Small Block Chrysler Vacuum Advance Question

[Marshall 357]

My friend has a 67 Barracuda with a 273 V8,NEW Holley 4 Barrel Carb and a New Mopar Performance Electronic Ignition Kit with Vacuum Advance Distributor. It ran great last year,now when the vacuum advance is conneted it runs rough and the engine huffs and puffs. Tried to adjust the advance like the book said , but nothing works. Disconnet it and the car runs good just does not have The power it use to. What do you think?

 

[Marshall 357]

My friend has a 67 Barracuda with a 273 V8,NEW Holley 4 Barrel Carb and a New Mopar Performance Electronic Ignition Kit with Vacuum Advance Distributor. It ran great last year,now when the vacuum advance is conneted it runs rough and the engine huffs and puffs. Tried to adjust the advance like the book said , but nothing works. Disconnet it and the car runs good just does not have The power it use to. What do you think?

 

[Spotteddog]

In the distributors spec's, does it list how many degree's of advance the vacuum diaphragm is capable of delivering?
Dependent on how many degree's overall advance the mechanical advance weights are capable of delivering, and at what RPM the weights are sprung to commence advance at, you may be "over timing" when the vacuum advance is in line. Have you checked with an advance timing light at the crank, how many degrees of advance from BOTH the mechanical and vacuum advance's the engine is getting at about 2500-3000 RPM? (I found Mopar A blocks preferred max combined vacuum and mechanical advance of 37-39 degree's)
Last thought. If she's not picking up detonation at her current initial setting and performs well on her current advance rate, you may wish to hook the vacuum advance up to a "metered" vacuum source. AKA, one that is NOT @ manifold vacuum. Look above the sole plate on your Holley quick and dirty, one of the ports higher on the carb's body will be a "ported vacuum" source. Meaning, it'll only get vacuum at above curb idle. Contrary to popular belief, the vacuum advance diaphragm is a fuel economy device, not one for performance. It allows the engine to operate at cruise speeds, with slight throttle opening, with an advanced timing in order to fire the highly lean mixture. Try a vacuum source higher on the carb body.
Good Luck get back to us with how she does!

 

[Marshall 357]

Ok thanks we'll give it a try

 

[sar4937]

It's not coming out of storage with a tank of old gas is it? Assuming nothings been tinkered with gas would be the variable.

 

[Marshall 357]

Oh no, that the bad gas thing again LOL!!!!! I am going through that one in my truck LOL!!!! Wouldn't it run funny with the Vacuum advance disconnected too?

 

[Spotteddog]

Yes it would!

 

[sar4937]

Since the vacuum advance simply advances the timing, nothing more, bad fuel could very well cause the problem. Or you are not being truthful about what been done to the car over winter. Think about it for a second. You park the car in the garage, it's running fine. You get it going in the spring, it's not. Gas does not go bad any one certain way, and ethanol does something different than straight gas. Racing fuel or additive thrown in can make it act differently. So I guess you need to explore what has changed since last fall. Having just retired from the auto repair business after 37 years, I have seen my technicians waste a lot of time on a diagnosis because they are sure it's not this or that. I generally don't buy the bad gas diagnosis on a vehicle driven regularly. And that is a different type of bad usually, stale gas usually looses some of the lighter components because gas is a blend of light and heavy chemicals that boil off and evaporate at different rates. This is why there is no typical bad gas blend. Was a fuel stabilizer added last fall? It could be you fuel will simply not tolerate the extra timing. Or it could be something completely different.

 

[Marshall 357]

Well, its not my car,I wish it was. Its strange to me too! There are some of the things we did. New fuel filter, Shot Gum Out through the carb, Checked timing about 5 time. Added a bottle of Dry-Gas,and carb cleaner to the gas. Checked wires,plugs,cap,rotor.Would the voltage regulator be bad? I am Stumped. If its the Gas wouldn't run funny all the time. Why does it run fine with the vac advance diconnected? There is nothing under the hood but a motor and a few wires. How long does water stay in the tank,before it is all gone? Come on now sounds like you know more about cars then I do LOL!!! Help us out. By the way the engine was rebuilt,about 5 years ago. Thanks

 

[7.62foryou]

Marshall 357 you say it runs ok with the vac adv disconnected. In addition to what spotteddog stated about vac ports on the carb, some of those dist. had a check valve inline on the vac hose to the adv. This delayed vac adv of the dist timing until the engine was at a higher RPM. Doing the same thing Spotteddog stated in a different way.

 

[sar4937]

The vacuum advance should be connected to a ported vacuum fitting, double check the instructions. That means that at idle there will be no advance, and the idle speed should not change when the hose is removed. Total degrees advance is usually controlled by the mechanical advance built under the pick-up plate, the vacuum advance just helps at part throttle and peters out as manifold vacuum falls to zero at W.O.T. Put the hose in your mouth and suck, you should hear the engine speed pickup. If the engine starts to misfire that might help diagnose the problem. Suck the hose and close it off with your tongue, it should hold vacuum. Shut the engine off, remove the cap and rotor. Get a light and look for contact between the pickup coil and the toothed wheel. There used to be an electronic voltage regulator that was required, but not furnished with this conversion. It was required to keep the voltage more constant. Is that there? Is this a Holley 4160, or the AFB Weber model. Do you have a timing light that can check total timing? If so set it around 38 Degrees. If the fuel does not contain lead, it would be easy to siphon it out and rule this out. Does the owner run racing fuel? Do they put lots of go-fast additives in the tank? Can you verify that the balancer ring has not slipped, causing the reset timing to be over advanced? Now I am reaching, but is the charging system functional? Or does the owner put a battery charger on it, by the way that regulator is behind the distributor and used to be blue with a wire coming in the side and a stud on the front of the box with a second wire.

 

[Spotteddog]

It's been long enough now, that my memory of the anatomy of Chrysler's electronic ignition distributor is fuzzy? But, is not the pick up coil mounted to the movable breaker plate that is driven by the vacuum advance linkage? If the inserts that keep the breaker plate in correct alignment are sloppy, it could cause erratic timing. Un-common but not unheard of, are breaks in the wires coming from the pick up coil. BTW, when checking the reluctor to pick up coil clearance it must be done with a brass feeler gauge (.006"-.008" seems to ring a bell?) Is the car being run with the correct rate ballast resistor? Don't forget, there's a different one for breaker point and electronic? Was it changed upon install of the E ignition?

 

[sar4937]

Yes it's as you describe, but I thought it's a new distributor. The pickup coil could be too close and touching the toothed wheel. The Mopar Performance conversion comes with a wiring harness, ballast resistor, control box, and a new distributor. It does not come with the regulator, but it does say that in the directions. I have run across cars with enough rust that the fenderwell that they usually locate the control box on is a poor ground. This can be cured with a ground strap running back to the block, I never could figure why Mopar didn't put a dedicated ground wire in the harness. If the pickup coil needs adjustment make sure to use a brass feeler gauge.

 

[Marshall 357]

Ok that is a lot of info,get back with you when I find out more . Thanks

 

[Marshall 357]

Just wanted to thank everyone for their help . He replaced the ignition system with a MSD ready to run system, and she is ruuning great. Its alot better than the Mopar one,no orange box or resistor and a 3 wire hook up, one to a 12 volt supply and 2 to the coil. We'll say it was something in the ignition system and move on to more important things like buying more guns!!!!. Thanks again. You guys are Awsome!!!!

 

[Spotteddog]

Glad you got a bit of info from it.
Print it up! The company that made it is circling the drain as we speak. It'll be a photo finish as to which assumes room temperature first now, Pontiac motor division or Chrysler?
Take a slow last look back folks. Neither we, nor any other generation will ever see their likes again.
R.I.P. Detroit.
I was very lucky to have known and been involved with you, when you were at your zenith.
Thank You!

 

[Bullet Bob]

And, Spotteddog, I think I can tell from your posts on this thread that they and the car owners were lucky to have known you!

It is true that growing old is not a total picnic, but with Roy Rogers as a pre-teen, GTO's as a teenager, and the good times of the '70's (including military service), I have no regrets. I'm sure everybody feels the same way about their own era, too.