Concentric v Monobloc Jetting

snatchpiece · Post by **snatchpiece** » Mon Aug 23, 2010 8:01 pm

My 59 G12 is fitted with a concentric carb, I toyed with the idea of replacing it with a monobloc 389 as per standard, but I thought 'if it aint broke dont fix it' so I left it on and it has worked fine. However now it is definately broke (see my topic 'FIRE') and I intend to replace it with a brand new 389 monobloc from Amal. Would the jet sizes be the same e.g if the main jet in the concentric was 160 should I fit that in the monobloc and so on for other jets or should I specify the original settings used when the bike was new?

Don Madden · Post by **Don Madden** » Mon Aug 23, 2010 8:59 pm

You should start with the original settings. Concentrics seem to be more efficient at high flow rates requiring smaller main jets. Original carb: 389/29, 1-1/8in; 400 main jet, (likely too large, I suggest 390), 30 pilot; #3 slide; .106 needle jet with needle in 3rd notch. A 160 main jet in a monoblock would probably melt the piston. Cheers, Don.