posted 09-27-2000 11:26 AM               

another question about vacuum advance...my rebuilt 351c is running fine and timed to 6 btdc...if I then hook up the vacuum advance my ignition timing go thru the roof (>35 degrees)...I was told to just leave it unhooked and plug the line as it doen't do much anyway but I was also told it could affect my mileage and emissions (we have to pass a yearly test in Vancouver)...this is the second rebuilt distributor I have tried as the first one did the same thing...it is a single diaphram Autolite, single points...I did the Petronix thing once but it crapped out on me after a year and stranded me in a dangerous traffice situation...can I adjust the vacuum advance on the autolight or should I get an adjustable unit? any help is greatly appreciated as this is one of those things that has been nagging me for weeks now...ps the original distributor was traded in for the core charge....thanks in advance (pun intended)

posted 09-27-2000 04:39 PM               

my vacuum advance is hooked into the carb...under acceleration it is supposed to advance my timing but as mentioned it does so (in a big way) at idle...I don't seem to have any problem off the line so I am assuming my mechanical advance is soft in that it is coming in early...does this sound right??

posted 09-27-2000 04:53 PM               

If you remove the plate that the points attach to, you'll see the mechanical advance weights and springs. These are attached to a peice that includes the cam that opens and closes your points. There's a clip in the top of the cam that holds this unit onto the distributor's shaft. If you take this off the shaft remove it, and replace it 180 degrees out, you'll probably be on a shorter advance curve. (your firing order will be 180 out too, so either move the wires around on the cap or remove the distributor and replace it 180 out) There's a slot on each side of this part that control how much the mechanical advance can travel. My guess is that your mechanical advance is advancing about 30 degrees all by itself and that it is on the long slot. The shorter slot will limit the mechanical advance and allow you to add some vac. advance. Check it out Our "rebuilt" distributor advances about 34 degrees mechanically (without vac connected) We run it mechanical only and are not worried about a smog test. Good luck, and I hope I explained it good enough

posted 09-27-2000 05:02 PM               

i will try it this weekend and let you know...can I actually see the two slots...this would allow me to determine if the slots are wrong and need to be changed...do you feel the lack of vacuum will affect a smog test???

posted 09-27-2000 05:09 PM               

I don't know how they test in your area, but they probably want it to be as it came from the factory. You might get away with plugging up the vac. hose and connecting it for looks Most of the performance guys run straight mechanical advance. Vacuum advance has it's place on a street car for sensing subtle changes in engine load (like going up a hill) while the mechanical advance only responds to engine rpm. For drag racing we don't encounter too many hills or part throttle driving for that matter. Good luck, I'll be racing this weekend but the rest of the gang will be here for you.

posted 09-27-2000 08:25 PM               

When the throttle plates open, the vacuum from the intake enters the carb and vacuum advance is triggered. When they close the intake is almost sealed off and vacuum advance shuts off.

You can see that if you have it backwards you'll have lots of advance at idle and it'll back off with throttle opening.

If you have a Holley hook it to the metering block vacuum source.

BTW, you can usually adjust vacuum advances by inserting an allen wrench through the vacuum port, but it should already be calibrated for your application.

posted 09-28-2000 01:00 PM               

It doesn't sound like an adjustment problem to me, its way too far off. You should have little or no vacuum at the hose at idle.

If you're sure you have the hose hooked up right, I guess its possible that a big cam or something is causing your carb to have to have the plates open pretty wide at idle. In that case you'll probably need mechanical advance.

I have vacuum advance on my 302 with a B303 cam & Holley 600 DP with no problems.

Anyway, good luck!

posted 09-28-2000 05:18 PM               

in Vancouver we have yearly emissions tests where they put your car on a "dyno" and stick a probe up its tailpipe...I only have to meet the emissions standards which were in force in 1971 and have previously passed with flying colours...I want to put a 4V carb and intake on soon and thus want everything to work as good as I can...I have a Summit K5201 cam (not too big 218/228) and Crane rockers...for some reason I am getting a lot of vacuum at idle...any more thoughts....