust the jet up or down with the external brass mixture screw. the carbs found on TR250s and very early TR6s have adjustable jets which aren’t suppose to be adjusted, but can be. These have a large plug with a hex head on the bottom of the carbs. Remove this, and the jet adjustment mechanism is exposed. In all cases, moving the jet further into the carb leans the mixture, while moving the jet further out of the carb richens it. Later carbs do not have adjustable jets, but have adjustable metering needles, which require a special tool for adjustment (Moss #386-310). On these carbs, lowering the needle leans the mixture, raising it richens the mixture. Once the mixture is set at idle, it will automatically be correct throughout the engine’s operating range. The mixture will be correct when, upon lifting the air piston no more than 1/16″ with a thin screwdriver, the idle speed remains constant. If speed drops, the mixture is lean; if rich, speed will rise.

Balancing the airflow on multiple carbs is not difficult if a Unisyn (Moss #386-200) is used. After balancing the airflow, it may be necessary to slightly readjust the idle mixture.

The idle trim adjusters provide very fine adjustment to compensate for the differences between a new “tight” engine and one which is run in, and to set the carbon monoxide emission level at idle. On earlier pollution controlled carbs this is a large brass screw on one side of the carb, located just above the lower end of the temperature compensator. On later carbs it is part of the “down-stream discharge idle circuit”, in an assembly screwed onto the side of the carb. These are not ordinary mixture adjusters. Any adjustment of either type should be checked with a CO meter, and the CO level set to factory specs.

Temperature compensation is found on all but the earliest Z-S carbs. In an uncompensated system, rising temperature adversely affects fuel metering performance and engine idle stability. This is unacceptable in meeting strict exhaust emission requirements. The earlier temperature compensators are oblong units with plastic covers. The only servicing possible on these is cleaning, and ensuring that the black plastic tapered valves do not stick. Do not bend or alter the flat metal “spring” – it is a bimetallic arm controlling the unit’s operation. Later carbs have internal emulsion tubes which need no attention.

Bypass valves have either external or internal adjustments. This is set at the factory to allow the valves to open at a precise vacuum. Do not alter this setting. The maintenance to be done on these units (aside from cleaning) is replacement of defective diaphragms (Moss #365-755).

The operation, maintenance, and repair of the “water choke” units is as complex as the units themselves, and is far beyond the capacity of a full article of this length. For information on these units, I suggest the excellent chapter on them in the previously mentioned Haynes manual.

-I’m not sure where this came from (a very old photocopy) but the following title was at the top of the page:
“Back To The Basics – More On Zenith-Stromberg Carbs” by Eric Wilheim, Technical Services