I have a 1956 Cad w/factory A/C, that has had a low speed miss for sometime. It has a Petronix unit in the distributor, and the spark plug wires are fairly new, as are the plugs themselves. The vacuum advance unit has been rebuilt by Terrill Machine, and the engine was rebuilt about 20,000 miles ago. I initially thought it was a fuel issue but I have now ruled that out. It misses when it is cold and the choke has the engine at the higher speed until it warms up, as well. The carb was rebuilt about 20 years ago. The generator light is always on at idle, but goes off when the engine speeds up.
Any ideas and thank you ?
Jim Greene
A possible cause for the Gen light to come on is a misadjusted regulator. Could also be that your idle speed is simply set too low. Does it have a tendency to stall? In theory your idle speed could be so low that it starts to miss. But that is in my opinion unlikely.
Is the carb adjusted properly? That's where I would start looking.
First thing I would try is a points distributor just to eliminate anything in that Petronix setup. Each car I have had with one of those I keep a standard distributor in the trunk for emergences. Their good but the components are beyond me should any part of them fail.
20 years ago on the carb rebuild? That is a long time especially with todays gas.
Seems like I remember reading about possible issues with the petronix related to the magnet gap between the rotor thing and the rest of the unit? Its one of those things people often skip because its extra work to get right. I have also read that the timing changes when you install the kit so needs to be reset after the kit.
One other thing I remember about fitting several of these to a friends cars is that Petronix specified a special coil for their system. Not your normal coil?? Just a thought.
On the other side of the coin, I have installed several Pertronix ignitions over the years and have never changed the coil. The coil will only put out what the system needs to fire the plugs --- a super duper powerful coil is overkill and underused for a stock engine.
" --- a super duper powerful coil is overkill and underused for a stock engine."
OK, maybe a slight oversimplification, for everyone's edification and education try this site ---
http://www.auroraelectronics.com/ignition_systems_-_basics_to_high%20performance.htm
and scroll down to read "More Voltage?", "Some Math", and "Misconceptions".
I talked to a mechanic and he said disconnect the vacuum advance
And see if the miss goes away. Disconnected it, and put a piece of
Tape over the vacuum hose and the miss seems to have gone away.
Does this mean that it is strictly relying on the mechanical advance?
I drove it to work today and it seems to be working just fine. What am
I missing here? Also, how much vacuum should be coming out of the
Vacuum advance hose, which feeds from the carburetor?
Thank you for reading this
Jim Greene
Perhaps the vacuum advance diaphragm is defective and that vacuum is being lost through the distributor?
Quote from: Steve Passmore on October 15, 2016, 05:21:12 PM
Perhaps the vacuum advance diaphragm is defective and that vacuum is being lost through the distributor?
Steve,
I think you hit the nail on the head. ;)
Vac should be around 19-21" a good running engine.
Bobby
You probably found that your vacuum advance chamber is leaking
PERTRONIX: there are 2 versions, the better version uses a SPECIAL OHM coil. It also allows the key to be left on and not compromise the modual.
The trigger ring in BOTH versions has 8 magnets glued into it with scotch tape over them. I had 3 separate issues with the magnets FALLING out in the heat of our AZ summers. Ultimately i removed the clear tape...glued magnets in with JB weld and coated the whole assembly with catalyzed automotive finish clear. (Several coats) No problems now
I tried taping off the vacuum advance vacuum hose and the miss basically went away. Then I replace some of the vacuum hose at the back of the engine, and then hooked up the vacuum advance again. The miss came back, but not as bad as before. Now I guess I should try to trace it under the dash. If the miss completely disappears by disconnecting the vacuum advance, wouldn't that mean the pertronix is not the issue.
Thank you all for your responses
Jim Greene
I would say it's not the pertronics, they are generally fool proof about not needing to be touched.
The vacuum is the issue, cap off the hoses that go from engine to vacuum manifold that is on the firewall, that caps it off and leaves the vacuum advance alone and then you will be able to see if it is the vacuum advance or hoses in the rest of system.
Then replace all hoses anyway, it only takes a pin hole to mess it up or a stretched hose connected, or dry cracked hose.
Thank you for the suggestion. I cut the other vacuum sources just leaving the carburetor vacuum to drive the advance, and the miss was still there. Does this mean the carb needs rebuilding? I tried spraying water around the carb base and the intake manifold, and I could not get the idle to change. Should the vacuum out of the carb be 19-21" of vacuum?
Thank you
Jim Greene
If removing the vac. advance hose and taping it off eliminates the miss. The vac. advance unit is faulty.
In order to get that 19-21" vac. reading Bobby gives, the base engine timing must be set to around 10-12 deg advance timing at idle speed. that has been my consistent experience with the 56 engines for over 40 years. They will also run like a bat out of ...... with 89 octane pump gas and get much better mpg.
Good luck. Larry
Quote from: Steve Passmore on October 10, 2016, 10:23:34 AM
One other thing I remember about fitting several of these to a friends cars is that Petronix specified a special coil for their system. Not your normal coil?? Just a thought.
Regular coil and Pertronix on my 58....working perfectly for almost 17 years.
Thank you for all of the replies. I pumped up the vacuum advance with a hand pump, and the vacuum held fine. I can find nowhere in the manuals what the vacuum pressure should be on the line coming out of the base of the carburetor to the vacuum advance. the 19-21 inches is from "booster pump line" coming out of the middle of the intake manifold. I had the vacuum advance redone by the company in Texas. I suppose the best bet here is to just change out the vacuum advance with another one and see what happens.
Thank you
Jim Greene
At idle the vacuum reading to the distributor advance is supposed to be zero. The carburetor butterflies should be completely closed. No vacuum until opening the buttterfly valves to speed up.
I think I may have found two issues on my 1956. The nut that holds the pipe in place on the advance (where the rubber hose goes) was a little loose, so that may have solved part of it. The car has a Pertronix unit on it, but what I was wondering about, is the ground wire from the breaker plate to one of the screws that holds down the vacuum advance? Mine (the original) was loose on both ends. I did a make shift fix, with solder on one end, but is that wire necessary with a Pertronix unit? I ran out of time last night so I haven't tried to start the car.
Thank you
Jim Greene