It also allowed extremely radical high lift cams that gave a very lumpy idle when unplugged in the open position or when using a full aftermarket exhaust. Yamaha continued to use the EXUP valve system on the subsequent Yamaha YZF1000R Thunderace and Yamaha YZF-R1 motorcycles.
The reference describes a system that allowed for extreme high lift camshafts, which resulted in a rough idle when unplugged or with an aftermarket exhaust. Yamaha continued to use the EXUP valve system on subsequent Yamaha YZF1000R Thunderace and Yamaha YZF-R1 motorcycles.