No it's the engine ain't big enough. Thats why they twin feed them.Save the engine working so hard. "They say" it's because Interpump have messed up with the pump design (ws202) but I don't believe this.The manufacturers will not drop it from their range as other people are making them...and it's the pump everybody wants.Why don't Duel pumps offer this option on a GX390 engine? They are missing out big time...Qwashers,jetmac all the small independents do one. I use this engine on the bigger pumps ...http://www.geargb.co.uk/product/3000-BRIG-14Q/Briggs_and_Stratton_14HP_2100_Series_Keyway_Shaft_Engine This pump http://www.lunesdale-pumps.co.uk/59-series-evo3-hollow-shaft-for-engines/19726-e3e2520c-interpump-59-series-evo3-pump-20-lpm-250-bar-1-hollow-shaft.htmlThis unloader http://www.lunesdale-pumps.co.uk/flow-sensitive-unloader-valves/1778-k1-unloader-valve-9-41-lpm-250-bar-brass.html