Ignition / Electrical Gremlins

[Gaz64]

Originally Posted by Gaz64

You have changed from an analogue 6A to a digital summit.
I know changing from 6A to 6AL causes issues with rotor phasing.
Get a spare distributor cap, drill a large hole in the top, and observe phasing with a timing light.

Just another thought, the analogue and digital may trigger on different edges, one could be rising, the other could be falling edge.

So drilling a hole in the cap, checking with a timing light is on the agenda.

[cycleguy55]

Originally Posted by Texasdoc

Usually popping on decel is lean fuel.

And the surge / miss feels to me like a classic 'lean surge'. It was worse when I finally got the car running after 3+ weeks, then seemed to fade after being driven a bit. The biggest improvement was in the first 5-10 miles, then less after that.

I wonder whether the 3+ weeks of not running and the hot weather we had evaporated the fuel in the carb and left some deposits / varnish behind that are causing the problem.

I'm going to try running some carb or fuel system cleaner through it and see if that helps or resolves the problem. If not I'll be pulling the carb and tearing it apart.

[Texasdoc]

Another thought is to make sure you connected the new Summit 6AL box to the distributor correctly. Does your distributor have green and violet wires or two black wires? MSD in their infinite wisdom made the wire colors wrong. If you have two black wires, make sure the black wire with the orange stripe connects to the violet wire on the Summit box. The black wire with the violet stripe goes to the green wire. The black wire with the violet stripe DOES NOT go to the violet wire!

[cycleguy55]

Originally Posted by Texasdoc

Another thought is to make sure you connected the new Summit 6AL box to the distributor correctly. Does your distributor have green and violet wires or two black wires? MSD in their infinite wisdom made the wire colors wrong. If you have two black wires, make sure the black wire with the orange stripe connects to the violet wire on the Summit box. The black wire with the violet stripe goes to the green wire. The black wire with the violet stripe DOES NOT go to the violet wire!

My distributor has one wire that's black with green and the other is black with violet. The black wire with the orange stripe is connected to the violet wire on the Summit box, and the black wire with the violet stripe goes to the green wire. The polarized plug and colour coding on the wiring is exactly the same as MSD so therefore pretty difficult to get wrong. That's not surprising considering MSD makes these boxes for Summit.

[cycleguy55]

I tested the resistance of each of the ignition wires, partially to see how they compared to the 'High Performance' wires, and also how they compared to each other. I have Belden 'Performance Silicone' 8mm wires, and here's how they measured up:

Cyl - Ohms
1 - 10K
2 - 8K
3 - 10K
4 - 17K
5 - 6K
6 - 8K
7 - 7K
8 - 6K

Summary:

1. Too much variance, wire to wire, even considering #4 is one of the longer wires - it's not that much longer.
2. Many HP wires are much lower in resistance - often 50 - 150 ohms / foot - a fraction of what mine are. MSD’s 8.5mm Super Conductor Wire is about 50-ohms per foot while theire Street Fire wires are about 500 ohms per foot.
3. Going to install a new set of wires.

One set of wires that seems to be well reviewed is these https://www.summitracing.com/int/par...r460/overview/

They're 9mm wires with the right ends and boots. It's not listed on Summit, but found elsewhere, is the indication these are 1,000 ohms/foot - much lower than what I currently have, but should I be looking for something lower? Taylor Spiro-Pro wires are 350 ohms/foot (https://www.summitracing.com/parts/tay-74299) and there appears to be no end of variety - from 40 ohms/foot up.

Thoughts?

[Texasdoc]

Those numbers do seem high.

I've run both the street fire and the super conductor wires with my non-resistor plugs. I didn't see much difference.

[Gaz64]

About 3000 ohms per foot is a good silicon lead.

Any spiral wire lead is an improvement, but think about the airgap (rotor to cap terminal) resistance. Obviously there are two sparks occurring at the same time for each cylinder to fire. When you can run the gap down to near zero, there is less loss in the cap.

Gary

[Dwight]

I buy and use the MSD kits that I have to make up one end. That way the wires are the correct length for what ever motor I installed them on. Much better looking that factory made wires.

The last set I measured was MSD 8mm and found two were twice the ohms reading of the others.

I removed the boots and check from conductor to conductor. On one the factory crimp was bad. The other wire I could not prove which end was bad, it might have been the end that I made up. So I replaced both ends and got a good reading.

I would suggest you measure the wire by putting the meter probe on the conductor not the steel terminals. If you get a different reading than terminal to termingal then check the crimp.

Dwight

[Texasdoc]

You have 16 base, 21 mechanical totaling 37. If you are using the stock MSD vacuum can, that will give you too much advance at light throttle - around 52°. You can either:

1. Run it without vacuum advance. Lots of people do. It is designed for fuel economy while cruising. Not a big need in a cobra. No need to switch out to your old distributor. You can leave the can disconnected and it won't have any effect. Just make sure to plug the carb side so you don't have a vacuum leak.

2. Change out the advance limiter in the distributor. If you use the black one, you will only get 18° of mechanical advance. Set your total to 34, still giving you a base of 16. Those few degrees *may* keep it from over advancing. (Doubt this will work with the stock MSD vacuum can. These number are for a SBF - YMMV)

3. Replace the vacuum can on the distributor with an aftermarket adjustable piece. I use the Accell 31034 unit (Ckicky - get the GM part, not the ford part) and can limit the vacuum advance to around 7° instead of the 15° the MSD one provides.

BTW, I recently switched to a CB Performance black box to control my ignition and can set any timing at any RPM/Vacuum.

Forget the inline smog vacuum reducer. Waste of time, money, and effort for the limited benefit it will provide.

[cycleguy55]

MSD says their VA is good for 10° of advance, but 47° is still too much. I could change springs to slow the centrifugal advance, but recommendations I've seen are pretty universal in calling for advance to be all in by 2500 RPM. I'll almost certainly be staying pure CA.

The curious part is why the VA and the same timing settings worked fine with the analog box, but not with the digital box.

[Texasdoc]

Most MSD distributors vacuum cans add about 15°, regardless of what they tell you in the phone.

I expect you would be fine at light throttle with 47° advance. You are probably getting quite a bit more. However, when you start to accelerate and the throttles open, the manifold vacuum should drop, decreasing the advance added buy the VA system. The problem with the cobra is that with such a powerful engine in a light car you don't have to open the throttle much to get light acceleration so the VA system doesn't drop out and you start to detonate.

Make sure you disconnect (in your brain) the amount of vacuum provided from when it is applied. The springs control WHEN the mechanical advance occurs. The bushing controls (limits) how MUCH advance occurs. You can still have it all in at 2500, but only get 18 degrees by changing to the black bushing. Leave the springs alone to keep the curve (the when) the same.

The two light springs are ok if your idle is less than 800. If your idle is over 800 you can start to get some MA at idle. I ran one light silver and one blue spring. That way the advance didn't start until 1200 but was still all in by 2800 (numbers from memory and are close but maybe not exact). If your idle timing fluctuates, try switching one spring to the light blue one.

Originally Posted by cycleguy55

The curious part is why the VA and the same timing settings worked fine with the analog box, but not with the digital box.

I agree. However, as Gaz64 mentioned, it has been noted there are significant differences between the analog and digital systems