Dec 1st, 06, 11:28 PM
Busy night..lots of questions. I was installing a new MSD kit in my stock GM HEI. Anyway I was curious when the Vacuum can started advancing (So I hooked up a Vacuum Pump). Anyway when I pumped it up it just sucked air and didn't build Vacuum or move the rod. Anyway there is a small hole on the underside of the can that when I put my finger over and I pump it up the rod moves. Just wondering what is up? The hole looks like it was originally there doesn't look like someone drilled it out. Am I missing something? Car ran fine when I pulled it out for the motor mods.
Dec 2nd, 06, 12:37 AM
The can has a diapham in it that is spring loaded...u have a hole in the diapham...
The hole in the canister is there to equilise the pressure(vac) behind the diaphram.
U need a new one...
just behind the canister, there are some numbers, one of these is the 1nches in vac that the arm moves at. Im not sure what the other number is.
The amount of deg of movement is the position and length of the slot the arm moves in.
Changing this slot at the bottom will change the vac the canister works at
Dec 2nd, 06, 01:24 AM
Thanks for the response....The number is 69120 does that have any meaning. (It is a stock GM can)...I was going to go with a Canister that came on around 8 Inches of Vac, as I am going to a 106 LSA cam, and Vacuum at idle will probabaly be an issue.
Thanks for the help!!
Dec 2nd, 06, 12:55 PM
There are serveral 100 diff Stock vac adv units, combine this with maybe over stock GM 1000 dizzys and one comes up with several 1000 Stock combinations
Source Delco Remy service manual.
I cannot find 69120 in the manual as a vac adv unit part number, they are listed as 111**** for Points and 197**** for HEI.
There are some older threads re curving dizzies/vac advance canisters and how to...They have several methods and ideas behind them, neither is wrong just diff approachs for different applications.
They also describe how to make adjustable (vac and deg range) vac canisters if u have one that is in the ball park.
To establish a curve centrifical and vac is unique to each engine
You will need timing light, in car vac gauge and tach and a lot of time, be it on a dyno or road test.
Dec 3rd, 06, 04:39 PM
Here are the four most common GM HEI distributor vacuum advance units (NAPA/Echlin part numbers);
VC-1838, stamped AR12 - starts @ 7"-9" Hg., 14* in at 10"-12" Hg.
VC-1843, stamped AR15 - starts @ 3"-5" Hg., 15* in at 9"-11" Hg.
VC-1853, stamped AR23 - starts @ 5"-7" Hg., 15* in at 11"-12" Hg.
VC-1862, stamped AR31 - starts @ 2"-4" Hg., 16* in at 6"-8" Hg.
There are about 25 others for the HEI distributor, but these four cover most performance applications - about ten bucks each. :thumbsup:
Dec 3rd, 06, 06:17 PM
Hey JohnZ your awesome writeup mentions the VC-1810
Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.