Vac. Advance

[roger]

I recently replaced dual points in Mallory dizzy with pertronix 2 ignitor on RV8.
I timed ignition at 21 degrees btdc @ 3000rpm with vac. advance disconnected as per set up last year on rolling road.
Can any one tell me why engine runs better without vac. advance connected than with ?
Thanks
roger

[wilf]

Following advice from here and other places, I put a lot of advance onto the RV8 in my old car. It ran like a raped ape as a result. Basically it got mechanically advanced until it pinked, than backed off a touch.

However - I found that the vac advance would make it stumble and miss at small throttle openings and low to medium rpms - obviously it was advancing too far.

I never got into the technicalities of restricting the vac advance - I don't think the POS Mallory dual point pain-in-the ar$e distributor allowed for that anyway, so I just disconnected it. Ran fine after that.

Vac advance is about part throttle economy and emissions, I leave it to you to decide if you care about that or not!

[roger]

Wilf
The symptoms you described are exactly the same as mine.
Perhaps I will only use vac. advance when it is mot time for emissions, and on the Le Mans trip for economy.
Thanks
roger

[Happy Jim]

Roger,

I had the same symptoms with mine and spent ages scratching my head and doing research (never figured it out though) - then took the same route as Wilf :thumb:

Cheers

Jim

[Bowtie]

I remember reading up a lot on this topic and finally plugged it right out of the box. Don't remember why but it did make sense at the time (running good so I will keep it plugged until someone can talk me out of it)...

[roger]

I have just read an interesting article on www.centuryperformance.com/timing
and they recommend ( on street cars)using the full time vac. port for the dizzy advance. They say there is plenty of advance at idle but when throttle is opened load increases and vac. drops. I haven't tried it yet.
Anyone any thoughts on this? - Wilf?
roger

[wilf]

Roger - interesting theory, but there is still a lot of manifold vacuum at low throtle openings - just about maximum at around the 30mph in 3rd or 4th mark, so if you are experiencing rough running as we desribed, I don't think it would help.

Manifold vacuum only really drops significantly with large throttle openings. Try hooking a vac guage up that you can see whilst driving to see what I mean.

[jools]

Guys, I am following this thread with interest. I am having problems running smoothly. My engine ran great originally but since having some work done it is lumpy & prone to backfires. The work was not engine related, however I am sure the vac connections are now hooked up differently. According to Holley they are correct. I am fairly sure that originally the top vac on the carb was sealed off. This is now connected to the dizzy. Is this the vac point you guys are discussing?

[wilf]

Jools - there are often two different vacuum connections on Holley and edlebrock carbs.

One "sees" the inlet manifold vacuum all the time, no matter what the throttle opening, and the other is "timed", which means that it only "sees" vacuum when the throttle is cracked open - i.e. the port is above the butterfly in the carb.
My understanding is that the vac advance probably wants the "permanent" port.
You can easily work out which is which by simple experimentation - which sucks when?

You ought to check the ignition timing on your engine with a good strobe light first (vac disconnected) to establish your starting point as correct or not, and only then start to doubt the vac advance.

I believe in setting the "all in at xrpm" figure correctly, and letting the "static" figure fall where it may. I don't know what engine you have but if it is a Ford small block, 34 degrees at 3000 rpm is a good starting point. If it is a Chevy, then ask someone else, as I don't know (or want to know ), and I have forgotten the Rover figure!

[osgood]

If you have half sensible cam, vacuum advance is not required, in-fact due to the depressions in the inlet manifold will not allow the engine to rev! take it from one that knows Osgood