Carburetor Rebuilt - Bogging and back-firing

Joined
Jan 7, 2011
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55
Location
Peoria, AZ
 
‘70s are a special year. You have an extra port on the carb base by your idle mixture screw near the valve cover. Yours is open in your first pic. It needs to be capped to prevent loss of vacuum. Instead of capping, the port can be connected to a vacuum advance dizzy. However, your stock dizzy is vacuum retard.
@stoshzack - How can I tell if I have a vacuum advance? I believe it was swapped out by prior owner since it came with an extra dizzy. And I added a Pertronix ignition a couple years ago. Just want to triple check my vacuum connections and where I need to cap. Thanks for all the help
 
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Feb 14, 2013
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Wake Forest, NC
 
@stoshzack - How can I tell if I have a vacuum advance? I believe it was swapped out by prior owner since it came with an extra dizzy. And I added a Pertronix ignition a couple years ago. Just want to triple check my vacuum connections and where I need to cap. Thanks for all the help
Typically vacuum advance have the diaphragm toward the front of the vehicle when the dizzy is installed, instead of the rear for a vacuum retard. Although this is not always the case. In the pic you posted, the diaphragm is clearly on the rearward side of the dizzy. That is stock for a '70. Check here for more info:
 
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Feb 14, 2013
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Wake Forest, NC
 
If you continue to have issues, I'd consider rebuilding the carb again yourself. PinHead has a series of YouTube videos on how to do it.

 
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Feb 6, 2002
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15,109
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OC, CA
 
 
 
 
Bogging down off idle during initial acceleration and backfiring through the carb is often due to a manifold vacuum leak or a lack of idle fuel. If your engine can run with the idle mixture screw turned all the way in, then that is good confirmation of the problem. If not, it could also be due to a weak accelerator pump squirt.
 

thebigredrocker

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Nov 6, 2013
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3,390
 
What was your vacuum number?
You can just plug the booster hose if you think the booster is leaking.

@stoshzack - How can I tell if I have a vacuum advance? I believe it was swapped out by prior owner since it came with an extra dizzy. And I added a Pertronix ignition a couple years ago. Just want to triple check my vacuum connections and where I need to cap. Thanks for all the help
To verify advance or retard, remove the dissy cap. Hook a vacuum pump to the dissy. With Vacuum applied a retarding points plate will rotate clockwise. Counter clock for advancing plate. If you don't have a pump you can suck hard on the dissy hose.
 
Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
Bogging down off idle during initial acceleration and backfiring through the carb is often due to a manifold vacuum leak or a lack of idle fuel. If your engine can run with the idle mixture screw turned all the way in, then that is good confirmation of the problem. If not, it could also be due to a weak accelerator pump squirt.
@Pin_Head It will run with mixture screw all the way in. I used two dry gaskets on each side of the insulator. Is there an easy way to see if I have a leak here? Thanks for the help.
 
Joined
Feb 6, 2002
Messages
15,109
Location
OC, CA
 
 
 
 
The fact that it runs with the idle screw turned all the way in shows that you had to increase the idle speed screw so that it pulls fuel from the main nozzle to run at idle. It also explains why it bogs down off idle: there is no fuel at the transition fuel slot then the butterfly starts to open.

Check every potential manifold leak point, like the manifold-head gasket, the brake booster, the PCV line and check for cracks in the manifold under the carb. You can spray carb cleaner or propane gas around all the leak points and see if the idle speed increases. Spray especially underneath the manifold where it meets the head.

You can check if there is fuel coming out of the transition slot by shining a flashlight in the primary with it "idling". The slot is just above the throttle plate on the engine side of the carb.

If there is no detectable manifold leak and no fuel coming out of the transition slot, then try blowing out the idle circuit with carb cleaner and compressed air from the idle mix screw hole and the idle fuel solenoid hole.
 
Last edited:
Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
Hey all, quick update - took the carb back to my rebuild guy and he hooked it up and ran fine. So, left off the gasket at the top of the insulator, made sure the bolts were torque to spec, cap'd the dizzy and advanced the timing and well it runs good. I need to tune it but am happy the stutter is gone. This sounds like I had a vacuum or timing issue.
 
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