I would check the timing belt before I set the ignition timing. No need to add to the confusion. Once you are satisfied that the cam and the intermediate shaft are in correct sequence, then check the timing. With the engine at normal idle, pull the vacuum hose off the distributor, and plug the hose only. The distributor will not be getting a vacuum signal, so It won't be necessary to plug the advance port on the distributor. Set the timing at normal idle speed to 12 degrees before Top Dead Center, and lock it down. You may need to use a drift and give the adnustment slip bracket a shot to loosen the distributor in the block. Connect the vacuum line, and go for a test drive.

I don't know for sure about the 79, but my 85 with the B-230 engine has sero degrees at the extreme right side of the timing pointer (where the pointer meets the cover). It has markings at 5, 10 and 20 degrees. I set mine at about 1/4 of the way between the 10 and 20 degree mark (and just fudge it toward the 10 degree mark). It almost sounds as if the ignition timing is very retarded, if the temperature goes up very quickly. This may be your whole problem. Normally, advancing the ignition timing will allow the engine to run cooler, and retarding it will warm things up quickly. Kids used to back the timing up on their chevy's so they would sound like they had a big cam. They would ease down the block, and bump the timing up to cool things down, and later come by with the timing way slow and have the lumpy idle again. Of course with the timing that retarded, my 245 could have blown their doors off.

Bill
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Those who beat their swords into plowshares will surely plow for those who did not.