I wanted to follow up on this discussion forum covering the “B28” vacuum advance and centrifugal advance mapping. I need tips about recurving my distributor and fine tuning the carb.
My ‘69 302 Z28 is basically stock except with a ’70 LT1 solid lifter cam, flat top pistons, so I estimate my C.R. somewhere between 10 - 10.25:1. I am using stock exhaust manifolds and exhaust system, and running a 180 degree thermostat.
The distributor is a std. Delco points style distributor converted to Pertonix electronic ignition and Pertonix Flame-Thrower II high output coil. The distributor was recurved about 10 years ago with aftermarket centrifugal weights and light silver springs, but I don’t know the brand. I recently changed the vacuum advance unit from a “B1” to a “B28” and mapped the advance curve. A black advance stop busing is installed limiting centrifugal advance to 24 degrees.
I recently gutted the A.I.R. smog pump and it now functions as an idler pulley and blocked off the air delivery tubes The vacuum advance hose is now connected to direct manifold vacuum, at the T-fitting located at base of carb were A.I.R. diverter valve vacuum hose was originally connected.
The carb is the stock 780 Holly, List 4053, with 68/76 primary/secondary jets, #85 primary power valve and block off plug in the secondary PV side. The #85 PV was selected to opening at 8.5” Hg, or about 1” to 1-1/2” Hg below the manifold vacuum reading. I using the #28 pump discharge nozzle, and the white (weakest) vacuum secondary spring and omitted the check ball. I run 93 octane pump gas.
My set-up was with the smog pump active and the vacuum advance connected to the “stock” ported vacuum source, but now connected to full manifold vacuum as stated above. Initial timing is set at 12 deg. BTDC with a +/- 0.5 degree of flutter with the centrifugal advance secured to not to influence timing. Idle vacuum is 9 – 10” Hg and idle mixture optimize to maintain 9 - 10” Hg. The B28 vacuum advance adds about 16 – 18 "crank" deg of advance, which I can adjust with a vacuum timing limiter cam from Crane Cams. Idle RPM is set to spec., at about 900 RPM.
With the centrifugal advance active, the advance it is coming in too early and causing a jumpy +/- 1.0 deg or more of timing flutter at idle. The springs are too weak and need to be replaced. Idle must be adjusted down to 600 RPM to prevent centrifugal advance from coming in. At 900 RPM, 9 deg’s of advance results, checked with a dial back timing light. The centrifugal advance map curve yielded the following degrees of advance at the corresponding RPM’s, checked by dialing the timing light back to the 12 deg initial and recording the advance: +9* @ 900, +12* @ 1100, +16* @ deg at 1300, +19* @1500, +22* @ 1700, and all in at +24* @ 1900. Thus total advance of 36 deg (12 init. + 24 cent.) is all in by a very early 1900 – 2000 RPM. The overall advance totals 54 deg at cruise conditions, adding the 18 deg of vacuum advance. I may limit vac advance to 16 to get overall advance down to 52 deg. I fear engine ping or detonation during the hot summer season.
Throttle response is noticeably better. During aggressive 1st, 2nd, or 3rd gear acceleration runs, the 302 power really pulls hard after 3000 – 3500 RPM to redline. At this point I can not tell if the power band is natural to my engine, the current distributor timing map, or the mapping of the Holley carb jets and secondaries. While in park and idling, stabbing the throttle to wide open, there is occasional lean condition back fire thru the carb, but otherwise no noticeable stumble. Changing the Holley primary jets from 68 to 72 will hopefully cure this lean back fire.
If the goal is to have a weight/spring combination that does not come on too early, but is “all in” by 2800 – 3000, what is a recommend source for centrifugal weights and springs?
What color spring or spring combination and advance map is recommended, or found to be ideal in the 302?
The MSD Ignition web-site does a good job showing various advance curves with their distributor, limit bushings, and spring combinations but I am not sure if or how this translates to my stock distributor. I have read mixed results about the quality of aftermarket weights, and recommendation to stay with GM weights. I am not having much luck finding advance bushing and advance kits locally for the old style points distributor.
What Holley Carb vacuum secondary spring is recommended?
Should the vac secondary check ball be retained or omitted?
My goal is to find the combination of parameters for the ignition and carburetor yielding optimum performance and drivability, yet retain the stock look of my 302.
Please excuse my long discussion but I suspect there are others with my same questions and goal in mind, yet have experimented with a set-up that has yield good results.
Thanks and your recommendations will be appreciated!