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Toby The IHC 304 V8 of that era was notoriously cold blooded! Was due primarily to the excessively lean calibration of the 2 bbl Holley carb I suspect as a quick and cheap way for IHC engineers to meet emission requirements at the time. Anyway we struggled with a balky 68 304 for many years. A few things we found that helped: Keep the ignition system in top condition. Run a larger plug gap than spec (.040 or more). An aftermarket CD ignition kit helped in this regard. Rejet the carb to permit a richer idle mixture adjustment (involves manually reworking the idle metering jets to enlarge the orifice openings several thousandths not for the faint of heart!) We also rejetted the high speed circuit with larger orifices obtained from a carb rebuilder. The penalty is a slight reduction of the already poor gas mileage. Modify the carb accelerator pump linkage to increase its stroke (to increase the volume of gas shot in as the throttle is opened). Switch to a smaller PCV valve to reduce PCV airflow dilution in the manifold. I recall we adapted a PCV valve from a Chrysler 225 slant six. Install the hottest thermostat you can find 192 deg if possible. Run a winter front whenever the temp drops below about 50 deg. Trick is to get the engine as warm as possible as fast a possible. Good luck aside from the factory carburetion the 304 is a great engine!
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