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Holley Carb Trouble Shooting

By Murray Hartley, Taranaki Mustang Club, New Zealand

# SYMPTOM TRY /CHECK
If the problem gets better, continue in the same direction.
If the problem worsens, go in the opposite direction.
1 Blackens Spark Plugs while driving Smaller main jets, lower float levels, lower power valve number, reset butterflies.
2 Blackens Spark Plugs at Idle Reset butterflies, Screw in mixture screws, lower float levels, lower fuel pressure, clean out air bleeds, use smaller number power valve, increase static ignition timing.
3 Backfires through carburetor Screw out mixture screws, increase squirter size, raise float levels, reset butterflies, increase main jet size, check engine timing.
4 Backfires through the exhaust Screw in mixture screws, decrease squirter size, lower float levels, reset butterflies, decrease main jet size, check engine timing.
5 Doesn't pull at wide open throttle Increase main jet size, raise float levels, increase fuel pressure
6 Float levels wont adjust Lower fuel pressure, clean needle and seat assembly, make sure floats move freely
7 Float bowls drain down after shut-off Retighten bowl screws and power valve, replace bowl metering block and power valve gaskets, clean out air bleeds.
8 Fuel drips out of boosters Lower float levels, lower fuel pressure, reset butterflies.
9 Fuel drips out of Accel/Pump nozzle Loosen tension on accelerator pump arm
10 Fuel leaks out Throttle shafts Lower float levels, lower fuel pressure, clean out and/or replace needle and seat assembly, reset butterflies, clean out air bleeds
11 Fuel shoots out Vent Tubes Lower float levels, lower fuel pressure, clean out and/or replace needle and seat assembly, make sure the floats move freely.
12 No adjustment on Mixture screws Reset butterflies, increase initial engine timing, lower float levels and fuel pressure.
13 Rich while driving Smaller main jets, lower float levels, smaller power valve, (For very rich check for blown power valve diaphragm)
14 Rich at idle Reset butterflies, screw in mixture screws, lower float levels, lower fuel pressure, clean out air bleeds, use smaller number power valve, increase initial ign timing
15 Runs on after shut-off Reset butterflies, lower idle RPM, check engine timing.
16 Vac secondaries do not open Install a lighter secondary diaphragm spring
17 Sluggish at wide open throttle Decrease main jet size, lower float levels, lower fuel pressure
18 Smells rich at Idle Reset butterflies, screw in mixture screws, lower float levels, lower fuel pressure, clean out air bleeds, use smaller number power valve, increase initial ignition timing.
19 Smokes at idle Reset butterflies, screw in mixture screws, lower float levels, lower fuel pressure, clean out air bleeds, use smaller number power valve, increase initial ignition timing.
20 Smokes under acceleration Decrease squirter size, lower float levels, lower fuel pressure, reset butterflies.
21 Stumbles or hesitates (Light Acceleration) Reset butterflies, raise float levels, reset mixture screws
22 Stumbles or hesitates (Hard Acceleration) Increase squirter size, reset butterflies, raise float levels.
23 Surges at idle Reset butterflies, screw out mixture screws, raise float levels.
24 Surges at cruise Reset butterflies, raise float levels, turn mixture screws out, install larger main jets, install a higher power valve number.
25 Wont idle down Reset butterflies, check mixture screws, lower float levels, check initial timing, check throttle linkage, check for vacuum leaks
26 Wont stay running below 1500 rpm Reset butterflies, check mixture screws, lower float level, check initial timing
27 Wont start Prime carb with fuel, pump squirters, check float level, check ignition timing
28 Wont stay running Reset butterflies, check mixture screws, lower float, check initial timing.

If the above advice doesn't help,  Bob's Carb page http://www.bob2000.com/carb.htm describes tuning from a different perspective - a good addition to the Holley literature.