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I have read with interst several recent posts about ignition timing. My SB has a 62 centrifugal advance only distributor (ie. no vacuum advance can present). I have the engine out of the car right now ( part of the restore process), but before it was pulled, the engine idled and ran fine. What would be some of the observable differences in running characteristics between this distributor and one with a vacumm advance also.
Very NOM. 65 Tonawanda block, 67 (462) heads, 62 distributor. The intake is GM aluminum, and the carburator is a pretty new Holley 4160 with an electric choke.(I think that number is right, it is not the double pumper)
I just tore it down to see how it was set up. In a previous post, you identified the pistons as replacements for the 340 (L 2166N, .040 over). No one was able to identify the cam from the numbers that I found on it. But since that post, I have measured the lobes. Both intake and exhaust show .30" lift at the lobe (1.533" - 1.233"). The rocker arms look stock, so that would make the valve lift .45". I haven't tried to measure the duration yet.
A side note on the cam. I measured all of the lobes. 13 of them measured 1.533" lobe and 1.233" base. 3 of them measured 1.458" lobe and 1.233" base. The 3 short lobes are #3, #13, #14 counting from the front. I wouldn't have been surprised to find one or two lobes out-of-spec, but finding 3 at the same out-of-spec measurement seems strange.
It's time for a new cam - .075" difference in lobe lift is about .108" at the valve at full lift (stock rockers are about 1.44:1 at full lift), which is WAY out of spec; sounds like several lobes have already started going away.
My recommendation is that you sell the dual point - may be worth some bucks depending on the number - and replace it with a single point. Rebuild the engine with the 151 cam, deck the block down to get a piston to deck clearance of zero to -.005" (piston crown above deck) and assemble with a .040" composition gasket. This will yield minimum quench clearance and about a 10.5:1 CR with a flattop piston. Use the 151 cam, and blueprint the distributor - verify that the bushings are serviceable, install a new wobble free breaker plate, with the high breaker are tension points (B-W A112HP or equivalent), a Echlin/NAPA V1810 vacuum can and shim up the end play to two to seven thou.
My engine is running an after market distributor with no vacuum advance. I did notice the the spring tensions were different and confirmed with Mallory that it is intentional so the distributor can bring in some advance quicker on acceleration to compensate for the missing vacuum advance.
My car runs fine too at idle, it pulls very strong off the line, but as Duke indicated, gas mileage is terrible and my engine does tend to run a little hot. I have this setup for 40 degrees total advance (distributor gives 24 degrees at 3000 rpm and initial is 16). I wish I could add vacuum advance to this distributor, but it doesn't appear possible. A new tach drive distributor is not cheap
Steve - I had a similiar set up with my '63. It had a dual point distributor with no vac can. It started ok, but stalled badly off the line and required a very high idle speed. I found a guy in Chicago who took my distributor in trade for a rebuilt original with single points and a vac can. Since this is a SHP engine, I needed the vac can noted in Duke's post. It now idles and runs very good. The swap for the rebuild dist ran me about $130.00.
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