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Ok, dumb question. The Road Demon carb has two vacuum connections on the passenger side. Probably one is constant and one goes with teh acceleration (as a guess).

Which one does the vacuum advance get hooked to? I currently have it hooked to the one that is to the rear of the carb and the engine runs pretty decent but I haven't had it out on the street yet.

Thanks.
 

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I have a Speed Demon 750 on my rod and the vacuum advance goes on the front nipple. The rear is a constant vacuum. To it I have a vacuum gauge connected as I am currently tuning the engine. The way you can tell for sure is to feel both nipples and see which has vacuum when idling, and which only has it when you rev the engine up. The one that pulls vacuum upon reving is the one for the advance.
 

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an easy way to figure it out is to go to the auto store and but a ten dollar vacuum gauge. I use mine all the time for tuning and **** like that.
 

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Actually, the vacuum source to the distributor should be constant vacuum, dropping on accel. I, like everyone else, through my high school years, thought that it should advance under accel, but, after 10 years of tuning fuel injection by computer programming, I'll tell you this, and it will make perfect sense to you.

Cylinder pressure is highest when the manifold vacuum is lowest. The same is true in reverse.

Ignition timing demand lowers with higher cylinder pressure-MORE CYLINDER PRESSURE, LESS TIMING, LESS CYLINDER PRESSURE, MORE TIMING.

Wonder why they tell you to block the throttle open on compression test? The pressure will be higher, and vacuum lower that way-try it.

Vacuum advance does just that-advances timing with more vacuum, if hooked to venturi port.

WHAT YOU REALLY WANT IS LOWER TIMING WITH LOWER VACUUM, AND THE CONNECTION MUST YIELD THAT RESULT, thus manifold vacuum(full [email protected]).

The total advance curve should be set low, then play around with vacuum advance, shooting for 6-12* of advance change from 20" to 0"-your application will determine what it will take, and will take a little time, but MUCH MORE DRIVEABILTY AND FUEL MILEAGE WILL RESULT.

I learned this fact 10 years back, and opened a whole new world of performance to me then, and it will you, too!!

jp
 

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WOW Madingo, you just freaked me out! It's good to see such deep thinking, if only it would rub-off onto others(self included).
 

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[ January 15, 2002: Message edited by: 1BAD80 ]</p>
 

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The time port on any carb has VERY low vacuume on idle and increases with speed, Thats the one the distributer is hooked to. If its on a full port the timing will be advanced at the wrong time.
Pull the line when its running the idle will change also,give it some gas and there will be a increase in vacuum,you can feel it with your finger, THATS the one you want.
 

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It's not really deep thinking, just learned fact, based on basic research, and ironically, from programming fuel injection systems.

There are two types of carb vacuum ports, regardless of the name. Forgive the names I may give these, but you will get the idea:

Ported vacuum: vacuum from below the throttle plates, is high when low load, low, when high load.

Venturi vacuum: As throttle is opened, and air velocity around the venturi and plates is increased, creates a vacuum signal-for 10 years or so, now, the only use I have found for this one is for evaporative emissions, or possibly an egr valve(though it needs to be carefully tweaked).
EGR opration is desired under moderate accel, or light load cruising, but not one extreme or the other, and not too far open, either.

The vacuum control on timing does make sense to ya'll, right? Just checking to see who's awake.
 

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Lol, madingo, I was about to jump all over you . Then you mentioned ported vacuum in the throttle blade. I got in a big argument over at chevy talk. Some guy named ignitionman doesn't catch on that there's two kinds of ported vac. I did a bunch of reading on theory and guess here is how it goes.

Venturi vacuums works as such. Carburetors are dependent on one phenomenon, the venturi effect. Looks at the barellels in your carb. See how they are big, then pinch in and get big again. What happens is that in order for air to squeeze through that tight spot, it has to speed up. This also creates a low pressure area, which means higher vacuum. Thats why when you crack the throttle, more fuel is pulled in. The manifold vacuum may be low, but all that air is getting stuffed down the carb that it makes alot of vacuum in the venturi which meters more fuel. If you have a vac port feeding from there, it won't be good for vac advance.

Full manifold vacuum is good for the reasons explain.

Ported vacuum that feeds from the throttle body of the carb is the best kind. It reacts just like full manifold vacuum, except at idle. The throttle blade just barely covers the port if feeds from at idle. When you crack the throttle, the blades uncover the port to vacuum from the manifold and your vacuum advance responds accordingly. So you have vac advance that behaves properly and won't advance the crap out of your timing at idle or very low throttle. Or something like that, <img src="graemlins/mwink.gif" border="0" alt="[mwink]" /> <img src="graemlins/spank.gif" border="0" alt="[spank]" />
 

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WHat the hell I just looked at my vaccum advance and it his hooked up to the thermostat housing then the thermostat housing is hooked up to the carb stupid 80s!!!!!
 

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Johnboys explanation is correct but for simplicity's sake there are no carbs I know of that have external porting for venturi vacuum. Basically that means two kinds of vacuum ports available...ported and non-ported (manifold vacuum).

There are some that feel that you can use either one but you cannot make an advance curve change accurately with light loads with non-ported vacuum (manifold), it's an all or nothing affair unless you balance the two against each other as some manufacturers did in the 70's with dual diaphram advance canisters.

Vacuum advance is connected to ported vacuum, recurve your distributor or limit total vacuum advance if you experience pinging with your optimum mechanical advance curve.
 

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capn is correct in that the front nipple is ported for timeing and this is what the instruction video that came with my demon 750 also said, so you should hook your vacumn advance to the front nipple.
 

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Lol, like I said, I'm not claiming certainty. I guess I looked all it up when people were bashing ported vac and using the venturi reason as why. Well its not in the venturi, so why bring that up. Actually I stumbled upon this 'cause I was looking up stuff on demon carbs. I think I'm gonna get one. Oh and stonedchihuahua, they hooked it up to the thermostat housing because there's a thermal switch in there. I think it makes it run on ported vac when cold to keep emissions because the choke is on and it switches to full after it warms up. Or maybe its the other way around, I dunno. I didn't mean to start any arguments, just thought I'd at least put to use what I spent a bunch of time reading up on.
 

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Hey Johnboy! this isn't arguing. Just normal civilized discussion, except for the shooting spitballs.

Dave stop that. :p
 
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