I've got a '68 Mustang, with a seemingly stock built low to mid compression 302. The engine was in there when I bought the car, so I don't know it's history, but it will pull about 17" vacuum at idle. Currently running a Holley 4160, 600 CFM, vac secondary, I think it's a list # 9834, but I would need to verify - nothing exotic. Electric choke - operating correctly.
The thing that befuddles me with this carb is the fact that in order to get it to idle smoothly, both idle enrichment screws are set about 1/4 turn out. Some days it will idle reasonably smoothly, other days it will idle rough and need another very fine adjustment. It seems to me that I must have some type of internal leak which is supplying a tiny bit of extra fuel at idle, leading to the relatively hypersensitive nature of the idle enrichment screws. Any thoughts on likely sources of this unmetered fuel? Maybe I am jumping to conclusions. This issue remained even after a carb rebuild. I've also tried various adjustments on the secondary throttle plate stop screw thinking it was being fed too much air / fuel from the secondaries at idle, but this didn't have a significant effect on the mixture screws, just forced me to adjust the throttle stop screw to get back to my desired idle speed.
Ignition is recently tuned, Carb is running #66 main jets, I assume stock metering block on secondary - carb is used also. Fuel pump and filter are recent as well. Recently R&R'd the carb thinking there might be a base gasket leak, and also re-torqed intake manifold with no change in behavior. 302 is hooked to a seemingly stock C4 and 3.00:1 open rear. Running 12 deg initial timing, but behavior was the same at 6 deg and 10 deg.
Any insights would be appreciated. I've live with this for years now, but it is very annoying to be having to tinker with the idle mixture on a weekly basis.
Thanks,
Andrew
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