Hitchcocks Forum

Hi.
Just a question regarding carbs, I have a 500 classic 4 speed with a 535 conversion and a standard head and free flowing exhaust, I was wondering which would be the best carb to fit as the micarb although its been rejetted as best as I can to suit I think a 30mm mk1 or mk2 concentric may be the way to go, I'm sure lots of you will have opinions as to the good and bad of each of them but all thoughts are most welcome.

Hi ive got the same setup as you with accralite 8.5 piston and close ratio cogs in the box .I fitted a 32mm jap mikuni with pod filter main jet 230 freeway running on long trips I go up to 240, goes like stink also 19 tooth gearbox cog and boyer ign never thought much of amals wore out way to quick I run 98 oct fuel.bike is a 94 iron barrel.only probyou have to get a slightly longer cable. Good luck.

Amal Mk1 Concentric would be fine for your set-up. They are versatile and easy to set-up and use. Thousands have used them with first-class results. Mikuni may be a better design carb in theory, but costs are much higher and tuning not so straight-forward.

Carb tuning is not that difficult, just time consuming!
Shown on the lower graph is my first seat of the pants attempt (Green trace) .
The Blue trace is after lowering the needle one notch having seen what was happening. Although the AFR figures are not shown, the broken Red line is at 13.5:1 and the dotted black line above at 13.75:1
Carb is a Mikuni VM28 and jetted to suit one of these...



PS upper graph is what happens after dropping in a set of decent performance cams

Hi ric - is that a Mikuni VM28 or a Mikcarb VM28?
And can you explain - 'the broken Red line is at 13.5:1 and the dotted black line above at 13.75:1'?
And how I could know that without a dyno or computer?

Hi badger man... as Scotty I ride with a Mikuni 32mm (flatslide). It's very precise in both tuning and working. The carb set up is 27,5 pilot jet, 190 main jet, series 389 P8 needle jet, needle in top groove. The engine set up is our hosts tuned head on a 500 cc iron barrel, free inlet and exhaust, 18 teeth sprocket. I ride at sea level. Everything works a treat... regards Jacob

Hi Presto, that was the (expensive!) Mikuni. You can't tell what AFR your running without special equipment but I put up the Dyno graph just to indicate what results are possible without any.
I didn't know how well the bike was fuelling until after I'd seen the results, only that it felt right. It was the Dyno Guy who advised repositioning the needle a notch to straighten out the line.