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I'm trying to diagnose a slight problem with my truck.
info
1950 chevy truck, engine 54 car 235. ignition pertronix.
history
truck has been running great since 2010

last October I was driving on the freeway and it shut down while going about 50mph. I found the distributor cap had come off.
I purchased a new distributor, cap, rotor and got her back on the road.
Ive been driving her pretty regularly the last six weeks or so and it didn't seem to be running as well as before.

sounded like a slight random miss.
as a test tonight I put my timing light on all the cylinders.
It seemed like #1 had a random miss ,

I changed the spark plug wire on just that cylinder and re checked, with no change.
I then changed the spark plug and no change.
the old spark plug had a white coat

tomorrow Ill change the cap and rotor and see if it goes away. I'll Continue diagnosing.

as I put the light on all the cylinders it seemed to me that #1 had dead spots
#6 had a slight miss in spark also but not as bad as number 1

2,3,4,5 all looked fine with the light.

Here is my question.
Is checking with a timing light really telling me anything?
or is it just an effect of something else.


thanks

Check the upper distributor shaft bushing for looseness by trying to move the rotor from side to side. It sounds like you might have developed enough of a wobble to let the dwell change from one place to another as the distributor turns, resulting in an intermittent miss. If the wobble was bad enough it could have let the rotor make contact with the cap and knock it out of position.
Jerry

I will check the distributor shaft tomorrow when I get back on this

I just installed this distributor in October. It's a reman from rock auto


Here's a pic of the number 1 spark plug
....spark plug pic...
I took a video of the timing light test I did
I don't think it transfers well to video because of the light
But I think you can hear the slight miss.
...video...

...slowed down video...

In true life I can see the difference in number 1

I am always skeptical of rebuilt distributors. The rebuilders seem to always leave the wear disc and spring out from behind the grease cup assembly. With out these the shaft can wobble causing misfire.
I have a post on my BLOG that touches on this topic.

That Pertronix system could be the cause of some of the problems. Try reinstalling the points temporarily if the shaft wobble isn't the culprit.
Jerry

Well I had a bit of success this morning

I removed the ...Rotor...
And ...Cap...
I'm not sure if that pattern is normal?....

I noticed quite of bit of wobble in the whole distributor. So I removed it.
I did not notice to much wobble between the shaft and body but there was a little bit.
...pic...
So I decided at add the disc and spring from pre68dave blog.
Then as reassembled I noticed a lot of play in the vac advance. So I tried to tighten the screw the holds the to pieces together and found out the small fine thread bolt was stripped and would not tighten.
...video...
My guess is this was causing excessive play once the distributor was installed?

I had a spare vac advanced and reinstalled the distributor with new cap and rotor.

I lost my time but once I got her retimed the truck sounded much better. I set my rpm to 450, advanced the distributor to max vac reading, and was able to turn in the mixture screw a bit.

Tested spark at each wire and had a nice constant spark.
Went for a test drive and it ran much better and sounded great.
At one stop I came to I had to rev the engines just to make sure it was still on. It was.

Thanks for the suggestions
I think this came down to excessive play in the distributor.?

Looks like you nailed it- - - -Sometimes the little details make a huge difference! Any variation in the firing impulses at the distributor doubles the timing change at the crankshaft. Since the crankshaft turns 2 degrees for every degree of distributor rotation, it doesn't take much wobble or wear to result in big timing changes between cylinders.
Jerry

Follow up question on spark plugs

In the truck now I have delco r46sz spark plugs

With new cap rotor and wires I was installing new spark plugs

They are autolite 303

I noticed the autolites are shorter. The probe will not be in the cylinder as far.

...pic...
Autolite on left, delco on right

Does that distance make any difference?

The important distance is the thread depth, which appears to be the same for both plugs. Extended-tip plugs put the tip of the insulated electrode out into the incoming fuel mixture, which tends to make the tip run cooler. The Delco 46 series is one step "hotter" than the original-equipment plug, (original was a 45,IMHO) so that should keep any deposits burned off the extended tip. I don't see a problem with running the Delco plug.

OOPS! I misread which plug you have now- - - -the short Autolite, right? If it's the equivalent of a Delco 45, you're still OK. The short plug with the equivalent heat range to a Delco 46 will be good for short-trip driving. It might tend to cause a little detonation on a long haul under heavy load.
Jerry

Thanks

Ill try the new shorter autolite 303 that I just bought.
The delcos r46sz do show the white deposit I posted a pic of in the ...third... post.
Not sure why I put those r46sz in

In the good old days that plug would have been dark brown or sooty. Too rich. Tough to "read" plugs with modern fuel. About what you would have expected with a miss on # one.