I agree with Cliff. It seems to be a rear carb mixture problem of some sort. How many turns down are the nozzle screws? However, that doesn't explain #5 and #6 looking like #4. As I recall, #4 alone was the problem child. @jalexquijano What was the outcome of changing the valve seals? Was one of the seals on #4 damaged and did you change them all or just #4?

Just changed the seals on cyl 4

 Well, it would appear we're back to the rear carb. How many turns down are the mixture screws on both carbs?

On 9/25/2021 at 7:53 PM, Mark Maras said:

 Well, it would appear we're back to the rear carb. How many turns down are the mixture screws on both carbs?

Ill check on this today. Anyway yesterday while parking the car i decided to turn on the air conditioner and the engine started to accelerate randomly. If i turn the AC compressor off, the engine will go back to idle speed. Could the electric solenoid i installed a couple of years ago be faulty? 

Just now, jalexquijano said:

Ill check on this today. Anyway yesterday while parking the car i decided to turn on the air conditioner and the engine started to accelerate randomly. If i turn the AC compressor off, the engine will go back to idle speed. Could the electric solenoid i installed a couple of years ago be faulty? 

How do i check on this? 

The simple way would be disconnect the wires to the solenoid. Do you have any theories why #s 4, 5, and 6 are fouled now? As I recall #4 was the only fouled plug previously and changing a couple of valve seals doesn't explain #s 5 and 6 getting worse. Was anything else worked on or fixed during the valve seal installation?

On 10/11/2021 at 7:31 PM, Mark Maras said:

The simple way would be disconnect the wires to the solenoid. Do you have any theories why #s 4, 5, and 6 are fouled now? As I recall #4 was the only fouled plug previously and changing a couple of valve seals doesn't explain #s 5 and 6 getting worse. Was anything else worked on or fixed during the valve seal installation?

Nothing else. Ill check on the fuel mixture knob to verify the amount of turns. 

Checked on the mixture knobs of both 3 screws round SU CARBS and they had 4 turns clockwise each. I checked on the balance to verify both open at the same time and lowered the mixture knobs to 3.5 turns each. Now the car has less power and started popping / backfire in the exhaust when releasing the gas pedal. I believe it should not backfire has the mixture is leaner. Any thoughts? 

If you went from 4 turns to 3.5 turns, you made it leaner!

 

Then why the backfire when releasing the gas pedal if its leaner? 

1 hour ago, jalexquijano said:

Checked on the mixture knobs of both 3 screws round SU CARBS and they had 4 turns clockwise each. I checked on the balance to verify both open at the same time and lowered the mixture knobs to 3.5 turns each. Now the car has less power and started popping / backfire in the exhaust when releasing the gas pedal. I believe it should not backfire has the mixture is leaner. Any thoughts? 

 When you say clockwise, is that viewing the mixture screws from the top or from under the carbs? Your thoughts on 3.5 turns down vs 4 turns being leaner is correct. You mention both carbs opening at the same time,which could be linkage related, are pulling the same amount of air at idle?

From the top lowering the jet to allow more fuel.