Hi All,
I've read many of the threads on distributors and vacuum advance options, but have a question or two still.

I've set my timing, w/o vacuum, to 12* BTDC. When I connect vacuum to distributor, ported vacuum (about 2" - 23" hg as rpms increase), my timing advances to 24* at idle. Then when rpms increase (3000ish rpms) my max timing goes to about 46*. My question is does this mean I need to add heavier springs to my distributor to limit the amount of advance from ported vacuum? Is this "recurving" the distributor?

I disconnected vacuum from the distributor and get about 10* of mechanical advance when I rev the engine.(So, my timing went from 12* at idle to 24* at 3000 rpms.) I've read that this is about right, so that says that springs may not be the adjustment I want. Is there a vacuum pot, the device that vacuum advance connects to on the distributor, that could be adjusted to get the amount of advance to give me the 36-38* that I've read about as max timing advance desired?

I've retarded the timing to keep max timing to 36* total timing, but the engine was a dog and was difficult to start and gas mileage sucked. When I set it at 12* it starts easily and accelerates well, but I'm getting about 46* timing advance.

Any advice would be appreciated.
Thanks in advance, Bart

 

I like to run about 42-44 degrees myself, but some cannisters are adjustable or you can buy one from crane cams via summit racing.

 

One of two things for the timing advancing when connected to vacuum. Either you are not connected to ported vacuum, because with the carb set properly there will be no vacuum at idle. The other situation is the carb mechanical throttle stop is set to far open, causing the throttle plates to be to far open allowing the port vacuum port to see the intake vacuum. In either case you need to make some changes.

 

42 deg. btdc sounds like alot, don't know your mtor secs but I only run 38 deg. in my race motor at full advance and I run 110 octane race fuel. Before I changed the dizzy in my truck I left the vaccume advance unhooked because it would cause severe pingging when hooked up.

 

42° vacuum advance is nothing, I was running 50° vacuum + mechanical. The thing is the vacuum advance is only there when the engine is unloaded. At WOT he will only see the mechanical advance. If your vacuum causes pinging chances are you need to get an adjustable vacuum can and back it off. No vacuum advance severely and adversely effect mpg.

 

x2 :)

Alvin in AZ

 

First, Bear I've set the plates so that ported vacuum is 0" at idle and tune the carb this way while I was at it.
Second, at initial setting of 12* BTDC I was getting pinging, so dropped back to 10*. Still pinging, then 8* and pinging has stopped and I have good acceleration and manners.
Third, with manifold vacuum I'm getting 45* BTDC at cruising speed. I've ordered a crane vacuum advance kit so that I can adjust my advance to greatest advance while cruising. Essentially to find the "sweet spot" of my engine for best mpg. The kit comes with springs to recurve the distributor which I may do for exercise and learning. The engine was easier to start when I had 12* initial setting, but pinged. Maybe with recurve I can get that and stop pinging as well move to ported vacuum. I'm using manifold vacuum now because it's giving a better idle quality.

Thanks for the input gentlemen.