65 L78 initial timing

[Mark E.]

Re: 65 L78 initial timing

It looks like the #1 wire is in the proper cap tower, but the gear engagement is wrong, which is causing a weak spark at the plugs due to rotor-cap tower misalignment...

A couple of weeks ago I started a thread on the CF...

Duke

Duke,

Your CF post and other articles by you on this topic provide a great step-by-step guide which will help ensure the distributor is oriented as originally installed by the factory.

A couple of quibbles about the quote above and this statement on the CF thread you mentioned:

(No GM service literature mentions proper dimple orientation, which has resulted in MILLIONS of improper distributor assemblies over the years and without proper gear installation, proper distributor installation is IMPOSSIBLE, and there are still plenty out there, today!) Improper distributor assembly and/or installation is likely to result in not being able to achieve proper timing and/or significant operational problems. The engine might run, but likely not very well!


No "rotor-cap tower misalignment" issues or timing issues or operational problems occur if the distributor housing is clocked differently, or if the plug wires are clocked differently in the cap, or if the drive gear is installed with the dimple 180 degrees from the rotor tip. These changes just result in a different housing orientation in relation to the engine.

Sometimes a "custom" distributor orientation is desired for things to fit, for example when swapping a Chevy engine into a different chassis...Vega, Corvair, Jag, whatever. In these cases, flipping the drive gear, changing the wire orientation or dropping the housing in a different orientation can sometimes help with clearances and/or make wire routing easier.

Another example with Corvettes- I've occasionally changed the clocking of the distributor to straighten the tach cable after the distributor was recurved.

With this said and to your point, an owner who doesn't really know what they're doing can run into interference problems when they twist the distributor to adjust timing with a non-original orientation.

[Lawrence S.]

Re: 65 L78 initial timing

I installed the distributor exactly as Duke's paper spells out. Car fired right up no issues. After testing and tuning ended up with 14* initial and 40* WOT timing. I am running a 201-15 VAC. With this can am experiencing some low speed low load minimal pinging. I will try the B26 VAC.
Thanks for everyone's help on this!
Lawrence

[Mark M.]

Re: 65 L78 initial timing

Lawrence I have an original 111093 distributor here. The pole piece has 734 ccw stamped on it's bottom and the main shaft has 94 stamped on top of the cam (football) next to the spring post. I believe the centrifical weights are correct which are in the heavier range. If these were changed the curve would change. Hope this helps

[Leif A.]

Re: 65 L78 initial timing

I installed the distributor exactly as Duke's paper spells out. Car fired right up no issues. After testing and tuning ended up with 14* initial and 40* WOT timing. I am running a 201-15 VAC. With this can am experiencing some low speed low load minimal pinging. I will try the B26 VAC.
Thanks for everyone's help on this!
Lawrence

Lawrence,
What is your current vacuum reading with the engine at normal operating temperature and idle at 750RPM?

[Duke W.]

Re: 65 L78 initial timing

Duke,




No "rotor-cap tower misalignment" issues or timing issues or operational problems occur if the distributor housing is clocked differently, or if the plug wires are clocked differently in the cap, or if the drive gear is installed with the dimple 180 degrees from the rotor tip. These changes just result in a different housing orientation in relation to the engine.

I've run into this issue countless times in the last 60 years. The first time was when it happened to me. After removing and disassembling my SWC's 340 HP distributor in an attempt to find out why the ignition was breaking up at as little as 4500 revs I could only get about 10 degrees initial advance before the VAC hit the manifold. I knew this wasn't right. The dealer was worthless as were the Chevy engine "gurus" I consulted.
Their advice to to move the wires one position, which made the car virtually undriveable due to low speed misfiring.

Finally on the third removal I saw the dimple and it was a Eureka moment. The 1963 Corvette Shop Manual said nothing about it, and John Hinckley told me he had never seen any mention of it in any service literature - just distributor assembly drawings.

The most recent screwed up distributor assembly/installation I corrected was a L-78 whose owner had only driven it about 400 miles in ten years due to a severe case of "trailer hitching". I documented this both here and on the CF about two years ago.

Of the 208 possible combinations of wire indexing, gear meshing and dimple orientation, there might be others that maintain sufficient rotor/cap terminal alignment to not cause misfiring under some engine operating conditions, and with Corvette, especially C2 small blocks, I know of only one that works properly... the way Chevrolet did it.

One other combination that works is as on C3s, but I doubt if that would work on C2s due to the different cowl geometry and coil bracket.

In my 60 years of experience with this issue on Corvettes, both for myself and many others I consider the factory wire and dimple indexing and gear meshing to be the only way to install the distributor on C2 Corvettes. And this forum is about vintage Corvettes, not installing Chevy V-8s in Vegas and Jaguars. I think you are doing a disservice to the vintage Corvette community by claiming that wire and dimple indexing and gear distributor-camshaft gear tooth indexing can be arbitrary.

If you have a vintage Corvette that has different wire indexing, dimple and cam-distributor gear indexing and allows complete freedom of setting initial timing between 4 ATC and 18 BTC and doesn't have any misfiring, stumble or other drivability issues anywhere in the usable rev range, snap some photos of the distributor with the #1 wire clearly marked.

Duke

[Mark E.]

Re: 65 L78 initial timing

I've run into this issue countless times in the last 60 years. The first time was when it happened to me. After removing and disassembling my SWC's 340 HP distributor in an attempt to find out why the ignition was breaking up at as little as 4500 revs I could only get about 10 degrees initial advance before the VAC hit the manifold. I knew this wasn't right. The dealer was worthless as were the Chevy engine "gurus" I consulted.
Their advice to to move the wires one position, which made the car virtually undriveable due to low speed misfiring.

Finally on the third removal I saw the dimple and it was a Eureka moment. The 1963 Corvette Shop Manual said nothing about it, and John Hinckley told me he had never seen any mention of it in any service literature - just distributor assembly drawings.

The most recent screwed up distributor assembly/installation I corrected was a L-78 whose owner had only driven it about 400 miles in ten years due to a severe case of "trailer hitching". I documented this both here and on the CF about two years ago.

Of the 208 possible combinations of wire indexing, gear meshing and dimple orientation, there might be others that maintain sufficient rotor/cap terminal alignment to not cause misfiring under some engine operating conditions, and with Corvette, especially C2 small blocks, I know of only one that works properly... the way Chevrolet did it.

One other combination that works is as on C3s, but I doubt if that would work on C2s due to the different cowl geometry and coil bracket.

In my 60 years of experience with this issue on Corvettes, both for myself and many others I consider the factory wire and dimple indexing and gear meshing to be the only way to install the distributor on C2 Corvettes. And this forum is about vintage Corvettes, not installing Chevy V-8s in Vegas and Jaguars. I think you are doing a disservice to the vintage Corvette community by claiming that wire and dimple indexing and gear distributor-camshaft gear tooth indexing can be arbitrary.

If you have a vintage Corvette that has different wire indexing, dimple and cam-distributor gear indexing and allows complete freedom of setting initial timing between 4 ATC and 18 BTC and doesn't have any misfiring, stumble or other drivability issues anywhere in the usable rev range, snap some photos of the distributor with the #1 wire clearly marked.

Duke

Duke,

We'll never know why the ignition "was breaking up" on you SWC, but it wasn't because of some sort of "rotor-cap tower misalignment." No such problem occurs by changing how the distributor is clocked or the orientation of the 13-tooth drive gear. A poorly clocked distributor may cause timing adjustment issues due to interference- as you mentioned with your VAC hit the intake manifold.

For Corvettes it is important how the distributor is oriented, more so than other cars... ignition shielding, brackets, high rise intake manifolds, tach cables, limited clearance with the cowl all conspire to limit which orientation the distributor will physically fit.

But here's a thought experiment: Let's say we have a Chevrolet V8 on the dyno with lots of room around the distributor so that the housing can point in any direction without hitting anything. It's timed perfectly and runs great. Now let's change all the plug wire locations in the cap by one terminal clockwise, AND remove the distributor and turn the distributor gear around, AND reinstall the distributor so the rotor is pointing in any random direction. After initial timing is set (by being able to rotate the housing as much as we need without interference) this engine will run exactly as it did before, even after all of these changes.

Think about it.

So neither plug cable indexing in the cap, how the drive gear is installed nor how the distributor is clocked affects performance. But to your point, for our Corvettes, there may be only one way the distributor physically fits in the car that enables proper initial timing.

[Duke W.]

Re: 65 L78 initial timing

Duke,



But to your point, for our Corvettes, there may be only one way the distributor physically fits in the car that enables proper initial timing.

Enough said!

Duke