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Help me understand vacuum and carbs

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drooartz

drooartz

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Okay, keep in mind that I've got a 1275 and carbs from a 1974 Midget in the Tunebug.

I'm trying to understand a bit more about what the vacuum fitting on my distributor does in my situation. Vacuum comes from the manifold in this application (carbs have no fitting).

What is the vacuum providing? Is it advancing or retarding? Is it based on RPM or engine load? Why do you remove and plug the vacuum line when checking the timing? Would you ever check the timing with the vacuum line in place?

I'm trying to improve my understanding of exactly what is going on here so that I can approach my impending tuneup with more confidence.
 
The manifold has vacuum when the butterflies are partially closed. When you are accelerating, there is very little vacuum. The purpose of the vacuum advance is to increase the timing when there is no load on the engine (cruise). This is done to get better fuel mileage by starting the flame front earlier when there is a low volume of fuel/air mixture.

I suggest setting your timing at about 28 degrees at 3500 rpm. Maybe a little less depending on your CR and fuel blend. Set this with the vacuum advance plugged.
 
Thanks Trevor. I appreciate the explanation.
 
Forgive my ignorance on LBC's as I am more of a Chevy guy but we shoot for around 50-55* total maximum advance. That's made up of 12* initial, 17* vac and the balance of around 25* mechanical. If you are looking at 28* at 3500 rpms (made up of initial and mechanical), assuming it's all in by 3500* then it doesn't sound like enough by about 10* +/-. Help me to understand . Bob
 
28* total advance on an A series engine is about perfect.

But Drew, a 74 Midget used a vacuum retard dizzy with a direct line from the intake. this was a pollution control move.
Plug the vacuum line, set your timing and take it for a ride.
I bet it's quicker.
If the dizzy has an offset nipple on the advance pod, it is vacuum retard.
 
28* total advance is pretty close to where I have it right now. Hap has told me that may even be a bit conservative given the engine build (he runs them up to 32* or so) but it's close.

Since pictures speak louder than words, here are both ends of the vacuum line. Distributor was rebuilt by Jeff at Advance, and it was noted on my receipt that the vacuum advance was the "correct" one for my setup.

vac_intake.jpg

<span style="font-style: italic">Intake side</span>

vac_dist.jpg

<span style="font-style: italic">Distributor side</span>
 
That is an advance unit.
 
Totally differnt animal. SBC is going to see less totaL load per RPM at cruise and can handle more timing. Different combustion chamber and piston designs,cam profiles, CR's and fuels have different burn rates and require differnt timing. Don Garlits ran 50 degrees of timing on his Hemi's...and that was blown on nitro methane under full load.

IIRC, air cooled VW's require allot less than A series in static and total timing. Install a turbo or nitrious and all the rules change again ( no matter what it is).

It all depends.

Drew, did he say to hook it to ported or manifold vaccum?
 
kellysguy said:
Drew, did he say to hook it to ported or manifold vaccum?
Click to expand...
That's the magic question no one involved can really remember (distributor rebuild was in early 2008). My receipt just said "correct" vacuum advance. I'm pretty sure I told Jeff that all the parts (including the carbs) were from a '74, and he would know that setup had no ported vacuum available. When I called him, he was confident that everything matched.

I'd still like to know if there is a way to test what I have to make sure that it all is working as it should and is matched as it should be. If for no other reason than so I can learn these details.

receipt.jpg
 
Drew , you can call Jeff and I'm sure he would advise you on how to check the vaccum adavnce, for now, I would just run the engine up to 3000 rpms and set the timing at 30 degrees BTDC, by then all your advance will be in, and that is your real concern, you might can play with a little more than 30, but this is a good starting point.
 
Ported versus un ported is pretty immaterial. If you are happy with the way your car idles, then the set up you have is perfect. Ported vacuum ONLY affects advance at idle. The purpose of ported vacuum advance is to reduce air pollutant production at idle.
 
I had a very nice chat with Jeff at Advance and we determined that I do have the best vacuum advance for my application. Jeff has been great to work with, and gets the big thumbs up. I also now understand more about how these work, which was a big part of my intention -- it's easy to be unconfident about something when it's a big unknown. Now that I understand what the vacuum advance does, I can get about the business of tuning the car.

Thanks all for the thoughts and information. It is appreciated!
 
drooartz said:
I also now understand more about how these work, which was a big part of my intention
Click to expand...

I can exppalin it in a nutshell.


...vacuum SUCKS !!!!! :jester:
 
kellysguy said:
I can exppalin it in a nutshell.

...vacuum SUCKS !!!!! :jester:
Click to expand...
The question, I guess, was *when* does it suck. Now I know.
grin.gif
 
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