Hot Rods SBC, points ignition, loses spark

Hi folks
I'm stumped on my hot rod engine. Its a sbc with a '57 vette dual point distributor. It will cold start and run for 20-30 seconds then cut out dead. Will then start and run for a few seconds then die again. No sputtering or coughing, it just stops dead. I've tried four different condensers, thee coils, a second resistor. Cleaned and checked points, tried a different secondary coil wire, tested the primary negative wire for continuity. I still get the same results.
I've byp***ed the starter switch, and byp***ed the resistor. No change.
I put a fourth condenser on yesterday and it ran for a few minutes and i shut it down thinking i may have solved it.
I started it today and it shut dead off again.
If i pull the secondary wire off of the dizzy and arc it to the intake, it will spark well for a bit then stop, then start sparking again.
I have an electric tach on the car that went haywire last year. Thinking tha t that could be the problem, i removed the tach lead from the coil before i tried the last condenser.
I put the dual point system on last year when i put the tri-power carb set-up on, with great results , till this summer. Car has been down for 2 months and I'm about out of ideas.
What am i missing?

 

Are the secondary carbs closing completely at idle? Check it with the accelerator linkage disconnected. What your fuel pump(s) and and filter situation? What's fuel pressure and volume look like? Maybe take the secondary carbs off, plug their fuel lines and block off the intake flanges. See if it still misbehaves with just the primary carb. Have you got another distributor to swap in?

 

Carbs are in good order, the secondaries seal and are sorted out.
I took the primary air horn off and fuel bowl level looks good.
I have 2 extra Mallorys to try but i'm wanting to figure the Delco out. I took it out and checked the advance mechanism, it SEEMS fine and the bushing seems fine.

 

You might try putting a volt meter on the positive coil terminal then start it. When it dies do you still have 12v to the coil? I know you said you byp***ed the starter switch but not sure what you actually did there. I also seem to remember a symptom like this where the negative pig tail inside the distributor had an intermittent short from chaffing.

 

Tomcat, i ran a wire from a 12 v terminal on the firewall and byp***ed the ignition switch and ballast resistor and it still started and died.
I removed the negative distributor wire to the points and inspected it for chaffing and worked it around and did continuity test and the wire tests fine after a couple times testing it

 

I would put a single point in, An go from there...
Narrow it down.. Stays running...
Dissect the dual point...

 

Not a fan of trying to diagnose on line since there are so many possibilities.
It sounds like a connection is getting hot and opening up. Have you changed the wire from the coil to the dist? Is the point plate losing its ground (common Model A problem w/modern points)?

 

Modern condensers are junk. Battled this with my antique boat motors and tractors. Can you score a used condenser? From the 90's or before?

 

The only thing you didn't change was the distributor. Put in a different known good distributor and try again. Obviously all the things you changed didn't fix it so it's not those.

 

Sleeper: i have not changed it, but i have removed it, inspected it and checked continuity a couple times. I may try another wire though as you say, too rule it out.
I think the plate is well grounded to the dist. housing.

 

100% agree... find a used blue streak..

 

Get a voltmeter and diagnose the system. If it quits sparking when you scratch the primary wire across the intake, the problem isn't in the distributor. Either the coil is bad or it is losing power. A simple voltmeter check will confirm that. Or you could keep up the swaptronics.

 

I second 29 sleeper. do the wire harness,junction block,ign.switch,distributor wiggle test while running

 

I have been using a voltmeter, coils all check good. It sparks continuosly against the intake, then stops for a few moments then starts again.

 

It only runs for 30 seconds at most.

 

So i removed the old neg. lead to the points off the coil, and put a complete new wire on there to the points. It started and ran for a few minutes and i shut it down, then started it again and it ran again for a few minutes. Tomorrow,if its not raining again I'll get it out and test it better.
The old wire seemed to be a bit hard where it goes through the dizzy housing. It has a rubber grommet there. I wonder if the copper is broken down there and just working intermittently ?
Thanks to everyone that offered their help! I'll keep you posted on tomorrow's testing!

 

I think you are on the right track now.
I believe there is also a ground wire that goes from the breaker plate (the part under the points) to the distributor housing that can also get broken.

 

Hi Gene!
I hope so.
The breaker plate is screwed to the distributor housing in three places grounding it out there. I didn't see any other wires underneath. I hope it stays running tomorrow. I couldn't get the old wire to fail with the volt meter and i couldn't keep it running long enough to jiggle to a failure. Lol
Thanks for checking in Gene.

 

Does the voltage at the coil stay constant when it stops sparking? What is the reading?

 

Another vote on yesterday’s suggestion on checking the ground wire for the breaker plate.

 

I chased an intermittent ignition shutting off issue, turns out it was the HT wire from the coil to the cap...and it was new.

 

Especially today, when it comes to car parts NEW really is short for Never Ever Worked.

 

Does your distributor have only centrifugal advance? If it also has a vacuum advance I think it almost has to have a pivoting or rotating breaker plate for the vacuum advance to work properly. Any power or ground lead, and terminals, that move when that plate moves should be suspected and inspected. Maybe there's a ground wire or strap that's been missing all along.

 

Zero dollar test-Just for giggles, mount your coil upside down. I've seen where a bit of the oil inside had wept out ages ago, but still ran fine until one day the oil that was left broke down or could no longer keep it's cool, especially at the terminal ends.

-rick

 

So i completely replaced the primary wire from the coil neg to the points last night as tomcat11 and 29Sleeper had suggested and the car ran. I drove it acouple times today and it did well.

The voltage at the coil is fluctuateing between 8-10 volts but sometimes flashes down to 2 for a moment while it is running now.
The voltage going into the ballast resistor is bouncing between 15-4 volts. When i test at the positive firewall terminal it's a steady 14 .6 is this normal to be so erratic after the ignition switch?

 

The plate is screwed to the distributor housing in three places therefore making a good ground.

 

I hadn't considered a bad coil primary wire, but that could definitely be the problem. That explains why it would quit sparking as you grounded it on the intake.

The voltage going into the ballast should be pretty steady. You need to start tracing that wire back and find the point at which it remains steady. If it goes directly to the ignition switch with no other connections along the way, I would then test it right at the switch to see if it is steady. If it isn't, I'd jump to the power wire to the switch and see if the switch input power is steady. If it is dropping to 4 volts, you do have a potential problem.

 

Sounds (and looks) like there is no vacuum advance on this distributor. The point breaker plate is mounted directly onto the housing. Many of us automatically think there is a vacuum advance that would be moving the breaker plate, which would require the ground wire.

If replacing the wire from the coil to the points makes it run, but the voltage is fluctuating at the + coil, you still have a problem between the coil and the battery. Some voltage fluctuation is expected, but any drop under 6 volts can shut down the ignition system. You have been having trouble because you probably have been fighting more then one problem.

Now your voltage fluctuation problem could be the ignition switch, the resistor, or the wiring between any of those points.
Does the wiring go through a connection block at the firewall? If so, that connection may be the best place to start.
If instead, the wire bundle simply p***es through the firewall, I believe I would then check to see if the voltage before the resistor is fluctuating or not. If the fluctuating is present before the resistor, the problem is the wiring before the resistor or the ignition switch itself. If that voltage is stable, but the voltage still fluctuates at the coil, the problem would be the resistor, or the wiring between the resistor and the coil.
At this point, the old wiring in the car could simply be breaking down after years of over the road use and vibrations, any point where the wiring is attached to a body structure could be a failure point.

As much of a pain that it is, that would be the fastest way to find your current problem.

 

Gene, the voltage is stable coming off of the alternator , at a positive terminal at the firewall. But it is fluctuateing coming into the ballast resistor . I guess I'm gonna hafta be a contortionist again and get under the dash of that bucket !LoL!