Not likely issue. Hydraulic pressure at carb-inlet should be sufficient regardless since it's below outlet of tank. However, in low-vacuum situations such as cold-starts, every little bit helps.
Having some measured numbers would help:
- vacuum at petcock-inlet when cranking
- petrol volume flowed from petcock in 10-seconds of cranking (aim fuel-hose into measuring cup)
There have been instances of leaking/crimped vacuum-hoses limiting vacuum to petcock, thus it doesn't open enough for full fuel-flow. Or petcock is clogged/damaged such that it doesn't flow sufficient fuel. Ultimate test is just to measure output-flow in 10-seconds and compare to spec in manual.
Then you can rule out completely: tank, tank-cap vent, petcock, vacuum-line, fuel-line, fuel-filter.
Also do compression test.
After that, carbs would need closer examination.
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