L-76 Engine Performance w/H & W Valve Adj.

[George J.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Duke,
thanks. I will use the bolt as you and Stu have.

George

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Before you do, put it in fourth gear, pull on the parking brake hard and torque it to 80 lb-ft to be sure it's properly torqued.

Due

[Geoff B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Hi Guys,

I have a '64 L76 and am attempting to adjust valve lash for the first time. I got Duke & Johns paper on how to adjust them cold and Duke has given me guidance on how to determine #1 TDC. I just want to do a quick sanity check to make sure I understand. I crank the engine over until I see the inlet on #1 close and by pulling the distributor cap, the rotor should be pointing at #1 at that point. Then I crank the engine 110 degrees and that should have me at #1 TDC. Start there, mark the balancer every 90 degress and then follow the pattern spelled out in the paper. Is that correct?

Thanks for any help and I apologize for what is surely stupid qestion to anyone that's done this before.

On my first attempt I assumed that the timing mark on the balancer was #1 TDC and did the adjustment starting from that point, so you can see why I want to make sure I understand how to determine #1 TDC.

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

You don't have to watch the valve nor do you have to count 110 degrees, but these are optional references along with removing the spark plug and feeling for the compression.

Rotate the engine and watch the dist. rotor tip. As it goes past engine centerline the next tower on the cap is #1, so when the rotor is pointing to #1 and you have the balancer notch aligned to "0" on the timing tap, #1 cyl. is at TDC of the compression stroke, which is the starting point for the procedure. There is a good photo of a properly installed distributor with the wires numbered in the 1963 Corvette Shop Manual.

OF COURSE, THE ABOVE ASSUMES THAT YOUR DISTRIBUTOR IS PROPERLY ASSEMBLED, PROPERLY INSTALLED, AND THE WIRES ARE PROPERLY INDEXED ON THE CAP!

There are a lot of distributors out there with the gear dimple 180 out and bubba inspired wire indexing - TOTALLY SCREWED UP!

Then it's just a matter of keeping track where you are in the firing order as you rotate the engine in 90 degree increments.

Using a tablet on a clipboard, I write down the sequence with spaces for the clearances I measure before I make any necessary adjustments, then check off each line as I complete the two valves for that crank position.

Write down the date/mileage, put the sheet in the file and you have a record that tracks valve clearance behavior over the years. I have valve clearance records on my vehicles that go back decades. I like to see what they're doing over the miles and years.

Duke

[Geoff B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Thanks again Duke! As painful as it is for you veterans, hopefully this will help some of the other rookies out there.

I just cranked the engine over and watched the inlet valve cycle, placed my thumb over the spark hole and then cranked the engine until I felt the compression exhaust. I stopped cranking at that point, checked and saw that the rotor was point to plug wire #1. I'm assuming I'm at #1 TDC and ready to start adjusting.


Thanks for the pointer on keeping the measurement history. I'm sure that'll come in handy later.

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

It's exactly at TDC #1 when the rotor is pointing to #1 AND the notch on the balancer is aligned with "0" on the tab.

You're allowed +/- five degrees from that exact indexing at all eight positions.

Duke

[Geoff B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

The notched timing mark on the balancer is about 22 degrees from 0 on the timing tab at this point. I'm guessing that means the distributor is not installed correctly. I'll get my hands on the 63 shop manual and take a look at the diagram.

[Larry B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

You could possible have the wrong balancer or timing tab. I had a 350 timing tab on my 327.

[Stuart F.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Just a little tip; many of us don't do an adjustment very often so we
are inclined to be a little rusty. Besides keeping good records/notes as Duke suggests, one must also be careful with your tools. On my last adjustment, I went to about two or three positions before I discovered I had a .004" gauge stuck to my .008" gauge. I expected to see some difference in tolerances, but it didn't dawn on me that it was that excessive. That's when you start talking to yourself; "Fox! You dumb idiot!!"

That's what I get for applying a little engine oil to my feeler gauges.

Stu Fox

Well, off to the winter meet again today, and probably some time at the Mecum Auction. Expect to meet up with some of our fellow NCRS'ers today.

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

The notched timing mark on the balancer is about 22 degrees from 0 on the timing tab at this point. I'm guessing that means the distributor is not installed correctly. I'll get my hands on the 63 shop manual and take a look at the diagram.

Assuming you have a correct balancer/timing cover/tab, #1 is at TDC of the compression stroke when the notch is at zero on the tab. The rotor orientation is a reference and not precise.

Your distributor is probably assembled and installed correctly if the VAC is about half-way between the interference points and the wires are indexed properly. If it's jammed up against the manifold or back against the plug wire support the gear is probably installed backwards, and you'll have to remove the dist. to properly orient the dimple in the same direction as the rotor.

This issue has been discussed many times. If you have a mongrel cam, it may have to be installed backwards, as some cam vendors don't orient the gear on the cam properly.

Duke

[Paul Y.]

Re: L-76 Engine Performance w/H & W Valve Adj.

I like that Duke, "mongrel cam"

[Geoff B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

The VAC is just about half way between interference points and timing is set to 10 degrees. When checking the diagrams in the 63 shop manual, it looks like the #1 plug wire should be in the spot just clockwise of the distributor window. My #1 plug is just CCW of the window, then all the rest of the plugs are in the correct firing order, but everything is off by one spot. I did rotate all of the wires to the locations indicated on the diagram, but then could not get the car to start.

In the current locations it does start and seems to run well. Though now after having adjusted the valves I am getting an occasional backfire when letting off of the accelerator. I haven't done any adjusting to the carb yet, so that on the slate for this afternoon.

Does anyone have any suggestions about the plug wire placement on the distributor? I've had the car about 3 years and it's always run well. I do know the engine has been rebuilt so I don't know if the plug wires were moved on the cap to accomodate some other change that was made.

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

I'm not surprised. It's all f..., ah, bubbaed up!

Here's what you need to do to set the distributor assembly and installation correct. THERE IS NO OTHER WAY on a C2! The backfires may be because of misfires due to the rotor tip being too far from the cap towers, so you are loosing a lot of spark energy.

1. Set the engine at 10 deg. BTDC #1.

2. Make sure the wires are indexed IAW the shop manual.

3. Remove the dist. and check that the dimple in the gear is pointing the same direction as the rotor tip. If not, knock out the roll pin enough to rotate it 180 degrees, index it correctly and reinstall the roll pin.

4. With the dist. rotor tip pointing approximately along the engine centerline reinstall dist. As the gear meshes the rotor will move about 20 deg. CW. If the oil pump drive won't engage, tweak it as necessary with a paint mixing stick to get the slot lined up with the pin as the gears begin to engage. You might want to pick up a new dist. gasket.

5. Once properly installed, rotate the dist. housing until you observe the points just beginning to open, then back a hair, and snug the bolt, slightly.

6. Start the engine and set the timing and tighten down the dist. bolt.

Your Mickey Mouse dist. installation had no effect on your lash adjustment, but when you get it right - no more backfires and the engine will run much better.

Duke

[Geoff B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

I pulled the distributor and the dimple on the shaft was pointing the same direction as the rotor, so I did not have to pull the pin and rotate it 180. By following the steps you detailed, I was able to re-install the distributor, set the plug wires per the shop maual and get the car started and timed. No more backfires while running in the garage. Only bummer is we got rain all day and I was not able to go out for a drive yet.

Can't wait to get the car on the road and see how it's really supposed to perform. It ran well before, so this should really be enjoyable.

Thanks again for all of the help!

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Very good! Now if you want to play some more, advance the timing to about 16 degrees. If it doesn't detonate, fine. If it does back it off in two degree increments until the detonation goes away.

If it will tolerate more initial advance you should notice more low end torque.

At 16 on the tab the initial is really only about 14, and you can verify with a dial back timing light.

The centrifugal curve on your '64 L-76 is very aggresive and should be all in - 24 deg. by 2350 RPM (check to see if this is the case), so if it's still OE you can just rev it to 2500 or a little more and use a dial back light to set the timing at 38 with the VAC disconnected and let the initial fall whereever.

It's tough to set initial on these engines because you have to get the revs down to 700 or less and the engine will barely keep running at this speed.

Duke