I'm running 32mm Mikuni round slide with gx390. With 40mm intake and 36mm exhaust. I was told by a friend that with those size valves that i was suffocating my motor and that I should be running a 40mm carb. I was just wondering if its true. It sounds good running but if it is suffocating I guess I should switch or any suggestions will help thanks.
Stock GX390 (88mm x 64mm) 389.4cc. Stock 11.7hp@3600rpm using 8.2cr! 81% Volumetric efficient! Your 32mm Carb can Flow 68.1cfm so I would suspect your CAM.
So what CAM Lift/Dur are you using?
What CR are you using?
389cc at 90% Volumetric efficient at 3600rpm = 15hp and makes 22cfm.
389cc at 95% Volumetric efficient at 3600rpm = 16hp and makes 23cfm.
389cc at 100% Volumetric efficient at 3600rpm = 16hp and makes 25cfm.
389cc at 100%:
At 5000rpm = 23hp and makes 34cfm
At 6000rpm = 27hp and makes 41cfm
At 7000rpm = 32hp and makes 48cfm
At 8000rpm = 37hp and makes 55cfm
At 9000rpm = 41hp and makes 62cfm
389cc at 110%:
At 5000rpm = 25hp and makes 38cfm
At 6000rpm = 30hp and makes 45cfm
At 7000rpm = 35hp and makes 53cfm
At 8000rpm = 40hp and makes 60cfm
At 9000rpm = 45hp and makes 68cfm
So as your Volumetric Efficiency goes up, your cfm needs also go up!
Peak Flow/HP Calc.
https://www.mk5cortinaestate.co.uk/calculator3.php
Mukuni Carbs CFM.
Look at this 460 Dyno Sheet I found. Notice Max HP at 5000rpm, but it was losing HP after 3600rpm. Probably Stock GX390 CAM.