The Stovebolt Site Forums The Shop Area Engine Shop Points burning and vacuum advance question

49' 3600 with a 56' 235. After 111 miles from a restoration the truck suddenly ran like crap and I barely made it home. I found the points had a buildup from arcing. I threw in a new set ran like a champ for about 20 minutes and went to heck again. I burnished the contacts and it fire right up and ran like a champ again. Would a failing or failed condenser be taking out my points?

Vacuum question, my understanding and conventional wisdom is that the vacuum advance should be hooked to full manifold vacuum and only operate at idle. At higher RPM's it drops out and mechanical advance takes over. Mine kicks in at high RPM's and does nothing at idle. Seems backwards to me.

12v system?
(If it is, do you have a ballast resistor?)

It is 12v and yes I have a ballasterrible resistor

Answer to your first question, possibly and note that condensers purchased nowadays are quite often garbage right out of the box.

Question #2, most of the more modern vacuum advances operate off a port on the carb. Don't know if it is full manifold vacuum or not. However, your operational theory is correct.

You don't say what carb you are using, but Rochester's do not supply vacuum to the advance at idle.

Which port to source vacuum advance from is a question, not a statement - there are several answers.
The purpose is to work in parallel with mechanical advance as engine approaches cruising rpm with high vacuum, and drop off completely leaving only mechanical advance with WOT.

The vacuum makes good discussion but has nothing to do with your problem. It could be a bad condenser or a short in the coil. It might be the resister is not matching the coil. With the engine not running & the key on & the points closed, check the voltage at the pos. side of the coil. It should be about 6-8 volts. Also its possible the resister bypass is stuck giving full voltage all the time.

FWIW....Resistors fail open. I would think if the ballast resistor were to fail, it would kill the engine.

Point arcing is why the condenser is in place. In any other application it would be called a capacitor(just a hangover from early days). The condenser acts like a sponge. Any sudden change in voltage, gets "absorbed" in the condenser as an electrical charge. When the value of the condenser is too small, material is pulled from one point face to the other(I used to know if it went plus to ground but part timers has taken that bit for a ride). If the value is too big the material is pulled the other way. We used to at one time play with "capacitive discharge" systems and this was how to decide on how big the condenser we needed. Any way to get to your issue, the condenser has failed. Change it and the points should last much longer. Yes, some new ones are bad(common complaint) but they are cheap. You did admit to a ballast resistor, and a previous post pointed out that resistors fail "open". That would be no spark so get the condensor and it should fix the problem.

I was leaning towards the condenser being the culprit after ruling everything else out. I knew it wasn't a resistor problem or I would have no or extremely low voltage at the coil. It's nothing more than a basic RLC circuit and without the capacitance in place to soften the blow of the inductive kickback from the coil the points were taking the brunt of it. So today I started at square one. Gapped the fresh points at .022 per the shop manual, reset the distributor which was also off, replaced the condensor. It fired right up and settled into the smoothest idle it ever has. Set the rpm at 500 with no vacuum advance and found my timing to be 3 degrees advanced and it was running smooth as silk. Drove it for a bit and it runs like it never has before. Beautiful throttle response, Tom Langdon holly weber carb, strong acceleration. It's a completely different truck now.