L-79 distributor blueprint overhaul

[Duke W.]

I posted the following in the nearby distributor ID thread, but feel it deserves its own thread with some additional editing. Given the poor condition of the breaker plate ground wire on this relatively low miles car, it's something owners should be aware of. If this wire breaks, you are stranded. The distributor is the most overlooked component on the engine, but if it doesn't deliver properly timed spark under all operating conditions, the engine will never achieve peak performance and fuel economy. We set up this distributor using the spark advance map I recommended in my San Diego presentation for the L-79 to achieve peak performance and fuel economy.

I did a "blueprint overhaul" on a '65 L-79 1111087 single point distributor, today, for a local chapter member. Jim purchased the car about a year ago - 90K miles, and it had been stored for quite sometime, but was unmolested.

One glitch was the breaker plate ground wire broke off the terminal that is secured with the forward VAC mount screw as I was removing the screw, and the wire was rock hard. We had to improvise a fix. I had some black 12 gauge wire and some generic terminals. Don made a new lead using the fresh wire with a generic lug on the end that's secured with the VAC mounting screw. He was able to loosen the crimp on the terminal that's soldered to the breaker plate and recrimp the new wire.

My advice: Buy a NAPA LW42 breaker plate ground repair wire and install it now. It costs about two bucks and has indexed terminals on both ends. It can be installed with the distributor in the car between the forward VAC mounting screw and a breaker point mounting screw. The ground wire is constantly flexed by VAC action, so it work hardens and will eventually fatigue and break, which can leave you stranded. In a pinch, a test lead with alligator clips can be secured to the breaker plate through the window to a ground. Disconnnect the VAC and plug the hose so the breaker plate doesn't move around and knock the test lead off. This will get you home.

There was virtually no detectable shaft side play, so the housing bushings were not worn. End play was only .043" (I typically recall .060-.090"), and there was some visible spark scatter. The football was "71", cam assembly "730 CCW". The weights had no numbers, but looked OE, not aftermarket. They had a hole drilled on the big ends as in the post #42 photo. ( https://www.forums.ncrs.org/showthre...-a-Distributor )

There was a "56" on the inside base of the housing under the breaker plate, and an O or zero externally with what looked like a square symbol.

The breaker plate fit nice and snug with no wobble, and the advance bushing was okay. The upper bushing grease well had some wet solid flakes, but at least the tach drive gears still had grease and were in good shape.

Once we got everything cleaned up I packed GM 1960954 distributor and starter motor grease into the upper bushing grease well, and also used it on the tach drive gears. We replaced the OE 236 8" VAC with a B26 (12") VAC, and Jim had already installed the gold springs from the Mr. Gasket 928G kit that we believe are the lightest. (BTW, compared to these the OE springs look like they came off a dumptruck.) A .030" and .010" shim from the shim kit brought end play down to the .004-.005" range. Jim had already installed the Echlin CS7860 32 oz. point set with the trick little lubricator wick, and I dabbed on some of the 1960954 grease. This is exactly how I recommended setting up a L-79 distrubutor in my San Diego presentation.

It fired right up after having to tweek the oil pump drive to get the distributor to seat. Centrifugal starts at about 700 and is all in at 3000-3500. The L-79 centrifugal is 30 maximum (at 5000 OE), but it's non-linear, so most of it is in by about 4000. Of course, the gold springs quicken it so it's nearly all in at 3000.

I set the total WOT advance at 39 @ 3500 and the initial is 9. The timing mark is dead steady at any RPM. That's what tight end play does - gets rid of the spark scatter that the sloppy OE assembly produces, which can result in a noticeably smoother engine. Idle mixture screws are out one turn from the seat, and it pulls 14-15" at 750, so I have no reason to believe is doesn't have the correct L-79 cam.

Given the car's history I thought the heads may have never been off, but the gasket measured .036", so they have been off. In my brief road test I couldn't get it to detonate lugging up some steep hills. Start up from a dead stop with minimal clutch slippage was smooth with no stumble, and it was tractable down to less than 1000 revs. Low end torque was a little weak (as expected compared to the 300 HP enigine), but it was strong and linear from 3500-6000 even though it might have lower than OE compression ratio. Jim's next step is to do a regular unleaded fuel test to see if it will run detonation free on regular.

Jim and Don have reported that the engine ran smooth and strong on the trip back up the world's busiest freeway to the San Fernando Valley.

Duke

[William F.]

Re: L-79 distributor blueprint overhaul

So, Duke, you're saying the only thing you need to change on a stock L79 distributor regarding getting best advance curve is to substitute the lighter springs and make sure you have a B-26 vac can? No problem with stock "football or the weights themselves?
Thanks,
'67 L79
'67 L36

[Duke W.]

Re: L-79 distributor blueprint overhaul

The OE football and weights are what give the centrifugal advance its "curve", which is non-linear on L-79s.

An interesting experiment would be to modify a '64-'65 365 HP distributor slightly to see if it improves the bottom end of the torque curve. Since the L-79 has less overlap, it doesn't need as much and may not tolerate without detonation as much low rev advance as the higher overlap mechanical lifter cams.

My thinking is to grind out the slot to allow 27 degrees centrifugal, which with the OE springs from would yield about 27 @ 2700 compared to the original 24 @ 2350, and both "curves" would be essentially linear starting from about 700. It would also include the 12" B26 VAC, not the 8" that was used on the 365 HP engine.

The three degrees lesser maximum centrifugal would require the initial timing to be increased to 11 nominal +/- 2 degrees which would yield 36-40 total WOT advance, and if the engine can tolerate 39-40 without detonation, that's where I'd run it. Total idle timing would also increase about three degrees.

My underlying thinking is that '65-'67 L-79s have the same centrifugal curve as the '66-'67 300 HP engines, but it would work best with something between these late 300 HP engines and the 365 HP engine.

This would involve some work including molesting a 365 HP distributor. The question is, would it improve the bottom end torque? Frankly, I'm not sure I'd bet that it could be seen on a chassis dyno outside the normal run to run variation of about one percent.

For the 20 bucks in bolt-on parts that were spent to dial in this L-79 distributor, I think it's pretty well optimized, however, if some lighter than the gold springs could be found, I'd recommend trying them out.

Duke

[William F.]

Re: L-79 distributor blueprint overhaul

Duke,
Thanks for reply. Still want to make sure I understand what you recommend as best 'quick and dirty" setup for the factory L79 distributor, not using a 365 hp modified one. Just use lighter springs to let full centrifugal advance come in early and make sure you have a B26 vac can and set initial to get the total advance you specified? Is this best for general street use and engine longevity or more of a race setup, realizing you've always said that racing and street setups are different?Will I be risking detonation, possibly inaudible but still causing undue ring wear? Why did GM have the full amt. of centifugal coming in at 5100 rpm rather than later?
Thanks

[Wayne M.]

Re: L-79 distributor blueprint overhaul

..... Jim had already installed the Echlin CS7860 32 oz. point set with the trick little lubricator wick, and I dabbed on some of the 1960954 grease. ......

Duke -- is this the trick wick you're referring to ? (red arrow -- 1st thumbnail). This non-Delco point set was on a '65/6 passenger 396 distr 1111109 I recently bought. No stampings to indicate manuf. I'd never seen one of these before. For comparison, I show a stock D-106P Delco set, above, which shows the normal cam rubbing block. This distr. still has its original square felt wick (green arrow).

I also include underside pic of the rotating (weight plate) cam and the autocam (football) for those who collect these numbers. (2nd thumbnail).

[Timothy B.]

Re: L-79 distributor blueprint overhaul

Wayne, What's interestering is that I think I see a snap ring around the centrifugal advance stop bushing on the main shaft. I have never seen a snbap ring here before even though every distributor I have ever worked on has the groove for it.

Willian, I can't speak for Duke but I think what you state about the light springs is correct in that it's a easy way to pick up performance for a street driven car. The reason for the slower factory advance curve is because people drive cars differently in that some will lug a engine rather than downshift like us hot rodders :-).

[Duke W.]

Re: L-79 distributor blueprint overhaul

Yes, that's the trick felt lubricator I referred to. It's only included with the Echlin CS7860 32 oz. point set, and from the colors and construction features I can see in your photos, I think that's what you have. I recommend this point set for all engines with redlines over 5500. None of the other Echlin point sets I inspected included this felt lubricator, but they all should. I wrote an article about Echlin point sets that was published in the The Corvette Restorer a few issues ago.

As I stated in my San Diego seminar, in most cases OE spark advance maps are a compromise for the widely varied conditions that owners drove their cars in. The only one that's right on the money from Chevrolet is the '64-'65 L/76/84 with about 14 deg. initial advance.

Most other engine configurations can use some help in the form of a more aggressive centrifugal advance and in some cases, a different spec vacuum advance. One of the PowerPoint slides has recommended starting points for the various vintage Corvette engine families.

Optmizing the spark advance map usually results in noticeably better fuel economy, more low end torque, and sometimes more top end power, particularly on emission controlled engines that had not only lazy centrifugal curves, but they were short of total WOT advance, due to short curves and low initial timing values.

Duke

[Wayne M.]

Re: L-79 distributor blueprint overhaul

..... I think I see a snap ring around the centrifugal advance stop bushing on the main shaft. I have never seen a snap ring here before even though every distributor I have ever worked on has the groove for it. ......

Tim --- it's present on all distribs with a vacuum advance (plate). both points and T.I., AFAIK. I've got an NOS plate /shield package assembly and it comes with a new retaining spring. Wish I could get a few more, because they seem very flimsy and I'm thinking that they should really be replaced each time removed. Alas, can't seem to find them at GM [Gr 2.835, #1932024]. I wonder if an auto parts replacement exists ?

[Duke W.]

Re: L-79 distributor blueprint overhaul

I assume it's a generic C-clip - probably 1/16". Measure the pin diameter.

I didn't see a clip or the groove on this distributor. It could be the bushing, which looked original hid the slot and clip. We had a new bushing from a Corvette supply house, but I wasn't sure if it was exactly like the original, which was in good shape, so we decided to leave it alone.

Duke

[Jim T.]

Re: L-79 distributor blueprint overhaul

Enjoyed your above posted information Duke. In the future I will be removing my 68's 327/350 L79 distributor to install a new main shaft because of tack gear failure. I will inspect the ground wire. After reading your post about the ground wire available from NAPA I gave my local NAPA store a call to see if it was available. It is available, has to be ordered and take a few days. Cost is less than $2.00, however they have to order it and it comes from the manufacturer and I will have to pay shipping. Shipping cost $6-$7.00. I shimmed my 68's distributor many years ago when I learned what the clearance should be. I have a after market distributor spring and weight set I bought about 42 years ago that I have never used, I am going to locate it and see if I have gold colored springs in the selection. It very well could of been a Mr Gasket product.

[Duke W.]

Re: L-79 distributor blueprint overhaul

I think your '68 L-79 has the same centrifugal curve as '67. I'm not sure about the VAC. Check the CSM and AMA specs.

Your '68 probably has ported vacuum advance since it's an emission controlled engine. Convert it to full time with a B26 VAC as long as it doesn't have to be emission tested.

Duke

[Jim T.]

Re: L-79 distributor blueprint overhaul

Duke the ported vacuum advance I have been aware of for 42 years. It was that long ago when I went direct full time vacuum advance on my new 1970 bypassing the TCS solenoid. The 68 also has full time advance and the air injector pump still works. I have a NOS NAPA Echlin VC1810 B28 I bought 10/23/2003 for the 68 to install when the original vacuum unit quit working. Would the B26 be a better choice? Do not have emission tests or car inspections to be concerned with.

[Duke W.]

Re: L-79 distributor blueprint overhaul

L-79 with the OE cam idles at about 750 @ 14-15", so a 12" VAC meets the Two-Inch Rule. I don't like to see a more aggressive VAC than necessary to meet the Two-Inch Rule because it may cause part throttle acceleration or upshift transient detonation, which is why I recommend the 12" B26 for L-79s.

The quicker centrifugal with light springs will increase total cruise timing, since most cruise conditions are at greater than 12", but the B26 will back off quicker than the B28 and prevent detonation if you add a little throttle to increase speed or are climbing a grade while comfortably maintaining full vacuum advance at idle.

L-46/82 with manual transmission has essentially the same idle characteristics as L-79, so the B26 is the best spec VAC for those configurations when converted to full time vacuum advance. L-82 automatic will generate less vacuum idling in Drive, so it probably needs the 8" B28. L-82/auto owners should check manifold vacuum idling in Drive and apply the The Two-Inch Rule.

Duke

[Wayne M.]

Re: L-79 distributor blueprint overhaul

I assume it's a generic C-clip - probably 1/16". Measure the pin diameter.

I didn't see a clip or the groove on this distributor. It could be the bushing, which looked original hid the slot and clip. We had a new bushing from a Corvette supply house, but I wasn't sure if it was exactly like the original, which was in good shape, so we decided to leave it alone. ....

Duke -- here's a little more detail. The NOS clip called "breaker plate retainer washer" in the instructions included with the Delco plate package, is spring steel, very thin and wavy. There's probably a technical name for this, but I believe the purpose of the wave is to exert downward pressure on the plate.

You can see that there's a bevel on the lower edge of the clip groove of the upper bushing. That might figure into the design, somehow.

F.I. distrib's might be different. It was 40 years ago, and I never dismantled the two I had.

[Duke W.]

Re: L-79 distributor blueprint overhaul

Jim's distributor had what was probably the original "wavy spring washer" retaining the breaker plate. In fact, beginning with my inspection of the gear retaining roll pin, I doubt if this distributor had ever been disassembled. The small parts kit he bought, had a new one, but it was not wavy, so I reused the old one.

Same with the upper bushing grease well seal. The "new one" was just a flat nylon washer, not cup-shaped like the original, so I reused the original. Both this seal and the breaker plate retaining wavy spring washer can usually be removed non-destructively and reused. In many cases the old OE parts are better than replacement parts, which are just generic designs.

I should have used a magnifying glass to inspect the pin and advance limit bushing more closely. With just my reading glasses, I could not see a spring clip on the end of the pin, but it's quite possible that one was there - just hidden because the bushing was butted up right next to it.

Duke