Flathead vacuum advance question???

How much vacuum does it take to fully advance a 8ba distributor? I am at 7 is that enough? That is at the carb at 2500 rpm's...Thanks in advance

 

Coupla things...
1. At low throttle openings, this works much like the vac advance on a more typical distributor.
2. At full throttle, ported vac becomes NOTHING, and it functions from the very small amount of depression from the venturi signal. This gives the full throttle advance curve, and frequently does not work well at all.
3. Testing the thing by revving it up in neutral tells you NOTHING AT ALL about your curve, though it does show the advance is capable of moving under ported vac. Revving it up without the load of moving the car will give entirely different vac results than running on the road under light load, under heavy load, etc. The thing responds to LOAD basically at part throttle, and airflow at full throttle.
4. There are procedures for testing on a distributor machine at specific vac levels, but again the specified vac levels may not match what your car is producing on the road.

If you really want to find out what it is doing...you may not like what you find...I think only realistic way is to run the car at a bunch of different load and throttle conditions woth a sensitive vac gauge teed into the carb to distrib line (NOT manifold vac) and record the readings.
Park it, run at idle, and pump up the recorded vac levels on the SAME vac gauge with a Redi Vac type pump, line disconnected from carb end, and see how many degrees advance a timing light reveals. You will also need to measure your pulley and draw in more degree marks on pulley.

 

What is the rule of thumb for " measure your pulley and draw in more degree marks on pulley". Since they don't make timing tape. It is a stock 8ba pulley

I was measuring vacuum in neutral and from what i read most say it is cut in half under a load..

 

Measure diameter of flange that has the timing mark on it, compute cir***ference and from that length of a degree (or 5 degrees) there. 5 degree marks out to about 30 will give you plenty of swing and close enough readings to estimate in between readings.
These things typically work well at part throttle but very frequently do not give adequate full throttle advance.

 

I was measuring vacuum in neutral and from what i read most say it is cut in half under a load..

Not really related.
Idle vacuum is highest with no overlap, high compression, dual-plane manifold.
Full throttle is far more affected by how much carburetor area you have.
A small cam 9-1 motor may have 17-19" vacuum at idle, and show 2" WOT with a big 4 bbl.
A big cam 6-1 motor may only show 10" at idle, and 6" WOT if the carb is too small.

Important: vacuum advance must come off the moment the engine pings, slightly before is best for safety. A heavy truck pulling a hill with 3.08 gears needs a completely different setting than a 2,000 lb. T with 4.56 with the same engine.
Many engines need 15-20° vacuum on top of their entire mechanical + initial setting.
How to tell when you have too much? The engine surges and behaves like it's lean, throttle movement has little effect.

 

Ok, engine does not surge, it's a truck that is light with 411 gears, above stock compression < not a stroked engine < heads milled < standard bore... Fenton 2 pot intake.. Either way I just ordered a mech advance distributor.. The cam just so people know is .365 lift and 255 dur...

The reason I asked this question is because my plugs are getting fouled out, yes i understand flatties are supposed to run rich at idle to cool off the valves << am i correct? Yes, i checked the plugs running at high rpm's and shutting it down and pulling a plug and it was rich < so i know that has nothing to do with the idle mixture screws.. So it's either jets or not getting full advance.

 

There is an EXCELLENT chance that you have no or very little advance at full throttle, even if advance works well at part throttle. Try this: Measure off about 20 degrees on your pulley. With engine warm and ready to go, disconnect vac line so you don't get over advanced at part T...set your now locked timing at 20-22 and go for a drive using lots of pedal. See if plugs look better...do note that you may under this test setup be running a little retarded at PART throttle now, so read a plug after a long hard burst up a hill or something. Plugs run retarded mimic over rich look.