Big Time BACKFIRE and then engine stalls - Suggestions??

Creatures from outer space??  Holy carburetor Batman, I have no idea.  We get all sorts of weird shyt here, but this one is out there.  Most will say ignition. Done anything there??  Heat related scramble of the signals in there??  What is the ignition system?

 

 

OK, I'll start the ball rolling.

Most likely cause is dirt somewhere in the carbs, which floats or drifts free when you shut down, and then eventually finds its way to plugging up whatever orifice it's clogging after you restart.

Maybe have a carb guy (if you're not familiar with carbs) pull the tops of the carbs off and have a look in the float bowls. The brand new carbs on my brand new VS came pre-loaded with lots of junk in there.

Another thought - did the problem start when the new exhaust was installed? The stock VS installation uses those funky compression 'donut' connectors instead of honest-to-gosh bolt-em-down flanges. If you look up 'backfiring' in the dictionary, I think there's a little picture of one of those freakin' donuts there.

 

Questions: does the car "burble" in deceleration? When it goes BANG are you on the gas, off the gas, at a steady cruise?

Any other changes in the way the car drives or feels or sounds before this started happening?

Is it getting worse/more frequent or not?

Tell ma a little about the ignition system? Points/condenser? Electronic ignition (magnetic pickup - no points).  What coil are you running? 

Hard to say, but doubtful that its an exhaust leak. Because the car is dying, sounds spark related. 

That said... My model A does something similar when it leans out due to lack of fuel in the bowl.

Mechanical fuel pump or electronic? 

We used to have a big nasty c10 that we would force a backfire like that everytime we would go under a overpass. As we would head under the overpass, we would turn off the key... Pump the gas a few times... Then turn the car back on...

Kapooow!

Sounds like a coil that gets hot and shuts off

or weak mechanical fuel pump getting worse?

what about a manifold gasket sucking air

I have had all three and the symptoms were somewhat close to what you described... 

only backfiring problem i ever had was caused by a burnt intake valve

My 2 sense: Sart with the basics.  Idle air passage blocked. Throttle diaphragm torn or just damaged. Since it stalls/ loses power after the backfire, check the timing/ points.  Then, the valve adjustments. Check the color of the spark from the coil. A weak coil is dim, yellow I think. A good coil is bright blue.  These are all easy fixes in you garage. Sooner or later a problem will reveal itself. In the meantime, it won’t go away or just get better on it’s own.  

   All running problems are fuel delivery or electrical! ;-0.   I am waiting to hear the cause & solution.  Thanks for keeping us posted.

Sounds electrical ....check the wired to the coil connectors  also when it back fires ( exhaust or up through carb?)  and dies I would feel the coil to see if it's hot, also do a voltage and ohm check, or simply temp. wire another coil and see what happens.

 

A compression check is easy to do with $25 tool -- it will quickly show a burn valve with low compression. Also bad rings and maybe pulled head stud (head leaking).  Exhaust valves like to stretch just before the head breaks off causing serious damage.  You said tuneup recently - what were valves like?  Was one or two loose?

barncobob posted:

only backfiring problem i ever had ...  

 

First thought that came to mind..... Was when I had Montezuma's Revenge

 

Also if it has points eliminator aka Pertronic ignition, check the screw that holds said ignition to the plate in the distributor, clean, re-install. It may be loose and/or corroded.

IaM-Ray posted:

barncobob posted:

only backfiring problem i ever had ...  

 

First thought that came to mind..... Was when I had Montezuma's Revenge

 

Ray, cut it out! My orange juice almost came up through my nose and onto the keyboard...

Thanks to all of you for your suggestions and "pointers." I just met with my mechanic and he believes the electronic ignition & coil are the prime suspects. After he checks everything out and, hopefully, confirms that suspicion, I'll have him replace both. Unfortunately, this won't be happening until next month, but when/if all is well....I'll report back with the results.

Thanks again!

Paul (plwinc)

Where is Stan?   He would say what I’m about to say:

None of this sounds like a fuel or valve problem.  

What you described is classic “heat soak”, where an electronic component, in this case that Pertronix module, has a component that becomes sensitive, over time, to a raised temperature level.  Lot of technical reasons for this to happen, just understand that there is a lot of heat being generated by the engine and that can cause a temperature-weak ignition module to instantaneously quit and restart for different failure durations once the engine bay temperature rises.  When that happens, the carbs are still pushing gas into the cylinders with no spark and when the spark comes back an instant later, BAM!  The feeling that there was a reduction in power right after the backfire is very common when this happens with a module failure.  

My money would be on the idea that the coil is probably OK, but the ignition module should be replaced.  I am not a big fan of the Pertronix in our engines, mostly because of the heat back there and the other fact that Pertronix has had high temp heat soak issues for years, but there are few other options out there so I would suggest a new Pertronix module, but buy two and keep the other in the car as a spare so you won’t get stranded somewhere. 

gn

Gordon Nichols posted:

Where is Stan?

Present, Mr. Nichols.

As they say, 90% of all fuel problems are spark. My money is on Gordon's hypothesis. I'd bet money it's the points replacement module, especially if it's a Pertronix. 

Gordon Nichols posted:

I am not a big fan of the Pertronix in our engines, mostly because of the heat back there and the other fact that Pertronix has had high temp heat soak issues for years, but there are few other options out there

Compu-Fire

More gooder-better. 

This is an interesting one! I'm going with Gordon's scenario of an electronic component (the Pertronix being the main suspect, but as mentioned, could be the coil) overheating and cutting out as well. As an aside, here is another reason to make sure there is not only enough air getting to the carbs and fan, but there is enough airflow through the engine compartment to carry away radiated heat off the sheetmetal, alternator and other parts so the engine is ingesting ambient air. Air is not a dense cooling medium like water, can only absorb so much heat and hotter air going into the fan isn't as efficient, forcing the heads and cylinders to run hotter. The tune goes away sooner so there isn't as much power available and if run under these conditions long term, reduced engine life is the result.

A lot of guys running electronic ignition control boxes (MSD's and the like) mount them outside the engine compartment for this very reason. Al

@Napa Paul- Have you thought of putting a remote thermometer in the engine compartment (mounted on the fan shroud near the coil maybe?) so you know how hot it is when the engine cuts out?

I'll go over your suggestion with my mechanic when he works on the engine next month.

Thanks again for your feedback.

Paul (plwinc)

"9 yr. old VS 1915cc with only 16,000 miles"  

If it has been running OK for the past 9 years and only acting up now, then the Pertronix is probably becoming toast.   Pertronix modules are designed/built with standard commercial-grade electronic components.  They're cheap.  They also don't like heat.  You can't prevent the heat inside of the engine bay that affects them - they're screwed to the inside of the distributor which has heat transmitted to it directly from the engine case and up the disti shaft.  Nothing really cools it other than the swirl of air coming in the grill and most of that gets sucked into the fan - the Distributor is a lonely orphan child and doesn't get much, PLUS the cap keeps the heat in.  Getting the picture?

Designers can go up a grade or two in part quality (there are engineering designations for this all the way up through higher military grades) and you pay a bit more for each next grade.  As you go higher, they usually can tolerate more and more heat, as what's inside is more rugged (they have a higher tolerance of transistor junction melting in use).  That's why the industry calls it "Ruggedizing" or "Hardening" the device (although "Hardening" involves a lot more at the equipment level).

The failure symptoms are not usually like it's working and then stops for good.  Instead, the transistor junction acts like a fuse wire that heats up, bends and distorts until it finally pops open, but then the stretch and curve of the wire lets it go back together when it cools and re-makes partial contact.  After that, it works but not as well as before and can repeat the failing mechanism whenever hot enough. (It's actually a little different inside of a transistor, but the fuse idea is a good visual for ya.)

THAT is what's happening with yours.  Going to a Compu-Fire is certainly a good alternative.  You don't hear about many failures with them and it should work with everything else.

Now, on the coil, it is either a dry coil or an oil-filled.  The oil is meant to cool it and is especially helpful if the coil is mounted on the fan shroud, closer to heat.  If the coil is mounted away on the inner fender liner away from the engine heat a bit, it'll probably last forever, whatever type it is.

Hope this helps.  And keep that Mechanic - he's a good one!

 

Remember the current mlimiting resistor that the Big Three once used to lower the coil supply voltage, once the engines were started???   Several of the "Pertronix' wannabes require a dropping resistor.....  Without them, life span is measured in minutes or hours.....    For what its worth....  

Normally these are just wired in series with the coil....   !.5 to 3 ohms seems to work well....  

IIRC, that was a special, temperature reacting, "Ballast" resistor that increased it's resistance as it worked (and warmed up).  It was supposed to give a hotter spark at start-up and then lessen the spark voltage to "running mode" or something because it essentially used a 6 volt coil in a 12 volt system.  Another memory from the dim, dark past, Leon!

I ran a ballast resistor for a while. Then I got coils that didn’t need them to work right.

Honestly— you can replace every component in the ignition system for the cost of a couple hours in a commercial shop (spent looking for the issue). I hate it that this is what the world has come to, but here we are. 

You're absolutely right.  My Dad had a fleet of GMC school buses and we had two get stranded by failed ballast resistors.  They were like, $3 bucks at NAPA (1965 pricing, here).  After the second one failed and the kid's schedules were messed up, I spent a Saturday swapping out 18 resistors and coils for 18 12 volt coils.  $350 1965 bucks for 18 buses and never had another failure.  

I think we will see that Gordo/Stan-the-man-from Stanistan have this nailed.  Heat soaked pnp transistors, a squirrel diode too hot, or some such.  Where I'll place my bet.

@Napa Paul

Any cure yet?

I'm out of town right now and won't be taking Speedie into my mechanic until March 21st. Thanks for keeping me in mind, though!

First of all: Thanks to ALL of you for your comments and suggestions!

Yesterday, I picked up my "IRS Dependant" and I look forward to putting on some shakedown miles...but it does look like the problem's solved: Replaced ignition coil and electronic distributor.

Assuming (Gawd, I hate "assuming" anything at my age!) she's completely up-to-snuff, I hope to see some of y'all in SLO for the June 9th festivities!

Thanks again!!

"Assuming (Gawd, I hate "assuming" anything at my age!) she's completely up-to-snuff, I hope to see some of y'all in SLO for the June 9th festivities!"   - Paul Watts

That would be great if you could join the rest of us knuckleheads in SLO, Paul!  @Napa Paul