Vacuum Advance sticking?

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Longgone

Longgone

John/68 Barracuda & Dart
Joined
Jul 12, 2005
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What would cause the vacuum advance to stick when the engine is revved? This is a Mopar Performance electronic distributor and if you rev the engine over about 2300 rpm the engine will come back to approx. 2000 rpm and stay there until you disconnect the vacuum line from the advance. You can plug the vacuum lines and the car will return right back to the proper idle speed. I`ve noticed one of the springs(the larger ,silver one) on the advance is very loose. Any help out there?
 
Longgone,
The spring that your talking about is normal, sounds to me like you have a vacume problem at the carb, where the line going to the dist. has vacume on it all the time and not timed vacume as it should be. What kind of carb are you useing? and where do you have the vacume line connected? If it's a Holley carb the port just above the set screw on the pass side of the carb is where the line gets connected, this port is timed. If it's a Edlebrock, or carter then you'll have to look at a diagram for where they hook it to. Depending on the carb it will be different. Hope this helps.


BJR Racing
 
Are you confident it's plugged onto the right port? There are a few nipples...one is ported vaccum, that is, no vaccum present at idle, but as the throttle is openned, the vaccum is present..It's only meant to be "on" at cruise, not idle. There are usually one or two other nipples that are manifold vaccum, which has vaccum present whenever the engine is running, regardless of load or rpm. If the hose is on the right one, I'd say you have the trottle plates open too far. Possibly a vaccum leak, or low initial timing and/or poor idle mixture. It is common for some GM centrificals to stick, but because the mopars' is way down in the "well" of the dist, I'd say it's more likely your throttle is open to the point it's uncovering the transfer slot and vaccum advance passage over the throttle plate. Unplugging it removes the source of vaccum, and the timing goes back to base, the centrifical is closed properly at that point. Slow down the idle (back off the speed screw) until you cant feel any vaccum present, then reset initial timing to 10* at least. If it cant idle, then you may have another issue.
 
Thanks you guys for the input!!These carbs are 3 x 2 Holley 2300`s. After going over all the vacuum line plumbing to see that everything was in the correct port and checking for leaks, I took to the timing light.I had about 3* BTDC base timing, the car likes more,the book says TDC, but the compression is near 11-1 and with octane boost and premium gas the engine will ping if I advance it too much. I went ahead and bumped the timing to 10 before and backed off the idle.The engine would rev and come right back down to idle. I think Moper nailed it with the vacuum passage being uncovered at the throttle plates.Weird thing is this is basically a stock 440-6 car with a stage one port on the heads,284/484 cam and dome pistons. Why does the timing have to be so far advanced to get everything in sync?Factory spec was TDC.Oh, and now when I put the breather back on the engine ,speed increases about 200 rpm. Is that richenening the fuel mixture now that I`ve advanced the timing so far BTDC.Spark knock may become an issue as well,I don`t want to buy my fuel from the airport.
 
Domed pistons and a lot of overlap in the cam make it want a lot more initial timing than factory. What you need to do is re-curve the distributor so you can run a lot of initial, but not a lot of total advance. It may still ping, the domed pistons will hurt you as far as ping goes. you may be too lean at idle too, which would explain the rise in idle speed..Also, make sure you're not hitting anything linkage wise with the air cleaner (I'm assuming that's what you are referring to as a breather).
 
The car came with a nice set of TRW forged pistons that were in great shape, so I just couldn`t see the point in replacing them.Now I`m thinking maybe I should have. I`ve read somewhere that you can remove one of the advance springs to get a faster initial advance. Is this true? This weekend I`ll get a chance to tweak the idle mixture. I thought maybe that was the case.Linkage and all is fine,(no clearance issues),that was one of the first things I checked. Do you know what is average idle mixture setting on these Holley carbs? I`m at 1.5 turns out? I`ve tried to set them with a vacuum gauge and tach but a good base point would be helpful. Breather= air cleaner, southern slang!I thought everyone used that term.Thanks again for your help.
 
Longgone,
The way that you described would be the best way, but there isn't any set starting point other than to make the engine run. The way that I do it is to run the air bleed in until the engine drops rpms and then back it out until the engine reaches peak vacume, then go to the other side and repeat the process. You may have to do the bleeds another time just to make a real fine tune adjustment. This will put the engine running at it's leanest mixture, if you want power then do the process the same but instead of stopping at the where it reaches the highest point keep going until the vacume falls from backing the air bleed out then go back in until it reaches the highest vacume, this puts the engine running on the rich side but at peak vacume. So you have a lean and a rich peak, and the engine will respond better but so will the gas hand along with your foot.
Every engine is different unless it's a new engine as the rings and valve jobs will vary in sealing and this is what your tuning to to get the best performance out of your given engine. Then if the engine still runs lean then you'll have to do some jetting. Make sure the air bleeds in the top of the carb aren't stopped up or the readings will be wrong, trying to compensate one for the other. There are 4 in each carb under the choke flap the 2 that are on the out side is the idle circut and the 2 that are on the inside are the power circut. If these clog up or get dirty the carb will run rich and flood making you think that the floats are bad or the needle and seat is bad and it's neither. I've recieved so many carbs from people that don't know and claim that holleys are junk, then I go in and fix them and use them. Just some info that might come in handy.


BJR Racing
 
Hopefully I`ll get a chance to dial it in some this weekend, today was a washout and this thing called work keeps me from doing the things I`d like to do.You know anyone with the money can pick parts from a catalog and build an engine, but it`s the guys (like yourselves) that can fine tune the engines to get the most from them that seperate the men from the boys. I hope that if others have similar problems with their Mopars, that they can use these threads to help resolve those problems.Thanks again- Longgone
 
i had to file the pick up plates on my mp distributor because the roll pin for the reluctor was catching it.
 
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