I have a 1976 Chevy c20 pickup I am wondering if a intake manifold or a blown headgasket will make my not idol or run well can anyone help please

faulty reverse light switch on gear nox

check for vacuum leak for the idle or idle mixture screws settings
no power in 3rd gear can be low fuel level, filter problems , ignition timing ( static is 12 degrees BTDC idle )
more than likely a vacuum hose missing after assembly

There are a few internet forums that specialize in 60's and 70's Chevy cars and trucks. I would sign up to one of these and pose your question there with all the details you have on the vehicle including those details on the build plate in the engine bay if it has a plate. I'm sure one of the guys there can tell you if you give them all the details you have on the vehicle.

Hi
You need to remove and clean with a carb cleaning product the carburettor and air blow it
Catalin

Aaron, there has been a few problems with head gaskets, the bigger problem was with the intake manifold gaskets.
Check the attached links,
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Intake Manifold Gaskets May Leak Coolant or Engine Oil Buick Century...

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Thread how do tell if my headgasket or intake gasket is bad

1997 Buick Century Blown Head Gasket

Idle problem like what? too low? too high? If it is too low it could be because your air filter is way too dirty or the throttle body has a bunch of carbon built up in it around the butterfly valve and gets no air at idle. If it is too high you have a vacuum leak. air sucking in from a cracked hose or intake manifold gasket. Check all your vacuum hoses and spray carb cleaner around the intake manifold gasket while the car is running.

The intake manifold gasket is the first place to check. It is possible for the coolant to leak internally. Have the cooling system pressure tested and checked for exhaust fumes in the coolant with a block tester.

these cars have issues with coolent getting into the intake... headgaskets are not one of the issues . check plugs for coolent. upper plastic intake has coolent flow in to chambers if u will, and over time they get micro cracks and brittle ...lower intake manifold gaskets are plastic also. ..cyl. heads are cast iron not alluminum . lower intake manifold is alluminum and upper intake is plastic .. on my lesabre a plastic 90 degree elbow connected to the lower intake manifold to the heater hose housing. the belt tensioner is connected to this housing im refering to. not saying for sure, but if it is this is usually why...