MGB & GT Forum

Just adjusted timing today: 16 BTDC at idle, which gives 36 at 3250 rpm. Vac hose disconnected when doing timing.
Coil is new within the last year.
CEI ignition, which Jimmy worked on couple of years ago.
New plug wires within last year.

The ignition seems to be breaking up. This is new behavior. Car is sitting in the garage as I recorded this video:

Most of the rev changes above 2500 rpm are happening by itself. Especially noticeable around 14/15 sec mark/3000 rpm when it's almost like a switch.

I do notice that A/F ratio goes very rich when rpms change. At this time I'm assuming it's because the ignition is dropping out for some reason.

Suggestions?

Plan for tomorrow:
1) Refresh grounds for the CEI, and road test to see if same behavior on the road - first with vac advance connected, and then without vac advance.
2) Depending on suggestions, possibly swap in the points distributor and see if any change.

BTW, the car is supercharged.

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Pogo is right.
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The power of reasons is an illusion. The belief will not change when the reasons are defeated. The causality is reversed. People believe the reasons because they believe in the conclusion.

Most likely a grounding issue at the module in the amplifier or the wires picking up noise.

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jeff@advanceddistributors.com

What make/model "new plug wires"?

Also renew the ground connection on the interconnect cable from the module to the distributor.

The grounding lug is attached to the braided shield on the cable to keep it from picking up stray electrical noise.

The GM HEI module in the Lucas AB14 box is, first of all, grounded by the screws holding it into the AB14, and secondly by the box having a good grounding to the chassis. The large 'eye' on the distributor connection cable is solely to ground the shielding for that cable, NOT to ground the box! Ensure the AB14 box has a good grounding to the chassis. and also the cable shielding as well!

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soonerv8
Do it right, do it once!

Also keep in mind that ONLY the module screw that attaches to the end that receives the signal from the distributor is grounded. The screw on the input side of the module for 12V and coil ground is NOT grounded!!!

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jeff@advanceddistributors.com

Ok, took the amplifier apart and cleaned all the connections to/from the module, cleaned the bolts holding it to the case, and renewed the heat transfer paste between the module and case. Verified clean metal/metal contact between the amplifier and car body where the case bolts to the car, including the cable shielding ground. Verified distributor fully seated and clamped in place.

Started the car, let it warm up, and the problem is gone. I’ll take the car out tomorrow and see how it does under load.

Crossed fingers.

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Pogo is right.
—————————————————————-
The power of reasons is an illusion. The belief will not change when the reasons are defeated. The causality is reversed. People believe the reasons because they believe in the conclusion.

8mm MSD Street Fire Spark Plug Wire
Spiral core
500 ohms/ft.

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Pogo is right.
—————————————————————-
The power of reasons is an illusion. The belief will not change when the reasons are defeated. The causality is reversed. People believe the reasons because they believe in the conclusion.

Jeff,
I read an article some years ago, regarding the identifying the various "Families" of GM modules, disregarding the cheap imports. As below.
Question , IS THIS WRONG??

This is why I have always recommended making sure that both securing screws have a clean contact on the front of the AB14.

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soonerv8
Do it right, do it once!



Edited 1 time(s). Last edit at 2024-04-16 04:54 AM by allanreeling.

Alan, yes that's dead wrong. Its the other side that has a ground. I have an old pass/fail HEI module tester that GM offered to its techs. If you hook the ground wire to the wrong collet, it will fail 100% of the time. If you don't get a clean connection to the correct collet it will also fail. Its very finicky, and it holds true to OEM and all replacement modules I've tested over the decades, including 3 times in the last week.

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jeff@advanceddistributors.com

Wow, thanks for that Jeff. Amazing what is written authoritatively, but in error, but would assume the coil end securing "T" screw having a ground connection to AB14 box is not detrimental, otherwise it would be insulated in some way??

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soonerv8
Do it right, do it once!



Edited 1 time(s). Last edit at 2024-04-16 10:51 AM by allanreeling.

All this amplifier stuff is way over my head. But my first reaction to your post was that you had the timing set too high. I think the general rule is 32 before TDC at 3000 rpm. That is what I have set on the GT and I have the roadster at 28 to keep it from pinging. Might be worth a try to lower yours to that to see what happens. Quick and simple diagnostic test.

BH

I'm not sure I understand the question, but extra grounding is never detrimental. In a GM HEI, there is one other ground for the capacitor on the power wire that softens electrical noise. I've never seen them grounded in another location other than the obvious external ground on the box. The factory paint on the outside of the amplifier box can be problematic.

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jeff@advanceddistributors.com

BH,
I don't think the "all in"32 btdc is set at a specific rpm, as such. You rev till the maximum advance is shown, be that 3200 or 3500 or whatever. In other words it's the mechanical advance the bob weights are giving when they hit their stop. When that's done the idle advance is what you are left with, once the vacuum is reconnected. It's not always a failsafe, as timing is generally set to be as much advance as your system will take without pinking under load. (assuming you can hear it??)

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soonerv8
Do it right, do it once!

Several years ago I bought a NOS CEI ignition setup on eBay. I sent it to Jeff asking it be checked for operation and to be set up for a supercharged engine.

When it came back the bag was marked 16BTDC and plugs at .040. I set the idle advance to 16, and my dial-back timing light shows 36 above 3000 rpm.

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Pogo is right.
—————————————————————-
The power of reasons is an illusion. The belief will not change when the reasons are defeated. The causality is reversed. People believe the reasons because they believe in the conclusion.