I am having some problems with the idle on my '63 L76. Initial timing is at 10, dwell at 30 and the AFB idle screws are 1.25 turns out. I can set the idle at 750 at the curb, but the car stalls badly when releasing the clutch from a stopped position. When I set the idle speed to 1250, the car will not stall. However, the clutch must be released slowwwwwly when moving from a stopped position. Once moving, the engine runs perfectly. The AFB was just rebuilt. Any suggestions are always appreciated !
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Re: Idle Problems for ' 63 L76
Assuming you have the OEM type Duntov cam, 750 is too low IMO. It should be more like 900. The initial idle mixture screw setting is 1.5 turns from the seat, so the engine might be a little lean. A stumble upon releasing the clutch is usually cause my mixture lean out.
You will have better idle quality and stability if you install the '65 SHP/FI vacuum can that provides full vacuum advance at 8". With a Duntov cam the typical idle vacuum is 10" to 12", so the OEM can, which didn't provide full vacuum advance until 15" is not suitable. The vacuum can should provide full vacuum advance at a couple of inches less vacuum than what the engine idles at. Chevrolet Engineering didn't get this right in'63, which was the first year they configured a vacuum advance on a SHP engine. The running change to the 8" can occured during the '64 MY run.
With a CR four-speed and 3.08 axle my L-76 SWC was never easy to start from a dead stop. Some stumble is normal especially if you try to minimize clutch slippage. When I blocked the heat riser passages is was much worse and would stumble badly until the engine had been running for 20 to 30 minutes, so I removed the blocking shims.
I haven't tried this but just installing one shim will reduce exhaust gas flow through the heat riser, but will still allow some exhaust gas to migrate under the carb, but there won't be an end to end flow path so heat transfer will be reduced. This heat riser strategy might work okay in a warm climate.
Duke -
P.S.
To confirm that the stumble is mixture lean out, set the idle speed up to 900 and the mixture screws two turns out from the seat. If it is eliminated you can lean out the mixture screws until it returns and eventually find some happy medium.
The original spec vacuum can may or may not be contributing to this problem, but I still recommend you install an 8" can - Echlin V1810, and you can buy it from NAPA.
Chevrolet's recommended idle speeds for SHP/FI engines are artificially low IMO. There are no magic formula for idle speed, but the general rules I follow are to be sure that the vacuum can provides full vacuum advance at idle speed, then I set idle speed at "the lowest possible commensurate with acceptable idle quality". On a L-79 this is probably about 750/800, about 900 on L-76 engines, and FI engines will probably need about 1000 RPM to achieve reasonable idle quality.
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