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Vacuum advance issue
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Dale Davies
Well-Known Member
Dale Davies
Well-Known Member
Are you connecting the vaccuum hose to a manifold or ported vaccuum port on the carb? Does the RPM go up when you connect the hose? If you are using the ported vaccuum port, there is no vaccuum present at idle, but as soon as the throttle is opened you get vaccuum.
With manifold vaccuum, the idle will pick up as you put the hose on. Adjust idle speed with the vaccuum advance connected. If you check timing with the vaccuum connected it may be 30° at idle. Light load and cylinder density is low. This is OK. Do not change from vaccuum disconnected timing. As you open the throttle the vaccuum advance will back off and centrifugal will come into play.
If you have connected to the ported vaccuum, you are getting vaccuum advance added to centrifugal and a dense mixture in the cylinder with the throttle opened. Way too much advance. A vaccuum gauge on the port you are using will also clear up whether you are on the ported or manifold port.
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Last of the Fast
Active Member
Last of the Fast
Active Member
I have a 1970 Duster 340 that needs 100 octane gas (stock). In order to make it run on 91 octane (California), I disabled the MECHANICAL
Last of the Fast
Active Member
Last of the Fast
Active Member
I have a 1970 Duster 340 that needs 100 octane gas (stock). In order to make it run on 91 octane (California), I disabled the MECHANICAL advance and run initial at around 10 degrees, and then tailor the vacuum advance to yield the most advance without pinging. I outran a 1969 Hemi Road Runner with this car, so does it work? Ohhh you betcha!
Dale Davies
Well-Known Member
Yes it can work. The intended function of vacuum advance is to improve fuel economy under light load cruise conditions. Mechanical advance is to accomodate the fuel burn rate and engine RPM to attain max cylinder pressure at somewhere around 15° ATDC. Your cam and compression will dictate initial timing. Mechanical advance can be tuned for max advance and how fast it comes in.
Dan Georgino
Well-Known Member
Don't plug the vacuum advance. Total with vacuum advance is 34-36. Mine is idle 12-36 max with vacuum advance connected.
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Dan Georgino
Well-Known Member
Remember, if you changed your carb gasket, and used an open gasket, your idle vacuum port will be open to vacuum at idle if you have an open gasket. You want ported vacuum, not manifold vacuum. Let me know if you don't understand, I can send a picture of the different carb gasket.
Dale Davies
Well-Known Member
Initial and centrifugal should be 30° to 40° with the vacuum advance hose disconnected and plugged. At light load cruise with the vacuum connected the advance could be 50°. Remember many of the old engines only had initial and centrifugal advance.
The vacuum port used and advance pot must match.
It's really a very easy thing to do, yet people act like it's frikkin brain surgery. Just find you a copy of the Mopar Performance engine book and you'll have step by step instructions at your fingertips. They're still out there, you have to look. The latest is the ninth edition. Tells about taping a vacuum gauge to the windshield to find cruise vacuum, how to adjust, how to curve the distributor. It's really very simple. I've preached for YEARS on this forum that the first thing you should get is the MP engine manual but nobody ever listens. If you cannot find one, find a friend or member here with one and see if they will scan that section for you. It's VERY EASY to see how to do this when you can read it in black and white from the Mopar manual.
Dan Georgino
Well-Known Member
It's a 360, they all have vacuum advance, not an old engine, it replaced the 340. I'm going by what he said, and that was it's pinging, so he is probably getting way past the 34 with the vacuum advance, so he needs to be all in with the vacuum advance hooked up. He also probably has it hooked up to the wrong vacuum port, or wrong gasket on the carb. He could have also put to light of springs for the centrifugal weights and the come on way to quick. He could also stick a Allen wrench into the snorkel of the vacuum advance and have it come on later. Many possibilities.
Dan Georgino
Well-Known Member
Then it's no fun on here. If everyone did what you said, then I may as well retire.
Dale Davies
Well-Known Member
At light load cruise, you may have more than 34° advance without detonation or ping. Light load cruise has a less dense mixture which burns slower. This requires more lead for economy.
As the throttle is stepped on or opened, the manifold vacuum drops and the vacuum advance drops off. The cylinder density has increased, putting more air and fuel molecules in close proximity for the chemical process known as combustion to take place. This increases the rate of that reaction or burn.
Remember that the intent of tuning centrifugal advance and vacuum advance is to develop maximum cylinder pressure about 10° to 15° ATDC.
From reading the OP, my opinion is the vacuum line is connected to a ported vacuum source. I come by this by the statement that as soon as the throttle is stepped on it starts detonating like a woodpecker on crack. The rest of discussion about weight springs and adjusting the vacuum advance (if it is adjustable) is all a matter of tuning how each reacts.
Dan Georgino
Well-Known Member
Exactly, you proved my point. Vacuum advance is not used in racing, if racing, then full 36°, for street then include vacuum advance, and go full 34° with advance on. Ported vacuum has no vacuum at idle, and if you gradually press gas, it will have vacuum, but step hard on gas, you also don't have vacuum. And yes, all Dodge vacuum advance has an adjustable flow.
Dale Davies
Well-Known Member
Some do and some do not. If it works for DV, it is definately worth checking into.
Like him or not. I learned alot about ignition and carburetor tuning from Ehrenberg's tech in Mopar Action over the years. The guy knows his stuff.
I find myself referring to the Direct Connection manuals and his articles from time to time rehashing stuff.
Oh he's had his moments. I've never been crazy about him. It's that brash, yankee bullshit I cannot stand, mostly.....but every once in a while he's just SLAP WRONG a sin case and point the FMJ spindles on the A body. They flat out WORK and he won't admit that. That said, I do agree with you. He certainly has his strong points.
Like him or not. I learned alot about ignition and carburetor tuning from Ehrenberg's tech in Mopar Action over the years. The guy knows his stuff.
I find myself referring to the Direct Connection manuals and his articles from time to time rehashing stuff.
Then again, none of us is right about everything.
Dan Georgino
Well-Known Member
Yes, some use it, but it usually is not used if your foot is to the ground. Drag racing doesn't use it. Vizard uses it with manifold vacuum, not ported vacuum, so there is a different there also.
Dan Georgino
Well-Known Member
He uses it with full intake manifold. Which means he has vacuum at idle, it's not ported vacuum.
Dan Georgino
Well-Known Member
Uncle Tony's Garage on YouTube is my go to guy! Don't get me wrong, there are many others out there, even HotRod magazine had a good article on it. Mopar is not hard to figure out, it's not rocket science here.
Like him or not. I learned alot about ignition and carburetor tuning from Ehrenberg's tech in Mopar Action over the years. The guy knows his stuff.
I find myself referring to the Direct Connection manuals and his articles from time to time rehashing stuff.
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Two very good articles.
Mopar Action April 2000 issue: " A spark in time" Ehrenberg
Mopar Tech Special 1996: Zap, Crackle, Pop.
This! and the big red X you gave me for the articles I was referring too tells me you have no idea what you're doing. lol
Keep hooking your **** to full vacuum at idle and continue with poor fuel mileage and performance. lol
I bet your car runs like a dog, just like UTG's. lol
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