Hi all,

my distributor seems to b e the stock distributor which comes with the 289 engine.
It still has contacts (points).

I don't mind keeping the points, but what happened is that the vacuum advance doesn't work anymore.
It got to a stage that the metal plate which is pulled by vacuum advance fell off and the car wouldn't run anymore.
The point gap could not be adjusted anymore.
So in a quick fix I bolted down this metal plate with 2 screws and the dwell stays (better than ever) in it's range.
(only +- 0.1%) while running in idle.

I blocked the hose that comes from the carb to not pull false air here.

So now i have a distributor that doesn't have a vacuum advance anymore.
What are the downsides of it ? do I really need to get a new distributor ?
what am I missing out ?

Kalli

 

Hey Kalli, I did a quick Google for vacuum advance and came up with this info...

http://www.stoveboltengineco.com/howto/vac.htm

 

thanks for the link!
i just read through it and probably will get a new distri at some stage (probably breakerless).

only thing that I don't really get in the article is:
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The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in "retarded" spark timing and all the aforementioned losses.
================================================== =

When i have a gauge attached to the hose or when I was adjusting the timing with a strobe (on a car with working vacuum advance) I always experienced that when I accellerate sharply the vaccum grows and the ignition is always advanced further. (I know there is the advance per rpm as well with flywheel. I'm not talking about that).
This is totally contrary to what is said above.

kalli

 

This is how I understand it... If you are driving under light load (idling or cruising and not accelerating), there is less fuel in the air/fuel mixture (for economy) consequently, the burn process happens slower. To reach the point where the burn process is complete at the correct time, it must be started earlier (advanced). Without the vacuum advance, the distributor will only be able to advance the spark (using weights in the distributor) based on how fast it is spinning. So at regular speeds (distributor fast, and mechanically advanced), under light load (more air in the mixture) there needs to be the additional 20 +/- degrees of advance that the vacuum provides. When load increases (when accelerating) vacuum drops, which essentially retards timing because there is more fuel in the mixture (for power) so the burn process happens faster.

 

Hi,

that's a very good explanation. it's just that I'm nearly positvie:
"When load increases (when accelerating) vacuum drops"
is _not_ what's happening.

I must have been wrong. Next time I'm with the car I'll plug in a vacuum gauge to test again. if the quote is true it makes all perfect sense.

thanks again !

PS: if you adjust ignition timing with a strobe on a 289 engine, do you leave the vacuum advance plugged in or out ?

kalli

 

I don't think you'll be able to check that vacuum drops _under load_ by using a vacuum gauge unless you take the car for a drive with the gauge attached. Even then, I'm not sure that a standard vacuum gauge would register the difference. At idle or regular cruising speed, the car isn't under load. Driving up a hill or under acceleration are good examples of a car under load. I wish I knew for a fact that this happens, hopefully someone else will pitch in their two cents to verify it. That's the only way that it makes sense for me anyway.

I've always been told that to set timing on any motor, all vacuum lines need to be disconnected and plugged. Set the timing, then connect the vacuum advance and rev the car while watching the timing mark to make sure the vacuum advance is working properly.

 

So I've been told as well.

Thanks very much for your 2cents. Will get a new distributor so for the summer ;-)

Kalli

 

I had problems with my car pinging. I thought that I was going to have to reduce my compression 11:1. On the power tour I talked to Vic Edlebrock (I am running an RPM performer Carb) and he said that there was too much vacuum advance - cap it off. I did and the car ran perfect, so good that I broke the shear pin in my distributor.

 

You probably could have acheived the same end results with backing off your timing at idle. That would have made it so it didn't advance to the point of pinging, but still kept the vacuum advance functioning properly. If your timing at idle was already correct, I think some distributors have adjustable vacuum that you could have played with. Either solution, at least it runs good now!