... '65 L-78 owned by a friend of a friend. The engine was rebuilt ten years ago, and the owner has only driven it 400 miles since then. It had a severe case of "trailer-hitching" at 1500-2000 revs, light load and was not at all pleasant to drive. We removed the top distributor shield and I immediately saw a problem. The cap widow was rotated to the left rather than being near normal to engine centerline, and the wires were indexed one position CCW from proper indexing so #1 was to the left (as viewed from the driver's seat) of the cap window rather than to the right (passenger side). I figured there were likely other issues and suggested that we pull the distributor for a "blueprint overhaul" the following week which was yesterday.

After indexing the crank at 10 deg. TDC #1 compression stroke we pulled the distributor, and I lost the first bet because the gear was properly indexed with the dimple pointing the same direction as the rotor tip. One .030" shim was installed and end play was .065". The installed VAC was a 8" B28, and we had a 12" B26, but it was out of spec requiring 14" to pull to the limit, so we retained the B28 . Fortunately the shaft and housing bushings were in good shape, and the only significant problem was that the pickup coil pigtail wires had been spliced and wrapped with tape. One had a solder ball on the splice and the other was just twisted together. Bubba had definitely been there. Fortunately my buddy can solder so he took care of that with a nice solder and shrink tubing job while the owner and I cleaned parts.

Assembly went smoothly after filling the grease well with 1960954 Delco Distributor and Stater Motor lubricant that we also used on the tach drive gears. We added two .030" shims before installing the gear and end play checked out at .006".

After having to tweak the oil pump drive a couple of times with my "special tool" (a Home Depot paint mixing stick) the distributor seated and I rotated the base to line up the stationary pole pieces with the rotating pole pieces. We installed the cap and reindexed the wires one position CW that put #1 wire along the right (passenger side) of the cap window, and it was time to start. My reputation was on the line.

The carburetor was some replacement that has a non-functional electric choke (The owner as an overhauled original and that's his next project). After a couple of backfires with small fireballs, I was beginning to worry, and decided to manually close the choke. Then it started and we we were able to warm the engine enough to set idle speed and timing was right on 10 degrees, so we locked down the distributor, installed the air cleaner, and went for a test drive.

The trailer hitching was gone and the engine ran smooth as silk. The owner couldn't believe the difference. I wanted to check the entire spark advance map and try some lighter centrifugal springs, but we were getting short of time and agree to do that job maybe next Saturday.

I have no doubt that there are many more screwed up distributor installations out there. I've been running across them for over 55 years.

Duke

 

Nice diagnosis, explanation and result. Curious... What was the idle vacuum and RPM? I'd think it was right around 14"? Manifold vacuum on the L78 right? I'd think so.

The misindexing of the wires certainly hinted other flaws from before.

The questionable PU coil wiring could have been intermittent but likely not the trailer hitch problem, would you agree? It sounds like the main problem was the distributor end play, causing the stator to bounce in and out of horizontal range with respect to the pole piece.

One other note, the PU coil wiring is polarized. When he fixed it was he sure it didn't get reversed? I have the configuration written somewhere if needed.

Rich

 

The 1960954 Plasti-Lube Moly grease is no longer available. What do you recommend for a substitute?

 

Use a synthetic grease like Mobil 1. There is a hole in the bore that allows the grease to slowly seep in.

Dujke

 

Typical L-78/72/71 idle behavior is 14" @ 900 Some prefer a lower idle speed, which is less manifold vacuum, but IMO it's too lumpy.

Distributor end play of up to 90 thou is typical of untouched distributors. It causes spark scatter that you can see with a timing light, but I don't think it had anything to do with the severe jerkiness I described.

We had a problem identifying the pickup coil wires. The green strip was very faint, We had a spare TI distributor and the service manual schematic and finally made a decision, but knew that if we were wrong we could just reverse the wires in the connector. It's possible they were previously reversed and was part or all of the problem.

...bottom line is that there could have been multiple issues that contributed to the problem, but taking it slow and double checking everything with available sources got us to where we wanted to be... a perfect running engine.

Duke

 

Duke in your opinion what performance penalty is there for the distributor gear not being indexed correctly?

 

On a small block you can't get more than about 8-10 degrees initial timing before the VAC hits the inlet manifold with the 13-tooth gear 180 out. So you rotate the shaft one tooth CW and it hits the coil bracket with too much timing. Big blocks appear to have more rotational freedom than small blocks, but I prefer to assembling things IAW Chevrolet Engineering instructions.

It appears some aftermarket camshafts don't have the dist. drive gear indexed IAW Chevrolet drawings and to get proper initial timing you have to rotate the gear 180 degrees so the dimple points opposite the rotor tip.

Duke

 

SWC Duke, haven’t had a distributor opened up in awhile so I’m trying to picture this (I’m the visual learner type :-)), I believe I understand what u are talking about when you are aligning the pole pieces up your basically putting the dist. cam/points in a position to get ready to open so @ the 10deg. U have the engine set on before pulling out the dizzy, points are positioned to open & coil is ready to react with spark, hopefully I’m following u thus far?

HERE’S THE QUESTION: do you have a pic of the pole pieces in that alignment procedure u are referencing?
Back in the day (many,many years ago) i would use a self powered test light and turn body of dizzy to look for either the light turn on/off (cant remember that process), I like your method way, way better...
By the time it would take me to find the light, article/notes on How To, your method would be done especially if your working at someone else’s home/garage with limited tools.
Thanks
Chalie

 

Duke will come in for your answers but this is a L78 which uses the mandatory Transistor Ignition (TI) system, not a points system.


Some info & Photos


HERE


may help describe some of it.

Rich

 

Don't have a pic, but what Duke was referring to are the "teeth" on the TI equivalent to a points block and centrifugal advance cam, and corresponding teeth on the inner rim of the TI pickup coil. When the distributor is turning, a small voltage pulse is induced in the pickup coil when the teeth on the advance cam get close enough to the teeth on the pickup coil.

 

What you describe is called static timing the engine. By setting the crank at the initial timing point, when you reinstall the dist. and rotate the base until the points just begin to open, the initial timing will be very close to where the damper notch is on the timing tab. You can do this either with a meter or a test light, or just visually.

Lining up the rotating and stationary pole pieces on a TI magnetic pulse distributor is the equivalent of the points opening. I don't have a picture but both pole pieces have eight projections that nearly touch, and when the rotating pole passes the stationary pole a voltage is generated in the pickup coil that tells the main power transistor to shut off primary current, which is the equivalent of the points opening on a conventional point distributor.

Duke

 

I posted a link in post #9 to a thread which has the photos.

 

My bad wasn’t aware u guys were dealing with a T.I. Set-up..
Thanks

 

No problem... the TI system was a mandatory option with all mechanical lifter big blocks...L-78/72/71/88, LS-6.

Duke

 

Rich -do you know what happens if the pickup coil wires get reversed?

Duke

 

Having tested this pickup in my lab with an oscilloscope, here's what I saw: The pattern showed a very steep downward slope, crossing zero voltage, as the points of the reluctor pass the points of the stator. A few degrees past alignment, the voltage curve starts a slower climb upwards passing zero at about halfway to the next alignment and then plunging rapidly a few degrees just before the next alignment. The rapid zero-crossing change at alignment is used to trigger the TI Amplifier. It is also noteworthy that the rotational speed of the distributor increases the magnitude of the pattern. The rapid crossing at alignment is used for triggering, but, if I recall correctly, the Corvette amplifier and the MSD CD systems use opposite patterns, in other words, one uses rapid upward change, the other uses rapid downward change. To swap the direction of change at alignment, merely switch the green-white wire and the white wire. Another way to switch can be made is by flipping the magnet, but this is not recommended because of the difficulty of re-aligning the stator plate with the magnet's base.

If one wonders what happens when trying to run an engine with the wires reversed for any TI ignition using magnetic pickups like this - the timing will be WAY off (halfway between the alignments) and may jump to the wrong post of the distributor as rpm is raised due to the centrifugal advance affecting the alignment relationship of the rotor and the ignition wire posts.

Below are 2 pictures - one not-aligned and one aligned at firing:

 

Thank - you for that analysis. It may explain some strange observations. When I first looked at this engine the incorrect carb appeared to be set up for ported advance, but even with the VAC hose removed and plugged I read about 25 degrees initial timing. That's about what total idle advance would be with 10 initial and full vacuum advance that may explain why it idled okay, but once centrifugal advance is added it would be way over-advanced, which could explain the roughness in the 1500-2000 range, no or light load. Above 2000 it smoothed out, but wasn't very strong,.. not what I would expect for a L-78.

Then when it was assembled and reinstalled it appeared to have full time vacuum advance, so maybe in addition to the incorrect wire and shaft indexing those pickup coil wires were reversed. I should have kept better notes, but like I said we were getting short of time ,and when the test drive was successful we decided to knock off and work on optimizing the centrifugal curve later.

It seems to me that if the asymmetrical wave form your observed were flipped upside down the system probably wouldn't work very well, if at all.

To paraphrase Forrest Gump ...when you open up a 50+ year old distributor you never know what you're going to find.

Duke

 

As I recall I believe at certain RPM range it would exhibit erratic behavior, maybe high RPM only. I never experienced it but have read that somewhere or maybe when I was talking to Dave Fiedler years ago.

I'm on the road all day but when I get home tonight I'll try to find my notes and show the wire configuration for the PU coil.

I also did extensive waveform testing on the bench and will post some photos of that too in the morning.

Rich

 

The ac sinewave the pickup coil produces becomes inverted and the module interpets this misinformation then saturates the coil at the wrong time setting the spark line in the cap at the wrong time. I never recorded this bubba issue but have seen a few times in later model GM's.

 

Thanks Gordon. This makes sense as the coil position with respect to the magnet would affect it's output. Since the 2 wires are uniquely identified, the designers knew it would be a issue if reversed. It may even be somewhere in the CSM this is discussed.

Here is a image showing PU coil output(yellow top trace) and module output(blue pulse to Ign Coil). Note that the PU coil pulse triggers the Module output(coil+) on its falling edge.


So I went thru my old posts on the NCRS TDB. I did some bench testing on a Delco module then also on the 2 reproduction modules. My scope broke in the process and a member donated a digital scope to me and I did more testing. Full link to the thread HERE for those with access.

I also did a video with the old Tek scope before it died. It's kinda long but some of it is useful as lots of folks don't understand the basic concepts of how TI works.
Rich