Ignition Skips A Beat

[DonaldSmith]

My 47 DeSoto has a stumble in idling, maybe in running, too.  It seems that it's been this way as long as I've had it, but now I noticed something. 

 

When I put the timing light on, I can see each firing of a plug, bif, bif, bif, bif, and then a random skip, and the engine stumbles, then bif, bif, bif, etc..

 

It seems to do this regardless of what plug wire is being scanned.  

 

If I scan the coil wire, the bifs come six times as fast, of course, but I can still see the random skip (as the absence of light when it should be there).

 

The semi-automatic transmission has a function where it grounds out the coil momentarily, to do a shift, but I disconnected the appropriate grounding wire and the ignition still skips. 

 

It's hard to adjust the idle decently.  The engine appears to be running rich, with the smell that goes with it, because of the skipped plug firings, no matter how I try to lean out the idle. .  

 

Is there something wrong with the distributor, or are there any other clues which I should follow?  

 

I'll have to defer anything major for now, since I'll be meeting some of the guys in Mount Victory, Ohio, for a Memorial Day petroliana and old car show.. But then I'll  want to get it running right.

[TodFitch]

While not perfect yet, my engine started idling much better when I put in a distributor that had less play in the bushings and less wear on the cam. I think that the cam was wobbling some which affected the ignition timing and coil saturation. . .

 

Anyway, you might want to put your distributor on the bench and check the cam play with a dial indicator.

[greg g]

I had an idle skip on my 46 engine, ran fine at cruise and under load. When I rebuilt the 56 engine in it now, and swapped the distributor for one that set up perfectly on a test machine, added new plugs and new wires, I still have the skip, still runs fine at speed and under load. So every thing but the coil is differnt but the skip remains.

Have you tried the trick where you pull one spark plug wire at a time at idle (insulated pliers recommended) and see if you can isolate the cylider that is skipping? If a cylinder is firing correctly, pulling the wire will make a noticable change in the idle. when you get to the one that is missfiring either ther won;t be a change or the change will not be as drastic as with the properly firing ones. Then you can work that one, do a compression test, swap spark plug wires, put in a new plug etc. to try to get if back to normal.

Edited May 21, 2013 by greg g

[desoto1939]

Check the breaker plate in the dizzy.  If this is the original it could be getting wornout. See if there is slop in the upper and lower plate.  I had a friend with a 39 Desoto that had similar problem. We took the car to my local old time mechanic and told him of the issue. He took the dizzy cap off and then wa able to wobble the breaker plate assembly. He told use to replace the breaker plate.  I had a NOS breaker plate. Installed this and set the points and his issue went away.  Things do wear out.

 

Write to me and I can tell you which is the correct breaker plate assembly for your 47 Desoto.  I might have one but not sure if the number is the same as my 39 Desoto.

 

Rich HArtung

Desoto1939@aol.com

Valley Forge PA. 19403

[Don Coatney]

Do the dollar bill test. Hold a dollar bill near one end and position the other end directly behind your tailpipe. If you get an occasional suction that tries to pull the loose end of the bill up the tailpipe then your occasional misfire may be related to a sticking valve. Of course that would not explain the beep in the timing light.

[1941Rick]

Welcome to old world ignition systems......find a shop that still has an engine analyzer. they will find out where the miss comes from on the scope.

[Don G 1947]

Do the dollar bill test. Hold a dollar bill near one end and position the other end directly behind your tailpipe. If you get an occasional suction that tries to pull the loose end of the bill up the tailpipe then your occasional misfire may be related to a sticking valve. Of course that would not explain the beep in the timing light.

Don,

You must have a different engine than I do.  Whenever I hold money near my engine it just sucks it in then reaches for my wallet and takes my credit cards!

[Don G 1947]

Check the breaker plate in the dizzy.  If this is the original it could be getting wornout. See if there is slop in the upper and lower plate.  I had a friend with a 39 Desoto that had similar problem. We took the car to my local old time mechanic and told him of the issue. He took the dizzy cap off and then wa able to wobble the breaker plate assembly. He told use to replace the breaker plate.  I had a NOS breaker plate. Installed this and set the points and his issue went away.  Things do wear out.

Can someone provide a little more detail on how to check for a worn distributor? I have the exact same problem that has been eluding me for a while.

 

Thanks,

Don

[P15-D24]

Try plugging the vacuum port on the carb and see if it stops. 

[greg g]

The bearing or bushing at the top of the shaft will sometimes wear oval over time.  This will allow it to wiggle a bit side to side and change the dwell and point gap and even to some extent the timing on a cylinder or two.  grab the rotor and see if you can wiggle it side to side o a couple different points of the compass.

[LAKOTA169]

Do the dollar bill test. Hold a dollar bill near one end and position the other end directly behind your tailpipe. If you get an occasional suction that tries to pull the loose end of the bill up the tailpipe then your occasional misfire may be related to a sticking valve. Of course that would not explain the beep in the timing light.

Didn't have a dollar bill, so I tried 4 quarters.

[DonaldSmith]

I got the distributor rebuilt and adjusted on the Sun machine.  The bushings were fine. The centrifugal advance was fine.  The vacuum chamber had a leak, so I got a rebuilt one.  And new points.  After getting the timing right, the engine runs better.  There is still an occasional skip with the timing light, but the engine doesn't seem to stumble as much.  I guess I'll live with it. 

[Plymouthy Adams]

when you going to learn you can't do things in bits..

[Don Coatney]

Have you replaced your spark plugs and spark plug wires?

[DonaldSmith]

Yep, new plugs and wires.  The PO had the distributor 180 degrees off, so i took advantage of the opportunity to reset the distributor and do the plug wires accordingly.  Next to-do item: pnion seal.  But that's another thread.

[JerseyHarold]

I didn't see the ignition condenser mentioned in earlier posts.  Has it been replaced with a known good/new one of the correct value?  It may be breaking down intermittently causing the stumble.  

[Tom Skinner]

Donald Smith,

I agree with Jersey Harold.

Condenser.

Or just call her Skippy.

L.O.L.

Tom

[DonaldSmith]

Hmm...  Makes sense.  An electronic thing that gets tired every once in a while.  The distributor guy didn't replace the condenser.  Maybe they bench test OK, but then on the car they get fussy.  

   

So I'll be going to Condenser City for a new one.  Can't cost that much.  And it's one way to find out. 

[DonaldSmith]

OK, I spent the 7 bucks for a new condenser.

The skips in the flashes of the timing light seem less often, but that may be a placebo effect.  

 

New coil, plugs, wires, points, and now a condenser,  Seems that ought to do some good.  Throw a few bucks at it. Sometimes it seems that if we make propitious offerings to the gods, they might smile on us. 

 

The engine is running well now.  In my search for perfection, I'm settling for pretty darn good.  

 

Now to await the pinion seal and window glass.  But those are other threads.

[pflaming]

I'm no mechanic, but I had a miss also and found that one hose to the heater would draw a spark, I tied it away from the plugs and the skip disappeared. FWIW. Maybe get a used heart pace maker. 

[JBNeal]

My first thoughts are this skipping problem sounds like mechanical wear within the distributor.  I recall doing some research awhile back and learned that new spark plug wires can be a problem in itself if the replacement wires are the wrong kind.  The universal kit that can be bought through NAPA has the carbon core which can be problematic on stock 6V systems, whereas the original 6V system had copper cored wires.  I've read varying opinions on the usage of carbon core wires vs. the copper core wires, and it seems that the carbon core wires are prone to internal arcing, etc. when they get older and/or have been handled too much.  Conversely, the copper core wires are prone to excessive resistance at the terminal ends due to corrosion.  I reckon the hot rodders running 12V prefer the modern carbon core wires because the 12V coil can deliver the voltage through them efficiently, and traditionalists prefer running copper core wires with the 6V systems.

[DonaldSmith]

I put in copper core wires and resistor plugs. 

 

After dark, I can look at the engine for fireworks, unless the jumping is within those two fancy tubes that the wires run through.  Always wondered about those. 

The timing light was connected to plug wire no. 1 when I checked the timing today.  I could switch to the other wires, to check the flash.  Or even the coil wire. 

On the other hand, it's running well enough.  Maybe some other time I'll get curious again.  

[busycoupe]

With new wires you shouldn't have any arching, but an easy way to check for it is to watch the engine run in pitch blackness at night.  Any arching wires will be easily visible.

Dave

[Tom Skinner]

I think someone has suggested this but:

Here goes again.....

A Vacumn Gauge Test will usually tell if a Valve is sticking, weak spring etc.

Usually if its not the coil or condenser spark plugs wires etc - in other words

everything you have thrown money at, then its time for Vacumm Gauge Tests

Compression Tests, Leak down tests, You will most certainly find the culprit then.

Good Luck! I hope its not serious, or just live with it until you have to "go in" to fix

something later down the road.

[Mikemomd]

Friend of mine suggested switching to an ignition ignitor such as manufatured by Pertronix.....any suggestions on this idea to help performance.