Vette-in-the-Veins

This is for a 72 LS5.

On Feb 2017 I rebuilt the distributor with new bushings, shaft, tach drive button, points, condenser, springs, etc...

When putting the distributor back into the engine I noticed oil coming out of the top of the coil. Great.

So I order a new ignition coil. When it arrives it is an AC Delco U505. The older unit has 503 stamped on it. Not thinking much about it, I put in the new coil.

Everything started right up. However, since that time, the engine seems to load up(get rich) at idle and continues to feel rich unless I rev the engine up a bit to clear it out. Then it runs just fine until the next idle cycle.

While reading some this evening it seems that there are different ignition coils for a SB versus a BB. SB = 1115270 while a BB uses 1115287.

Is there truly a difference between the two coils as far as construction?

Is the AC Delco U505 the correct coil for a 72 BB?

Any assistance or insight is greatly appreciated.

71VetteLover

Chevrolet distributor gears have 13 teeth so if you happened to put your gear back on 180 degrees off it would affect the position of your distributor housing by 1/2 of 27.69 degrees (or 13.345 degrees) from where it's supposed to be. As you're thinking it's loading up I would think it's running 27.69 degrees retarded from where it was before you pulled it out. The only way to know for sure is to check your timing with a timing light. The OEM gears have a "dimple" on one side and I seem to recall the dimple is supposed to be on the side the rotor is pointing to but don't hold me to that.

Vette-in-the-Veins

71VetteLover, thanks for the thoughts on that.

I should also mention that after reinstallation of the distributor, I did set timing and even set the dwell to the specifications listed on the firewall emissions decal.

MelWff

AC Delco currently makes only one ignition coil for a 72 Corvette and that is the U505. You are correct that in 72 there were two different part numbers for SB and BB. If you need a coil with the correct markings you will have to get it from a Corvette vendor. The ratings for the two coils are identical so I can not see how they would be functionally different. Have you adjusted the idle mixture screws?

Vette-in-the-Veins

MelWff, I have adjusted the idle air mixture screws to give me the highest vacuum reading while the engine is idling, if that is the proper way. It does idle beautifully though.

It seems like it misses badly when starting to engage the clutch from a stop. If I have the rpms a little higher than I like or rev the engine just a little before I begin the clutch engagement, then everything seems fine.

I was originally thinking a carb issue but it seemed to happen immediately after the distributor rebuild and the coil replacement.

As for the coil specs, I didn't think there would be a difference. I read in some thread on the forum here that something like 80%percent of carb/fuel issues are ignition issues thus why I was thinking it was something silly I did in the distributor.

Maybe I'll just open the idle air mixtures screws a half a turn each to see if things improve some.

MelWff

Is the car an automatic and if so are you adjusting the mixture in gear?
Do you have the old coil to see if reinstalling it makes a difference?

Vette-in-the-Veins

It is an manual transmission.

I do have the old coil. I tightened the center screw in the coil wire hole and that stopped the leaking oil.

I'll try the old coil first followed by the carb adjustment if things haven't improved.

MelWff

isnt your coil in an almost vertical position? how can it leak oil out the center?

Vette-in-the-Veins

Yes the coil is nearly vertical.

As for the oil leaking, there is a screw inside the coil tower. I presume it holds the terminal in place and in contact with the internal connection. It was a little loose so I tightened the screw and no more oil.

I did put the old coil back on yesterday evening and the "Loading up" issue seems to be gone. I will drive it a bit more next week to confirm. Maybe the new coil was just a bad one.

One note on the new coil though, it was slightly larger in diameter. Not enough to present any mounting issues in the coil mounting bracket but large enough that it barely fit into the bracket with the clamp screw having only 2 or so threads in the bracket.

MelWff

sounds like you solved your problem by going back to the old coil.

marshal135

I had the same issue with the AC Delco replacement coil I purchased.
I had to open up the bracket ears a little bit and found that an electrical outlet/fan work box screw has the same thread pitch.
I used one of them which is a longer screw which gave me a few more threads out the other end and placed the heat reflectix product on the bottom of the coil as it sits almost on the intake. It's all hidden under the shielding anyway.
Nothing new is a direct fit it seems.
Marshal

Vette-in-the-Veins

True. Back to what to do if or when this one goes kaput.

Big sigh here