checking vacuum advance
04-09-2009, 02:13 PM
#1
2nd Gear Member
Thread Starter
Join Date: Jul 2007
Location: Laurel Hill
Posts: 226
checking vacuum advance
I've been fighting a low speed hesitation & drivabiity problems with my car lately. Carb is new Holley 570.
Found something today about how to test vacuum advance & tried it.
Car idling 1500 RPM with vacuum line unplugged. Watching timing mark with timing light (was around 20-25 I think). plugged in line to dist. vac. canister. Nothing changed. In fact nothing changes at any RPM when I plug & unplug line the vac line. Shouldn't this be having some type of effect on timing? Now I'm thinking my canister is not working at all & this may be source of my problems.
By the way, initial timing is around 12 & it advances (mechanically?) to about 35 @ 3500RPM.
One more question. Set initial timing in park or drive?
Am I on the right track or in left field?
Found something today about how to test vacuum advance & tried it.
Car idling 1500 RPM with vacuum line unplugged. Watching timing mark with timing light (was around 20-25 I think). plugged in line to dist. vac. canister. Nothing changed. In fact nothing changes at any RPM when I plug & unplug line the vac line. Shouldn't this be having some type of effect on timing? Now I'm thinking my canister is not working at all & this may be source of my problems.
By the way, initial timing is around 12 & it advances (mechanically?) to about 35 @ 3500RPM.
One more question. Set initial timing in park or drive?
Am I on the right track or in left field?
04-09-2009, 03:11 PM
#3
4th Gear Member
Join Date: Oct 2005
Posts: 1,195
I believe your dizzy vacuum is supposed to run to ported vacuum (no vacuum at idle). From what I understand, later models changed to manifold vacuum (vacuum at idle) to meet emissions requirements. If you want to check your vacuum advance, use a vacuum pump (or my ex-wife), and suck on the hose and watch the mechanicals with the cap off or listen to the motor while the car is running. You should see or hear the change.
04-09-2009, 03:24 PM
#4
6th Gear Member
Join Date: Oct 2006
Location: Cork, Ireland
Posts: 6,417
since you have a timing gun:
disconnect the vacuum line from the carb (this will cause lean condition, but that doesn't matter for this quick test).
check timing and when you suck (you can use your mouth) on the hose connected to the distributor you should find the following:
a) when you suck the timing advances
b) when you have vacuum there and block the hose with your tongue then it should not loose the vacuum and the timing should stay the same advanced until you let go your tongue
if either of that is not happening then your vacuum advance is shot.
when it advances with vacuum disconnected as you rev up then this is your mechanical advance.
35 degrees at 3500 rpm is good and 12 initial is ok as well for most engines. try 8 (both with vacuum disconnected)
what was written above ported vs manifold, i learned recently that ported is emmision hookup. for now it should not matter. disconnect, make sure you have 8-12 degrees at idle, then plug that thing back in.
that should throw you in the ballpark. when you just idle and have it connected to ported then there will be _no_ change in ignition timing as ported is picked up above throttle plate so you will have no change.
if you hook it up to manifold vacuum there will be _big_ change as this is hooked up under the throttle plate. when throttle plate is closed you have massive vacuum under it
disconnect the vacuum line from the carb (this will cause lean condition, but that doesn't matter for this quick test).
check timing and when you suck (you can use your mouth) on the hose connected to the distributor you should find the following:
a) when you suck the timing advances
b) when you have vacuum there and block the hose with your tongue then it should not loose the vacuum and the timing should stay the same advanced until you let go your tongue
if either of that is not happening then your vacuum advance is shot.
when it advances with vacuum disconnected as you rev up then this is your mechanical advance.
35 degrees at 3500 rpm is good and 12 initial is ok as well for most engines. try 8 (both with vacuum disconnected)
what was written above ported vs manifold, i learned recently that ported is emmision hookup. for now it should not matter. disconnect, make sure you have 8-12 degrees at idle, then plug that thing back in.
that should throw you in the ballpark. when you just idle and have it connected to ported then there will be _no_ change in ignition timing as ported is picked up above throttle plate so you will have no change.
if you hook it up to manifold vacuum there will be _big_ change as this is hooked up under the throttle plate. when throttle plate is closed you have massive vacuum under it
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