Intermittent problem with Backfires that occurs after 40 minutes

[70Challenger440]

My 1969 Barracuda 383 has developed an intermittent drive-ability problem. (I know it isn't an E-body, but this is the only forum I am a member of for Cuda's and Challengers so hopefully you will still help me).   The car is completely stock and has points. The car will start and run fine, but at some point after about driving 40+ minutes will begin to, on part throttle, make little multiple minor pop backfires. If you hit the gas a little harder it won't backfire but you can feel the engine is not running at it's 'usual" best. Then if you turn the car off, it won't be able to immediately restart as if the battery is weak (which its not).  The ignition will just 'tick' but can't turn the engine over. If I try to restart it  an hour later, the car will run perfectly again for another 40 to 80 minutes until it reoccurs. An electrical shop says they can find no shorts nor get the problem to happen for them due to how it occurs. I can tell they see it as too ambiguous to be worth their trouble. I will also try  to find another place  willing to work on this type of issue.

The parts on these cars are inexpensive and easy to replace. In a worst case scenario where it can't be determined the cause, what are all the parts to blindly replace and in what order? I assume the voltage regulators (both dash and firewall),  ballast, coil, & points, first all at once to save time. Then the  distributor, alternator, fuel-pump, carb one at a time. I bet I missed some parts, what are they? Thanks for your help and any other suggestions would be greatly appreciated.

[torredcuda]

Backfiring is usually caused by timing, ignition or fuel issue (lean), could also be getting too hot. Do you have an accurate, working temp gage? You need to check for fuel and spark/ignition voltage when it is acting up to narrow it down. When were rubber fuel hoses replaced last as they can collapse inside? Coils can get weak when hot.

[70Challenger440]

Backfiring is usually caused by timing, ignition or fuel issue (lean), could also be getting too hot. Do you have an accurate, working temp gage? You need to check for fuel and spark/ignition voltage when it is acting up to narrow it down. When were rubber fuel hoses replaced last as they can collapse inside? Coils can get weak when hot.

Yes there is a working temp gauge. Temps are fine. The weather has been in the 50s here so not too hot either. The fuel hoses are about 5 years old as is the carb (edlebrock). The car has a new fuel tank also 5 years old. The car ran well and had a tune-up about 1000 miles ago. I drive this car enough that it doesn't sit either (about 2k miles a year). Since March I put a few hundred miles on it running very well. Then this  started to just happen last week.

I usually just let my mechanic fix my cars, but this is a time when I think due to the intermittent circumstance I must check it myself when it occurs. I do have tools and a volt-meter etc (and am willing to buy any test device you suggest). I am more handy painting cars and interior work. Embarrassingly could you please, tell how I go about checking what you suggest. I will gladly continue to answer any questions you have. I am just not a good trouble-shooter and this might be a great crash course for me to learn so please bear with me.



                        =========================    =======================
Don't forget the oddity I mentioned when this occurs which might be very significant ... if I turn off the car, it won't be able to even crank the engine for an hour, and then cranks fine and starts as normal.

[Brads70]

Timing or exhaust leak when it gets up to temp? 

[70Challenger440]

Timing or exhaust leak when it gets up to temp? 

As to an exhaust leak. (maybe not) ...  note when it does begin to back-fire it loses significant engine power, and will stumble until I hit the gas harder, which makes the backfires stop and power resume.

[torredcuda]

Cannot be certain but sounds like a fuel delivery or mixture issue - filter or carb dirty/clogged, vacuum leak etc.

[Skdmark]

Is the vacuum advance on the distributor hooked up?

[70Challenger440]

Is the vacuum advance on the distributor hooked up?

Yes .. and it is connected to the passenger side port of the carb as it has been the last decade.  Neither side ever caused a backfire. Backfiring just started last week. Since you brought this up which of the 2 ports should I use?

[Bullitt-]

  timed vacuum port is on passenger side. perhaps disconnect & cap this line to remove as a variable


I wish I had clear recollection of the situation I have some memory of but it involved a hose laying on a heat source that only leaked after a period of time but would seal itself when heat was removed. It could explain a vacuum leak that could cause a lean backfire however I don't see how it would cause a starter problem...... Two seemingly unrelated issues

  @Chryco Psycho may have some feedback when he sees this

[dodj]

I like Bullitt idea too just plug the vac adv. Then go for a45 minute drive.  No I can't see that causing the restart issue but you may have a starter heat soak issue along with the ignition or fuel/ air issue

[BIGSHCLUNK]

I'm thinking over heated coil maybe 

[Skdmark]

My line of thought was pulling and capping the vacuum advance to rule it out.
If capping the vacuum advance has no effect, the coil would be the next thing I would look at.

[torredcuda]

I'm thinking over heated coil maybe 

That was my other thought.

[torredcuda]

Replace all rubber fuel hoses even if the outside looks good, it`s pretty cheap and worth doing every few years anyway as new fuel seems to eat them up a lot quicker.

[DeathProofCuda]

The fact that the car won't turn over when warm makes me think it is too much resistance in the primary electrical circuit.  Wiring warms up, resistance increases and then voltage to coil is too low for good spark and not enough juice to turn the car over.  Wiring cools, resistance drops enough to supply enough voltage for it to turn over and run well for awhile.

I'd go through and clean up all connections of the primary electrical circuit and see if that does the trick.

Carb and coil issues aren't going to keep the car from turning over.