Intake leak
Hi. I have a 79 super six. It runs rough and I'm troubleshooting. Everything checks out, but when I put my hand over the carb to choke it off the idle speed increases. This indicates an intake leak. All hoses are hooked up. Carb is tight. It's unlikely, but could the intake manifold be cracked. Or something else. Thanks in advance for any info, experiences.
It could also be that she's just not getting the right amount of fuel for whatever air it is getting. or
that the fuel is of poor quality, or
the air bleeds are not working right, or
the timing is retarded, or the PCV valve or hose is defective, or
the intake is cracked under the carb, or
the brake booster or it's hose is faulty, assuming you have one.
the point is this; your test only proves one thing; the IDLE-AFR is or may be, lean. I wouldn't be in a hurry to regasket it yet, especially as how, that intake can be a pita to seal in the first place.
For instance; as you advance the timing, the idle-speed will automatically increase. The usual fix is to slow it down using the curb-idle screw. This reduces the fuel being discharged from the Transfer Slot and the engine runs rough, so you give it more fuel from the Idle discharge ports. You may or may not get it to idle decently using this mucked up system. sooner or later, the idle screws become maxed out and the IDLE-AFR goes lean,..........
I'm not saying you have done this.
I'm just saying that this regularly happens, and is occasionally misdiagnosed.
This is just one of several possibilities.
I would start with the basics; put the timing back to stock, reset the carb as may be required including the fuel level, then if she still idles ugly, then; I would do a valve-adjustment and a compression test. Let's see where she's at.
That's what I would do, assuming you have properly proven that she ain't sucking air somewhere that she ain't supposed to be.
