Carbs 308GTB Popping past 5,000 RPM

I am trying to sort out a 1977 308GTB carbureted car and there is one remaining item I could use some help on. It runs OK up to 5,000 RPM and beyond that point it starts to pop (unsure exhaust or out the carbs) and the car felt like it could not go faster.

The car has Pertronix pickups installed on bank 1 distributor and bank 2 distributor acts as a spark delivery vehicle. So far, these are the things I have tried.

1. Set ignition timing at 10 BDTC at idle, and it does get to 35 BDTC at 5,000 RPM
2. Verify distributors are installed correctly with rotor pointing to the notch at C1 TDC and C5 TDC
3. Replaced the old ignition wires with new wires
4. Replaced rotors with ones that have wider blades
5. Replaced a suspect fuel pump with the correct Facet fuel pump
6. Readjusted fuel floats in the carbs to 50/58 mm

It is now running a lot smoother, easily gets to 5k RPM in 3rd gear, but it still pops past 5,000 RPM, especially on the accelerating side, and less so if the gas pedal is let up.

The distributor caps have some sign of old age but the new reproduction ones did not even make contact on terminal 3 so I ended up re-using the old caps. That is one potential issue yet to be resolved. Suppliers out of Europe don't come back on line until September...

So, other than trying a new set of main jets... and new dizzy caps from Maranello Spares in Europe, I am runing out of ideas. What's next?

 

Yes, still stock jets...

Here is another clue. Picture below, bank 2 is lean, bank 1 is rich

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So, I opened up the rest of the spark plugs. Pictures below, sorry there is no picture for C3 but it looks very much like Cyl 4.

Bank 1: A little richer than normal, except number 2, which is leaner than normal
Bank 2: All four holes are lean to very lean.

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Idle jets are 55, main jets are 135, Emulsion tubes are F36/220. Those are all stock settings.

I have some homework left to do, but suggestions are welcome.

 

I up the main jets on Cyl 5/6/7/8 to 140, next step from 135. Ran the car and it does not feel different. It runs out of steam right past 5,000 rpm and popping. Here are the pictures of the 5/6/7/8 plugs. They do not look different from bebore.

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It seems like the high speed circuit on these 2 carbs are having issues. On a separate note, do these plugs look too lean or just right for you?

 

I have also eliminated compression and timing issues. Compression is 160-170 psi, done warm, throttle open. That means the cam timing is also good.
Fuel is getting to the carbs. I saw the bowel full of fuel when I opened the top off to adjust the floats. Also pressing on the throttle linkages pumps gas into the carburetors.

Nobody has any idea what to try next? I am thinking up the jets one more steps on the lean holes, and down the jets one step on the rich holes and see if the pattern changes.

 

Yes, a restrictor in the return line.
I'll check the coils.
Plug wires are all spiral core, 7mm wires

 

I think I found the problem. I kept thinking why bank 2 is all affected vs bank 1. So I went back and checked the rotor positions vs the alignment notch at cyl 1 TDC vs cyl 5 TDC, and found something that can only happen with Pertronix.

Picture 1 is of Bank 1 rotor position at Cyl 1 Top Dead Center. (Picture taken via a mirror so it is backward)


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Picture 2 is of Bank 2 rotor position at Cyl 5 TDC. See the difference of 40 cam degrees ?
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With this Pertronix, all 8 ignition timing signals are generated and governed by Dizzy 1, and can be read using a timing light on Dizzy 1. Ignition timing via rotor position on Dizzy 2 for bank 2 cannot be verified with a timing light. So at idle, the rotor position is close enough that the spark is delivered to the correct cylinder. At 5,000 RPM, with an additional 25 degrees advance, the spark is delivered to the wrong cylinder and ... pop pop pop is the result.

I repositioned the rotor on bank 2 (pulling it out, turning it one notch, reinserting it) and then turn the dizzy so that the rotor has about the same position as the one on Dizzy 1. I did a few miles of test drive and 95% of the popping is gone. The remainder must have something to do with some tuning issues to be done.

This would not happen IF the conversion was done with two 4 cylinder kits individually installed on each distributor. AND then the timing of each bank can be independently verified with a timing light. That was the case on my old GT4 with XR-300 kits (2).

This is also a problem when you are working on a car that has been modified but you were not the person modifying it.

I removed the 5-8 spark plugs and they are definitely darker than before ... but I only did a few miles of test drive so more to be done.