Walbro WT-990 high-performance carburettor. This is the carburettor recommended by Zenoah for upgrading your RC-style engine. Engines with the standard carburettor (WT-603, WT-668, etc) tend to experience a lag in low-end acceleration when tuned for peak power. Carburettor specs: Walbro WT-990. 15.88mm bore, 12.7mm venturi, independent tunable high and low speed jets. This carburettor does not have an internal choke. 
Note: This carburettor now comes standard with modified throttle shaft, so it will work perfectly in most R/C applications such as HPI Baja, FG Modellsport, etc. 

The WT-990 Carburettor is the same as the WT-813 WITHOUT an internal acceleration pump.

Zenoah part# 575655001
Fg part # 07855