Troubleshooting

Before You Blame the Carburetors

Many performance problems are mistakenly attributed to carburetors when the root cause lies elsewhere. Common causes of hesitation and poor performance include:

• Distributor problems (advance, points, condenser)
• Clogged fuel line or fuel filter
• Incorrect fuel pressure (should be under 3 PSI)
• Fouled or incorrect spark plugs
• Bad spark plug wires
• Valve adjustment issues
• Air leaks at manifold or carburetor base
• Worn butterflies or worn carburetor throat

Strongly suggest that if you are working to fix a problem, after trying one cure, should it not work, you reset the carburetor to its original settings before you begin the next step. We see carburetors all the time where one thing after another has been tried, but when the first change didn't work, it was left anyway, until in the end, the carburetors are so out of spec that it is only by starting from the beginning that it can be sorted out.

Rich vs. Lean

NOTE: another symptom of running too rich is FUEL mixed in with your ENGINE OIL. When you are running too rich, a float has sunk, a needle valve has failed — CHECK THAT OIL and if necessary, change it immediately!

The Hesitation Trap

An excellent example of compounding problems is the matter of hesitation. There are a lot of reasons for an engine to be hesitating, usually, but not always, having to do with fuel availability. Most assume that it is a lack of fuel and begin making changes to deliver more. The pump/injection is increased, the float level raised, jets enlarged, and still the problem persists. At this point, there is so much fuel going through the carbs that the engine is being flooded, sometimes to the point of gasoline going into the crankcase.

When people call us about this, we start asking questions about the condition of the spark plugs, the exhaust, and when and how the engine is backfiring, because we usually encounter a RICH condition with hesitation, rather than a lean one. BUT a rich running engine runs cooler than a lean engine, so work for that careful balance.

Throttle Linkage

After carburetor installation, be sure that the throttle arms (from throttle levers to linkage) are properly adjusted. Please do not assume that because it was okay before carburetor work, it is okay now.

You should be able to lift the throttle rods about 3/16" after they are disconnected from the throttle arms. This allows for the expansion of the whole throttle linkage after it warms up. The engine expands, too.

You must also make sure that the air flow carburetor to carburetor is equal using your idle adjustment screw, then set your idle using the volume control screws.

Symptom-by-Symptom Guide

1. Engine Won't Start

• No fuel in the system: Check fuel supply lines. With ignition off, detach fuel line connecting pump with carburetors and be sure that there is fuel flowing from pump to carbs. If not, pump is the problem.
• If there is fuel, check the float needle valve, the main jets, and the injectors for fuel delivery. Make sure the carburetor pump system is working.
• Is distributor and/or spark plugs working to ignite fuel mixture?
• Too much fuel in the system: Check float needle valve, float levels, fuel pump pressure, injection.
• Hard start — modern fuels evaporate quickly leaving carburetor bowl empty. An electric fuel pump to prime the carburetors may be needed.

2. Uneven Idle

• Improperly adjusted idling — adjust idle mixture screws, idle adjustment screw.
• Idle jets or idle air jets or passageways plugged or uneven sizing — clean jets, check sizing.
• Air leaks — worn throttle plates, worn out gaskets, overly tightened carb bolts causing warping, worn throttle shafts.
• Defective idle mixture screws — points damaged with overtightening, threads stripped.
• Check distributor for proper advance and holding idle.
• Carburetors out of synchronization — use synchrometer.

3. Poor Power / Hesitation on Acceleration

• Worn throttle plates and/or throttle shaft.
• Idle adjustment too lean or too rich.
• Improper float level (too high or too low).
• Improper injection quantity (too high or too low).
• Shifting at too low a speed causing RPM's to drop too low.
• Carbs not synched.

4. Stalls on Throttle Close

• Improper idle adjustment — check throttle arm adjustment.

5. Runs Unevenly / Backfires

• Mixture too rich — check fuel pump pressure, float level, injection, float needle valve, distributor.
• Mixture too lean — check fuel lines, float level, clean jets, check distributor.
• Check for air leaks.

6. High Fuel Consumption

Recommend keeping track of your mileage/fuel. Seeing your MPG average drop (it will always vary by the type of driving) is one of the first indications of a problem.

• Carbs running rich or ineffectively — check all the above.
• Check distributor, points and plugs.
• Check driving habits.
• Check for fuel in engine oil — if present, change oil immediately.

Always reset carburetors to original settings between troubleshooting attempts to avoid compounding adjustments.

Idle Issues — A Deeper Look

By John Jenkins

Eliminate the Distributor First

Take a timing light and note when the distributor begins to advance. Watch the pulley TDC mark which should be at the 5 degree advance point at idle — about 1/4" ahead of the top dead center point. If it starts moving at a little over 1,000 RPM, between 980 and 1,020, you are okay. Check when engine is both cold and hot. Set static timing at 5° BTDC.

Before You Start

Take out the idle mixture screws and see if any of them have the "ham-fisted groove" on the tapered needle. If so, it is worthless and needs replacement before going any further. Check the idle jets too, and make sure they are not plugged.

Reading Idle Mixture Screws

You should be able to get an idle by setting the static timing to 5 degrees BTDC, opening the idle mixture screws 1-1/4 turns, and opening the butterflies by about .015". Normal position is approximately 1.25 turns out.

• Under 1 turn out = float level too high or idle jet too large
• Over 2 turns out = idle jet too small

Worn Butterflies

Worn throttle plates allow unmetered air past the butterfly at idle, making mixture adjustment impossible. If mixture screws have minimal effect and your float level is correct, worn butterflies are the likely cause.

Air Leak Testing

Use propane or nitrogen for safer air leak detection. A cigarette lighter (with no flame, of course) with the propane valve open and pointed in the right direction is a choice. Nitrogen gas is safe and will reduce the idle because it replaces the oxygen in the air. Beware of fire! Don't use gasoline, paint thinner, etc.

Throttle Rod Slack

You should be able to lift the throttle rods about 3/16" after they are disconnected from the throttle arms. This allows for the expansion of the whole throttle linkage after it warms up. The engine expands, too.