65 L78 initial timing

[Lawrence S.]

[Mark E.]

Re: 65 L78 initial timing

Start by disconnecting VAC to isolate and tune initial plus centrifugal advance. A good baseline is 38 to 40 degrees. How does it run? Idle okay? Any detonation? WOT performance? It's not unusual for high perf engines like yours to like 14 degrees initial but be sure initial + centrifugal advance doesn't exceed about 40 degrees. Experiment with what the engine likes. For example, maybe it likes initial at 10 degrees and total at 38... re-curve as needed.

Once initial and centrifugal are dialed in, THEN connect VAC and tune for that. I recommend using ported vacuum for better initial response and to avoid too much advance at idle. Test during cruise and part throttle, light loads which is when the VAC affects timing. If pinging returns with the VAC connected, use a more conservative can. They don't look original, but I like a variable VAC that allows fine tuning.

[Duke W.]

Re: 65 L78 initial timing

The free GM Heritage vehicle information package for '65 Corvette lists the L-78 centrifugal advance as start @ 1000, 28 @ 4600, so to get total WOT advance in the correct 36-40 range initial timing, set at less than 1000, should be in the range of 8-12, and the AMA specs say 10.

Of course, you need to use a dial back timing light to check the centrifugal because it's common that they were modified over the years.

So it's no wonder the engine detonates at 16 initial, and if the centrifugal is "stock" it may tolerate being brought in quicker.

Since the L-78 is essentially a L-72 with a smaller bore it should have similar idle behavior, 14" @ 900, so the OE 201 15 VAC does not pass the two inch rule, and since since it takes 15.5" to pull to the limit. Rule. The L-72 VAC IIRC is IDed as 360 12 and starts at 6" with a max of 12 @ 12"

A B26 is 16 @ 12", which passes the Two-Inch Rule, but big blocks don't seem to like as much low speed, low load advance, as small blocks so you may need to contact Lars and buy one of his vacuum advance limiters if the the B26 causes any driveability problems.

There is a 1965 Chevrolet Interim Shop Manual that a friend bought a couple of years ago as a condition of me working on his L-78 driveability problmes (It turned out to be a screwed up distributor installation.) This manual has all the L-78 technical and service information. Every L-78 owner should have one.

I documented the above both here and on the CF a couple of years ago and if the cap window of the distributor is not near perpendicular to engine centerline and/or the #1 plug wire is not in the cap tower next to the pass. side of the cap window your distributor assembly and/or installation is screwed up.

Duke

[Lawrence S.]

Re: 65 L78 initial timing

Duke is this distributor position to far to the driver side of the car? I am at 14* initial timing here.

[Chris H.]

Re: 65 L78 initial timing

I'm sure Duke will chime in, but yes the VAC should be pointing towards the passenger fender. The distributor install is off by maybe one, if not 2 teeth.

[Lawrence S.]

[Mark E.]

Re: 65 L78 initial timing

Yes, if clocked as per the factory the can should point more directly to the right of the car, but timing isnt affected by how the distributor is clocked. It's just a recommended orientation to allow enough movement to time the engine.

You need to look at timing; see my prior post.

[Larry E.]

Re: 65 L78 initial timing

Lawrence: If the engine is original timing light is not needed. It was done for you at
Tonawanda. Check in back of distributor for the chisel mark. JMHO>Larry P.S. See
picture below.

[Lawrence S.]

Re: 65 L78 initial timing

Lawrence: If the engine is original timing light is not needed. It was done for you at
Tonawanda. Check in back of distributor for the chisel mark. JMHO>Larry P.S. See
picture below.

Thanks Larry, I did see that chisel mark.

[Larry E.]

Re: 65 L78 initial timing

I would also use straight 100LL ;no blending. Believe this would help a lot. I did this
And my cars run so much better plus the gas will last (no gumming up) so much longer.
JMHO-Larry

[Duke W.]

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 with step-by-step instructions of how to properly install the distributor starting with how to index the crank before you remove it. It was very well received. This procedure applies to both C2 small and big blocks. It DOES NOT apply to C3s, which have different wire indexing in the cap and gear engagement resulting in a noticeably different final orientation.

https://www.corvetteforum.com/forums...sb-and-bb.html


Please look at the CF thread, follow the instructions for removal, verify proper indexing of the gear dimple and correct if required. Then follow the installation and static timing instructions. Lars also chimed in with an alternate method of getting the shaft to seat in the oil pump drive shaft slot if it doesn't drop right in.

Be sure to read the entire thread. Lars' post #13 includes a photo of the rotor tip and VAC orientation of a properly assembled and installed distributor with the crankshaft indexed with #1 at the proper initial timing point of the compression stroke.

If you have any questions, contact me directly.

Duke

[Lawrence S.]

Re: 65 L78 initial timing

Duke, thank you for writing that paper and all the other papers you have written. You helped me in 2018 on a distributor installation, first one, which I went back and re read the thread along with your recent paper on the CF and my distributor is now installed correctly and the car fired right up. I have not mapped the timing yet due to rain but will do so over the next day or so.
Larry, my chisel marks are lined up on the distributor and intake manifold. Thanks for that tip.

[Lawrence S.]

Re: 65 L78 initial timing

I am still working on getting my timing correct. Decided to pull the distributor because the gasket needed replacing and discovered the magnet had pieces missing. Can this effect the operation of the distributor?
see the pictures.

[Duke W.]

Re: 65 L78 initial timing

I don't understand why you are "still working on getting my timing correct". If the distributor is properly assembled and installed, set initial timing at 10 at below the centrifugal start point and you should be done.

Did you check the centrifugal curve to see it if it OE? Those springs don't look OE. At what RPM does centrifugal start and what is the maximum at what RPM.

Small pieces missing from the magnet might affect operation, possibly causing misfires.

Duke

[Lawrence S.]

Re: 65 L78 initial timing

Before post #12, I had fired the car in the garage ran fine at fast idle. I could not take the car out because of weather. Finally took the car for a drive and it detonated. Tried to get it to idle at 10* no luck. I probably should had done what Mark said above but decided after reading your paper decided to remove it and start fresh using your method. I know I am a glutten for punishment, but at least I would have checked all the boxes. I am learning more as I go through this though. I very well may end up at the same place and if so will just go directly to WOT timing.
I know with my other two cars when I did this I had no issue with getting static where it needed to be and then going through the process. But again will bypass initial and go to WOT if I have no luck with initial.
I have the OEM springs, I put stiffer springs on because of detonation while driving.