Could be that the air pilot is blocked in the carb
I had a similar issue with my 32A recently that has a 22mm Amal 626 on it. Bike would start but ran very rich and would hesitate when the throttle was opened. It also refused to idle with the air filter fitted. In my case, the bike had sat with petrol in it since the mid 70s and the petrol/oil had separated with some accumulating in the float bowl and elsewhere seeping out of the petcock and hardening on top of the gearbox casing.
I drained the old fuel out of the tank but there was still some crud in there which I tried to get rid of using a heavy duty degreaser, rinsed out with water and left to dry out in the sun. Looking at the pics of your decomposing tank liner, some of this may have dissolved into the fuel mix and found its way into the carb and is causing these issues.
I then dismantled the carb and removed the idle adjuster and the air control screw. I removed the jets and the air control jet in the float bowl and then sprayed carburetor cleaner into each of the holes where I'd removed these parts. I took a very fine piece of wire and made sure that the two small holes near the carb valve were also clear.
To cap it all off, I used some compressed air to blow out the places where I'd used the carb cleaner. Once the carb was put back together, I wound the adjuster screws in and backed them out about 1 1/2 turns and set the idle and it has been running fine (perhaps a little lean at 30:1 as the plug tip is a little light grey but the exhaust is still smoking lightly).
I think you might find a quick rebuild of the carb with a clean out of the air pilot may be useful. In my case, I think the contaminated old petrol had gummed up the interior passageways of the carb which the cleaning fixed. Your dissolving tank liner may have done the same.
I also found a really useful reference on the Amal carburetor website in the link below which may explain the differences in shape between the different carburetor slide valves you have (there are different shapes for 2 stroke, 4 stroke, alcohol etc.)
I had a similar issue with my 32A recently that has a 22mm Amal 626 on it. Bike would start but ran very rich and would hesitate when the throttle was opened. It also refused to idle with the air filter fitted. In my case, the bike had sat with petrol in it since the mid 70s and the petrol/oil had separated with some accumulating in the float bowl and elsewhere seeping out of the petcock and hardening on top of the gearbox casing.
I drained the old fuel out of the tank but there was still some crud in there which I tried to get rid of using a heavy duty degreaser, rinsed out with water and left to dry out in the sun. Looking at the pics of your decomposing tank liner, some of this may have dissolved into the fuel mix and found its way into the carb and is causing these issues.
I then dismantled the carb and removed the idle adjuster and the air control screw. I removed the jets and the air control jet in the float bowl and then sprayed carburetor cleaner into each of the holes where I'd removed these parts. I took a very fine piece of wire and made sure that the two small holes near the carb valve were also clear.
To cap it all off, I used some compressed air to blow out the places where I'd used the carb cleaner. Once the carb was put back together, I wound the adjuster screws in and backed them out about 1 1/2 turns and set the idle and it has been running fine (perhaps a little lean at 30:1 as the plug tip is a little light grey but the exhaust is still smoking lightly).
I think you might find a quick rebuild of the carb with a clean out of the air pilot may be useful. In my case, I think the contaminated old petrol had gummed up the interior passageways of the carb which the cleaning fixed. Your dissolving tank liner may have done the same.
I also found a really useful reference on the Amal carburetor website in the link below which may explain the differences in shape between the different carburetor slide valves you have (there are different shapes for 2 stroke, 4 stroke, alcohol etc.)
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