drdave69

My ZZ502 just recently started running a little rough and I am gonna start with checking the carb, plugs and timing. Does anyone know the specs for tuning this engine? I believe 10 deg initial timing is what I have found but I can't find anything else. Which plugs and gap? How many quarts of oil? Etc. This is in my 1966 convert with 4 speed if it matters.

mr.beachcomber

According to Chevrolet, your initial timing should be 8 degrees BTDC with 30 degrees total advance.

If you have the #12363390 oval port aluminum heads, you should be using an ACDelco Rapidfire #19307141 spark plug according to Karl Performance Parts (a Chevy Performance Parts dealer). The RapidFires are pre-gapped so all you need to do is install them. (Great plugs, I've used them on all my Chevy engines with aluminum heads both for street and auto-x.)

drdave69

Thanks. I tried to find the Rapidfire (#4) plugs in stock but they didn't have them so I got the Autolite 66 that were already in the engine. From what I can gather the gap should be .040. I'll replace the plugs and see how it runs then I'll double check the timing. The one plug I removed wasn't in very tight and the tip was half unscrewed. I'm hoping that the plugs are somewhat fouled and this solves my issue.

drdave69

Finally installed the new plugs (gapped at .040) and had to go ahead and install new wires. Engine still did not want to start and did not want to idle until warm. I will check the choke next time but I have a question about the carb secondaries. Are the secondaries supposed to open and squirt fuel when revving it in the driveway?

mr.beachcomber

If you have a double-pumper Holley, the mechanical secondaries will open and activate the secondaries' accelerator pump when the primaries are open beyond a specific point. The primary and secondary throttle plates will all reach WOT at the same time.

If you have a vacuum secondaries Holley and the back throttle plates are opening while reving the engine, you may have a weak spring in the vacuum pot allowing the secondaries to open earlier. Since there isn't an accelerator pump for the vacuum secondaries, any fuel you see is pull over from too high a float level.

Hope this helps.

drdave69

It is the vacuum secondary 850 (4150 I believe) that came with the engine. The secondary throttle plates are not opening and I can't see fuel squirting there when revving hard in the driveway. I assume this is functioning as needed. The front squirters are dropping a fairly big shot of fuel when revving.


I'll also check the HEI cap and rotor.


Thanks for any help.

drdave69

Replaced the HEI cap and rotor and the engine now runs well but still does not want to idle well. It seems happier to idle at 900 - 1000 rpms. I am thinking that the carb needs going thru and tuning. I just do not know enough to determine if it is running rich or lean. Any help about the carb would be welcome.