There is a small vacuum port on the progressive, but left alone it
produces way too little vacuum to be of any help. There is a fix however. The way we
did it was this:
Method 1) with the stock car or FI system read vacuum at the stock port with your
mini-mightymax vacuum pump and tester. Take this reading at the RPM where your
distributor is at max advance (on the TURD it was at 3100 RPM). Now install the carb
and read the vacuum at the same RPM. Enlarge the port using a selection of long and really
tiny bits until the readings match the stock set-up.
Method 2) Determine what RPM you want full advance, this may not be as easy as it seems.
Then hook-up the pump to the diaphragm and start pumping until the dizzy is at full
advance. Then start enlarging the port on the carb until you get the right vacuum at
your revs.
Some notes:
Drilling the vacuum port voids any thought of a warranty on your carb.
The port is at the point of tightest restriction in the carb throat. It can be tough
drilling and not damaging the carb. To do so, you will need to remove the brass
vacuum port fitting and attack from the outside using a selection of jetting bits. Be
really sure to clean shavings after every drill.
The vacuum level may rise quickly depending on how well your engine breaths. Once
you set your vacuum level, you really can't do it for a larger engine if you want to use
the carb on a different motor.
Be sure you're done with improvements to your motor before you do this. Better
breathing motors cause larger vacuums at lower RPMs, your dizzy may start to advance too
quickly if you did the carb before you install 1.25:1 rockers, bigger cam, larger jugs,
etc.
Take your time. It took us 3 days to get mine just right. |
