Erratic Timing at 3800+ ('67 427/390)

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

One thing that can cause ignition problems is a wobbly breaker plate. It was a problem on my SWC from day one and took me a couple of years to figure out. The dealer was worthless.

The breaker plate should fit very snuggly over the distributor housing. When you push down on the edge is should not dip. The force of the breaker arm spring places a significant downward moment on the breaker plate and, if it's not snug it can wobble, which reduces dwell that can result big spark scatter and misfiring.

Free rev it to 5000. Dwell should not fall by more than two degrees.

The standard 19-23 oz. breaker points should be satisfactory given the 5300 redline. What make/model did you install and what is the breaker arm tension?

How much does the timing vary up and down at various engine speeds (specify), and under load does the engine misfire?

Duke

[David M.]

Re: Erratic Timing at 3800+ ('67 427/390)

Do you have a good wire diagram of the OEM ignition & cranking circuits?

Cranking volts at 13.7 is correct = direct battery volts.
Running VDC is way too low at 5.24VDC. This is likely the problem. The plugs wont fire under increased load. The coil, the coil wires or the resistor are not properly matched.
I dont think a factory GM points system in a Vette requires external ceramic ballast resistors. I know All GM A-bodies of this era use a resistive wire in the harness. The ballast is there to limit current to the points while running which increases point set life. Running the ballast wire through another ceramic ballast is doubling the resistance of the circuit. OEM coils are matched to the resistor wire.

Try bypassing the ceramic ballast resistor.

If the factory resistor wire(assuming it has one) to the positive side of the coil is still intact and you are running that side through another external resistor you are doubling the resisting in this circuit.

An after market coil or distributer my or may not require external resistance(ballast resistor/resistor wire). OEM Points coils generally run around .02-.3.0 Ω, 5-20 KΩ on the secondary side.

Mechanically your distributor sounds ok.

Does the engine sound erratic when the timing visually goes erratic?

Your charging VDC is at the lower side the spec but is acceptable. Some charging system grounds may need cleaning. Same for both ends of both battery cables and the ground strap from the block to the frame.

The plugs look lean but they are new.

It sounds like you are pretty proficient with electrical/mechanical things.
Slow methodic elimination concentrating on each system while writing your findings will help to keep chasing your tail.

[David H.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen

Don't bypass that ceramic ballast resistor. If you do, you will be replacing your coil (fried). Purpose of ceramic ballast resistor is to drop voltage to coil during normal engine operation.

Starter circuit temporarily bypasses your ballast resistor. This is to temporarily increase voltage to coil to aid starting.

Good idea to check resistance on your ballast resistor re spec, but don't eliminate it from your ignition circuit.

Dave


BYW: was your distributor cap replaced during rebuild? Any evidence of internal/external arcing?

[Joe L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Do you have a good wire diagram of the OEM ignition & cranking circuits?

Cranking volts at 13.7 is correct = direct battery volts.
Running VDC is way too low at 5.24VDC. This is likely the problem. The plugs wont fire under increased load. The coil, the coil wires or the resistor are not properly matched.
I dont think a factory GM points system in a Vette requires external ceramic ballast resistors. I know All GM A-bodies of this era use a resistive wire in the harness. The ballast is there to limit current to the points while running which increases point set life. Running the ballast wire through another ceramic ballast is doubling the resistance of the circuit. OEM coils are matched to the resistor wire.

Try bypassing the ceramic ballast resistor.

If the factory resistor wire(assuming it has one) to the positive side of the coil is still intact and you are running that side through another external resistor you are doubling the resisting in this circuit.

An after market coil or distributer my or may not require external resistance(ballast resistor/resistor wire). OEM Points coils generally run around .02-.3.0 Ω, 5-20 KΩ on the secondary side.

Mechanically your distributor sounds ok.

Does the engine sound erratic when the timing visually goes erratic?

Your charging VDC is at the lower side the spec but is acceptable. Some charging system grounds may need cleaning. Same for both ends of both battery cables and the ground strap from the block to the frame.

The plugs look lean but they are new.

It sounds like you are pretty proficient with electrical/mechanical things.
Slow methodic elimination concentrating on each system while writing your findings will help to keep chasing your tail.

David———

All C2 Corvettes (except K-66) used an external ballast resistor. 1967’s used a 1.8 ohm resistor.

All C3 Corvettes used a resistive wire rather than a ballast resistor.

[Bill B.]

Re: Erratic Timing at 3800+ ('67 427/390)

Hi Owen,

Not to add another variable to this, but did you try another timing light? I had one years ago that would not fire steadily at higher RPM. Worked fine at lower RPM. I can't recall if it was erratic or not (too long ago) but when I used another timing light, the issue completely cured itself.

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Hi Owen,

Not to add another variable to this, but did you try another timing light? I had one years ago that would not fire steadily at higher RPM. Worked fine at lower RPM. I can't recall if it was erratic or not (too long ago) but when I used another timing light, the issue completely cured itself.

Ah, if this had been that easy. I started with a 40 year old Craftsman dial-back light and it was previously suggested that may be the issue so got an Innova 5568. Same thing. Yesterday I noticed that the timing light goes from almost imperceptible flashes to very strobed when it hits the rpm wall. The #1 plug is definitely misfiring and not steady -- didn't try it on the others but may as that could indicate a bad cap tower/wire/plug.

[Bill B.]

Re: Erratic Timing at 3800+ ('67 427/390)

Ah! Just to rule out an issue peculiar with #1, try connecting the timing light to the #6 plug, as this should also line up with the damper timing mark just like the #1 cyl should.

If the timing is not erratic above 3,800 rpm, then an issue with #1 for sure. If also erratic just like the #1, that eliminates a firing issue unique to #1.

(Firing Order 1-8-4-3-6-5-7-2 so #6 fires exactly 360 degrees of crank rotation after the #1)

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Do you have a good wire diagram of the OEM ignition & cranking circuits?

Yes though find them challenging to trace - plus, 'lectricity and all that.

Running VDC is way too low at 5.24VDC. This is likely the problem. The plugs wont fire under increased load. The coil, the coil wires or the resistor are not properly matched.
<snip>
Try bypassing the ceramic ballast resistor.

If the factory resistor wire(assuming it has one) to the positive side of the coil is still intact and you are running that side through another external resistor you are doubling the resisting in this circuit.

As others mentioned, the ceramic ballast resistor is the factory configuration, but what's in the car appears to have a higher resistance than the 1.8 ohms spec'd. To check coil voltage while running, do I merely touch one lead to one coil side terminal and the other to ground then measure the other terminal the same way?

An after market coil or distributer my or may not require external resistance(ballast resistor/resistor wire). OEM Points coils generally run around .02-.3.0 Ω, 5-20 KΩ on the secondary side.

From everything I've read, all three coils I have seem to be within the proper ranges.

Your charging VDC is at the lower side the spec but is acceptable. Some charging system grounds may need cleaning. Same for both ends of both battery cables and the ground strap from the block to the frame.

I'll have to investigate all of this. I know that the alternator ground is a bit rusty looking and definitely needs a cleaning. I'll need to investigate the voltage regulator connections and contacts too. Is there a Restorer article on regulators? (I have the original, but a replacement is in the car.)

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Ah! Just to rule out an issue peculiar with #1, try connecting the timing light to the #6 plug, as this should also line up with the damper timing mark just like the #1 cyl should.

If the timing is not erratic above 3,800 rpm, then an issue with #1 for sure. If also erratic just like the #1, that eliminates a firing issue unique to #1.

(Firing Order 1-8-4-3-6-5-7-2 so #6 fires exactly 360 degrees of crank rotation after the #1)

Excellent suggestion! (So one could time the engine off of #6 too - if one were a rebel?)

[Larry M.]

Re: Erratic Timing at 3800+ ('67 427/390)

Excellent suggestion! (So one could time the engine off of #6 too - if one were a rebel?)

If you marked your balancer with marks at exactly 90 degrees apart, you could time your engine off any cylinder.

I have done this in the past with a few engines............but it is not a preferred way. I remember timing a 1970 Jaguar OHC 6 off the #2 plug wire. But that is a story for another time. .

Larry

[Owen L.]

https://youtu.be/QPN5ZOhZYHA

I guess at this point all I have left is the timing chain.

[John P.]

Re: Erratic Timing at 3800+ ('67 427/390)

Hello Owen, If the timing chain is very loose it can cause the camshaft to "Walk" forward and back in the block changing timing at certain engine RPM. Remove your distributor cap, then turn your crankshaft balancer opposite engine rotation to remove the slack in the chain. Slowly turn the crankshaft balancer in the correct engine rotation while observing the distributor rotor movement. When the rotor begins to move, observe the balancer and see how many degrees it has moved to take up the slack. If you end up changing the timing chain set you may want to consider installing a cam button kit. This is simply a spacer-bearing that bolts to the front of camshaft and is held in position with the timing chain cover. This prevents cam movement in the block and also helps timing chain wear. Definitely not a requirement, but a consideration.

[Owen L.]

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Dang, pulling the harmonic balancer is aerobic exercise as well as works the deltoids!

The Grand Prize goes to Joe Lucia for being the one to call my attention to the timing chain in another thread:

A "non-invasive" "clue" to indicate deterioration of the cam sprocket is to observe the timing mark illuminated by a timing light. If the mark "jumps around", even a little, the timing sprocket is worn or deteriorated. If it "jumps around" a lot, you're "living on borrowed time".

Here it is, in all it's broken glory. Nylon is cracked in several places, missing in two, and every single tooth has worn through the nylon and into the aluminum "under teeth". As a matter of fact each tooth as the distinct impression of the timing chain links and comes to an almost knife edge. The chain was very sloppy. Now to order parts...

cam gear.jpg cam gear teeth.jpg

[John P.]

Re: Erratic Timing at 3800+ ('67 427/390)

Hello Owen, Well looks like you found the problem. On my 67 big block, all I had to do is drop the idler arm mount at the frame and lower it to gain enough clearance. I did however remove the complete center link and tie rods to give easy access to the underside of the engine. The cam buttons are designed primarily to fit under the stock timing cover. There are also ones designed to fit under nonstock covers for special race or design applications. Cam buttons are usually used on roller cams, however I have found older engines with flat tappet cams show timing fluctuations from cam walk due to worn cam bearing, cam lobe and lifter bases. Again, the cam button is one more bit of insurance. Check the listings on the "Summit Racing" or "Jeg's" sites for units offered. You may want to install the simple nylon design as you are not using it for race purposes. As you stated, it is a good time to inspect the crank rod and main bearings. Also inspect the rear main oil seal for leaks, as it's a good time to replace it. Don't you love how these old car projects always lead to other issues. Stick to it, you have a great and collectable car!