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Ok, I have a Gen VI 454 HO just installed in my 1970 factory BB 4 spd coupe. Carb is a Holley with 4 corner idle mixture adjustments.
The car has a factory TI ignition. Recently, the amplifier died and I replaced the amplifier with a unit that has the upgraded circuit board.
The car idles just fine but from 1100rpm to 1400rpm it now has a flat spot. The engine shakes as if it is running on 7 cylinders. Pulling away from a stop it lugs away just fine but once the rpm gets into the above rpm range it fails to accelerate. Once the rpm gets to 1450 it accelerates normally. Engine pulls strong at WOT.
I adjusted the idle mixture using my vacuum gauge. Closed the idle screws and backed them out approx 1 1/2 turns to the highest vacuum reading. Installed new plugs n checked all the plug wires for security. Pulled the cap n rotor to check that the fly weights were free n moved easily all contacts clean in the cap n rotor.
Have not checked the timing yet as my light died.
Only thing I can think of is the carb itself. Accelerator pump is working ok so it seems the issue is the transition from idle to run. Thought about pulling the carb n making sure all the fuel n air passages are flowing free ( blowing them out with air n carb cleaner).
What am I overlooking here?
Have you checked the gap between the accelerator pump cam linkage and the pump lever? Too much gap leads to off-idle stumble.
From Holley's website:
Make sure the accelerator pump linkage moves the arm to the diaphragm as soon as the throttle is moved. Clearance at the lever will create a hesitation. Adjust the spring-loaded bolt and nut until the clearance is eliminated. This should require only a slight adjustment to minimize the clearance. Do not preload the lever. Excess clearance at idle is a very common occurrence that takes a minute to repair and when adjusted drastically improves drivability.
Does the distributor have a vacuum advance? Have you tested if it’s functioning? I would make sure the ignition system is working as it should before making any carb adjustments.
Thanks for all ur input..
I will check the accel pump adjustment although before the amplifier died, it ran just fine. Power valve, maybe.
The car was running fine, shut it off, came back about 40 min later n nothing. The engine cranked , had plenty of fuel, but no spark.
plugged my spare amplifier in and it fired right up. Now the stumble off idle. The accelerator pump should cover the transition from idle to run so will ck that adjustment.
will pull the vac adv hose n ck that operation but the car idles fine. With the vac pot disconnected the idle speed would be off/ low. It idles at its normal 850-900 rpm. It has plenty of vac signal ( 16-17 in HG).
Separate question. Does the amplifier require a separate case ground or is it grounded thru the amplifier harness? The amplifier is mounted to the front of the left front fender well, all fiberglass/ no metal for a ground. I did run a ground wire to the amplifier case to ensure a good negative return.
OK, all u would be C3 mechanics, found the “ fail to accelerate ) issue with me Gen 6 TI equipped 454.
I kept thinking it was an ignition issue ( advance) not the carb. Removed the cap n rotor to check the flyweight’s operation. Sure enough, the old grease on the weights n pivots had thickened over time n were sticky. Cleaned the grease up, polished the weights n waxed them. Reinstalled n problem solved. Who of thunk it!
70 BB Carb or Ignition issue.
