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Aberdare · 12-04-2019, 11:58 AM

OK... interesting. The Primary Cause of Stalling at Low RPM/Deceleration is usually a vacuum leak and/or improper Carb and timing adjustments. Low Fuel Pressure can also cause the same issue. How about some tests?

1. Return all Vacuum Ports/hoses/lines to their original configuration(s) and check the Fuel Pressure. At Idle, the Fuel pressure should be between 3-6 PSI and Maintain this Pressure under LOAD (When you put it in gear).

2. Perform an ignition Timing Check. Timing for your engine should be 4 Degrees BTDC (But check your engine specs for an accurate spec value, as it could be 2-12 Degrees).

3. Check and/or adjust Engine Idle SPEED. Auto Trans should be 475rpm +/- 25rpm (NOT IDLE MIX - You'll do that later).

4. With a VACUUM GAUGE - check your engine MANIFOLD VACUUM. Any Port/fitting BELOW the CARB is acceptable to use as a test port. Engine Vacuum at Idle should be 18-22 in Hg (Inches Mercury)... although it may also be as low as 15 in Hg if your engine has high Mileage. If within range, proceed... if NOT, then you may have need to perform a Compression Test and/or check for a Manifold Leak.

5. Adjust the Carburetor IDLE MIXTURE, in accordance with your Carb's Mfr Specs. For most Carbs, turning in the mixture screw(s) until a 25-30 RPM drop is achieved, then back out 1/4 turn. Readjust Idle Speed as required to achieve proper rpm, AFTER you've finished adjusting the Idle Mixture.

6. Road TEST.

12-04-2019, 11:58 AM	#10
Aberdare Registered User Join Date: Oct 2017 Location: Manitoba, Canada Posts: 141	Re: Suddenly Running badly OK... interesting. The Primary Cause of Stalling at Low RPM/Deceleration is usually a vacuum leak and/or improper Carb and timing adjustments. Low Fuel Pressure can also cause the same issue. How about some tests? 1. Return all Vacuum Ports/hoses/lines to their original configuration(s) and check the Fuel Pressure. At Idle, the Fuel pressure should be between 3-6 PSI and Maintain this Pressure under LOAD (When you put it in gear). 2. Perform an ignition Timing Check. Timing for your engine should be 4 Degrees BTDC (But check your engine specs for an accurate spec value, as it could be 2-12 Degrees). 3. Check and/or adjust Engine Idle SPEED. Auto Trans should be 475rpm +/- 25rpm (NOT IDLE MIX - You'll do that later). 4. With a VACUUM GAUGE - check your engine MANIFOLD VACUUM. Any Port/fitting BELOW the CARB is acceptable to use as a test port. Engine Vacuum at Idle should be 18-22 in Hg (Inches Mercury)... although it may also be as low as 15 in Hg if your engine has high Mileage. If within range, proceed... if NOT, then you may have need to perform a Compression Test and/or check for a Manifold Leak. 5. Adjust the Carburetor IDLE MIXTURE, in accordance with your Carb's Mfr Specs. For most Carbs, turning in the mixture screw(s) until a 25-30 RPM drop is achieved, then back out 1/4 turn. Readjust Idle Speed as required to achieve proper rpm, AFTER you've finished adjusting the Idle Mixture. 6. Road TEST. __________________ RED SEAL CERTIFIED JOURNEYMAN SINCE 1993 My 1966 GMC Value Van Project 1951 GMC 9430 1-Ton I'm just going to put an OUT-OF-ORDER sign on my forehead... and call it a day.