73-87 Chevy _ GMC Trucks > Fuel Systems and Drivability

1976 C20 Choke Thermostat and Cold Start Drivability

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Henry:
Hi Guys:
I have a driveability problem now that we are in winter: my truck will not come out of high idle when the outside air temperature is below approximately 50F.

My truck: 1976 C-20, mostly OEM. 350 V8, T400 trans, A/C, Qjet 4MV carb (divorced choke coil), standard thermostat. Only mod is a 4-row radiator instead of 3-row radiator.

When the temperature is above the 50s, the truck goes through cold start normally where it is in high idle and you can kick it down to regular idle within a few minutes or drive away and it goes down to regular idle after suburban driving for 5 minutes.

When the temperature is below the 50s, the truck starts in cold start but stays there no matter how long I let it stay at high idle. It will not kick down and if I drive away it will not kick down even after 10 minutes of suburban driving.

Inspection of my carb shows all linkages to be intact, no hose leaks, and choke coil to be free and in good shape. On the surface everything seems in order. I will go make some linkage adjustment checks and do some testing of the vacuum break diaphragm to make sure it is really ok.

So when I first started the truck with the hood up I noticed that the fan was blowing a gale force wind on cold start and appears to be running at engine rpm...and this made me suspect it may be the culprit of keeping the engine and carburetor too cold in cold weather. I have the OEM fan and a OEM style thermostatic fan clutch, standard duty. When the engine is off, regardless of cold or hot engine, the fan has resistance to spinning: no matter how hard I spin, it will not feel like turning more than 1/2 turn. On a cold day (about 45F) I cold started it and at high idle when the engine was cold I attempted to stop the fan with a rolled up newspaper: it took considerable force to stop it while the engine was running. When I released it the fan just started back up to what appears engine rpm driving a gale force wind across the engine again. So I thought the fan clutch was bad and went to a big chain parts supply store and bought a new standard duty fan clutch. This clutch is a Wagner brand and it cost about $35. I put it in the truck and gave it a week to settle in by driving it around town in hot and cold weather. So now I go through the same checks of the fan clutch and it behaves exactly like the old clutch. So I am feeling I dont really understand how this fan clutch works or maybe I have a bad new fan clutch.

So I still have to do the checks on my carb, but can anyone tell me how to really check if this fan clutch is working correctly?

Regards,
Henry

JohnnyPopper:
The only thing to keep you at high idle is your choke/assembly.

What do mean by "divorced choke coil"?

bd:

--- Quote from: JohnnyPopper on January 25, 2022, 01:11:35 PM ---What do mean by "divorced choke coil"?

--- End quote ---

The thermostat mounts to the intake manifold versus a "married choke" where the coil mounts inside a housing on the side of the carburetor.


Henry, does the intake manifold become hot where the choke thermostat mounts?  Exhaust flows from head to head through a crossover passage directly below the choke stove.  Does the engine have a functioning heat riser valve/EFE valve?  Is the choke t-stat cover in place to help trap the heat?

Viscous fan clutches typically are engaged when an engine starts.  In really cold climates the fluid is so viscous that it can take some time to redistribute and release.

Edit: spelling

VileZambonie:
Divorced means you'll NEVER get along  ;D

They went to electric choke because those hot air chokes work for crap

Rapid Roy:
I have had mine since July 2015 and divorced choke still works great!

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