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1971 Cutlass 350 Distributor. This may sound dumb, but the shop rebuilding the trans, found a leak in the vacuum advance when connecting the downshift etc. The problem is they said what kind of HEI distributor is in there. Original was points. It has a brown top, Accell 140005 Coil. Other than that I don't know how to figure out which vacuum advance to buy. If a shop can't recognize it how do I? They are the best trans shop in the area so not half assed guys. Any ideas are welcome.
The universal HEI replacement adjustable, vacuum advance put out about 20 degrees of timing advance. That will work OK with your 71 350. I personally like the 30+ degree vacuum advance cans on low compression Olds V8's running manifold vacuum.
I'm struggling to understand what the vacuum advance has to do with "connecting the downshift". No GM trans uses vacuum for downshift. The TH400 is electric and the TH350 uses a cable. The vacuum modulator is connected to manifold vacuum. The vacuum advance could be connected to either manifold or ported vacuum, but typically has it's own port. Did they actually use a vacuum pump to test the advance can? Sorry, but I'm skeptical here. That leak would have been noticeable as a poorly-running car long before this.
Sorry, wrong terminology. The transmission was being rebuilt. When they were checking things, it had the full manifold vacuum running to a split w/ one tube to the trans vacuum modulator. The other to the distributer. Car had capped ported vacuum line at the carb that should go to the distributer. I assumed the vacuum modulator controls down shift, apparently not. I do have the cable at the accel. pedal in this 1971, 350. Not exactly sure what the vacuum modulator does here on my Olds. Upshifts? Checking the vacuum lines, he found a leak instead at the distributer. For now the distributer is disconnected & I need to get a new advance unit. Trans is connected to the manifold vacuum.
As far as poor running, it has & did run OK, but I have discovered some problems before this. This is the car that had it's HEI wired into the original ballasted harness. Now it has 12 volts. It idled rough & I increased the speed a little. Might have done more harm than good. Much better now w/ HEI at 12 volts. Smoother. Car was really hard to start cold. Cranked a lot. I found & replaced a seeping fuel pump. I assume the line got dried out sitting overnight. Haven't messed w/ an old V-8 for a while. Interesting project.
As long as I'm asking questions, another item addressed at the shop was installing a new starter since the engine would hang at TDC when cranking warm. I said just install one to be sure. Car still cranks very slow & hangs up. They are going to check initial timing & make sure it's not too far advanced, before I pick up the car tomorrow. Other than timing, anything else that could cause the poor cranking, w/ 2 different starters now. Battery is new & I took off & cleaned all my connections when I bought the car 2 months ago. Voltage is 12.7 w/ engine off. Shop just called. He said initial timing needed to be set back a little & they did this. However they discovered the springs on the advance were way too weak & beginning to advance at fast idle speed. So, another item to fix soon. How would this hurt them car?
A TH350 trans uses a cable that runs from the trans to the accelerator pedal, through a hole in the firewall. That controls downshifts. The vacuum modulator controls upshifts based on engine load (manifold vacuum is a measure of engine load - high vac=light load=early upshift; low vac=high load=late upshift). Too much advance too early will both cause the engine to crank slowly and potentially cause pinging under load (and you won't always hear the pinging either). Too much advance also hurts power.
I would ask them for the the initial timing setting. With the points distributor factory was 10º but the HEI has a different (lower) amount of mechanical advance, so the initial would be 16-20º in order to still have 32-36º total. Mine is set to 18º and the engine runs much better (smoother idle and higher vacuum) than it did with 10º. I had a similar issue with light springs letting the mechanical advance start at idle speed; one light and one medium spring corrected that.
A TH350 trans uses a cable that runs from the trans to the accelerator pedal, through a hole in the firewall. That controls downshifts. The vacuum modulator controls upshifts based on engine load (manifold vacuum is a measure of engine load - high vac=light load=early upshift; low vac=high load=late upshift). Too much advance too early will both cause the engine to crank slowly and potentially cause pinging under load (and you won't always hear the pinging either). Too much advance also hurts power.
