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Old October 14th, 2020, 12:17 PM   #16
DannoXYZ
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Name: AKA JacRyann
Location: Mesa, AZ
Join Date: Dec 2011

Motorcycle(s): CB125T CBR250R-MC19 CBR250RR-MC22 NSR350R-MC21 VF500F CBR600RR SFV650 VFR750F R1M ST1300PA Valkyrie-F6C

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MOTY - 2018, MOTM - Nov '17
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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