Vacuum advance problem

[Paul W]

I've been checking the timing and advance on my engine so removed the vacuum advance pipe to get the basic setup. When I reconnect the pipe it looks like I have a permanant + 10-15 degrees all the time ontop of the mech advance. When I first fitted the dizzy it was advance on idle and then gave no advance when you rev the engine. I'm wondering if the spring on the diaphram has weakend off over the last 6 months - its a mallory electronic dizzy. When you accelerate hard it seems to pull ok but is spitting back sometimes through the carb on startup. I'm sure that its connected on the correct vacum line on the carb as it was working fine i.e. vacuum advance at idle or part throttle / cruising along. The carb is an edelbrock performer 650 (it has webber stamped into the casting) and i only have the vacuum pipe to the dizzy - all other ports blocked off. I've checked the lines are clear and the diaphram seems to operate when sucking on the pipe ! Could the diaphram spring have weakend ? Any help appreciated.

[Ant]

So it used to be fine now its gone off some?

Deffo no other vac leaks? the one to use i think is the little vac connection on the carb not the big one (used for power brakes i think).


Do you have the full Edlebrock top end?

Friend of mine just got his rebuilt dutton with a crossflow through MOT this week, he asked mechanics to tune the carb (for cash ) it passed got it home. started slputtering on start up and idle. the vac hose was disconnected and the dizzy lose!!!! runs like a dream now. Severe case of mechanics arm

[Paul W]

Yes was ok and isn't anymore. I have removed a vac line that used to power my brake servos - plugged the hole back up. using the same smal vac line as it always has and I only have edelbrock performer heads and inlet manifold - not the rpm variety. My cam isn't particularly wild either as I wanted a smooth running street motor. I have fiddled with the idle mixture which may affect things a bit - but watching the timing light this seems way off to me.

[Jim S.]

Paul,

I hope I don't come across like I hate Vacuum advance, but the only thing you really gain from running it is a little bit of fuel economy at cruise. Other than that on one of these cars it doesn't really make since, it really doesn't. If you run it you usually have too much advance at idle and when you tip into the throttle if it's not enough throttle the vacuum won't drop then you end up with spark knock. So it doesn't make since at idle really, so the next thing is part throttle cruising to gain fuel economy and that usually isn't enough gain on this type of car to warrant the hassle.

The best setup for race/street car is to set your initial advance for what you need at idle then setup your springs (Centrifical Advance) to give you a good quick advance curve. Personally I like to have it all in by 3000 to 3500 rpm if the engine will take it with out pre-ignition or spark knock.

If it wasn't a race car, but a car like an old Galaxie or something I would say maybe go ahead and mess with it, but for this type of application it just doesn't make since.

Good luck with what ever you decide to do.

[Paul W]

Just thought I'd follow this thread up for anybody else who suffers the same problem. The canister on my distributor spring has maybe weakend or bed in and giving too much vac advance. Anyway after a great deal of searching online I found out that you can adjust the amount of advance by turning an allen headed bolt inside the canister - just pop the allen key in where the vacuum pipe connects to the carb and turn, simple when you know how !

[John]

Most 4 barrel carbs have two types of vacuum, manifold and ported. Manifold vacuum is what you have in the intake manifold and is high at idle. Ported vacuum is only available when the throttle is opened and the engine is under low or no load at an RPM somewhere above idle. Ported vacuum it taken at the venturi, and is the one usually used to connect to the distributor vacuum canister. It sounds like you may have your distributor hooked up to the wrong vacuum port. Usually the manifold vacuum port is located below the throttle plates and the ported vacuum signal is above the throttle plate. HTH, John

[Paul W]

John, My car is now running great - I've used the manifold port as I only want the advance at idle or part throttle, which is as you say when there is most intake vacuum. I was only following up this thread for others benefit in the future.