Tuning 389 Monobloc

[roadrocket]

Hello

I have recently put on a 389 Monobloc as a substitute for the previous Concentric. The Concentric worked great, but I like the more original look of the Monobloc, and it is an original Birmingham item in good nick. I have a Super Rocket head, 356 cams. In the carb I have a 250 main, 106 needle jet, throttle valve no. 3,5, and needle in second groove from top, pilot jet is no. 25. All these parts are new. I had to grind the throttle valve to fit in the bore, sp no slop there. I have a pancake type air filter. At low speeds the plugs get sooty, and acceleration is poor, but at about half open it runs fine. Any suggestions? An even bigger throttle valve cut out?

Regards, Otto in Denmark

[bsa- bill]

with a 356 cam I asume you have a Road Rocket ( could be a clue in your name  )
here are two sets of settings from the BSA booklet ( does not give cam number though )

A10 Gold flash
choke 1 1/8, Main jet 250, Pilot jet 30, slide 3 1/2, needle position 3, needle jet 106

Super Rocket ( probably 357 cam )
choke 1 1/8, Main jet 420, Pilot jet 25, slide 3 1/2, needle position 2, needle jet 106

Hope this is of some help

All the best - Bill

[roadrocket]

Thanks, but I think this would give an even richer mixture. If I had an air leak somewhere, could this give an over rich mix? I would rather suspect the opposite. Only dodgy part is the top, for which I have made an insert ring of plastic, to make an effective (?) seal due to wear. Burlen was out of tops.

[G/F DAVE]

Hi, Otto you could check the jet beside the main jet holder ,the one with the chrome cap.This has a taper fit sometimes the jet gets screwed into cap then into carb body so it doesn,t effect a seal, or it could be blocked? If you remove cap first then screw home jet with screwdriver then replace the cap and fibre washer ,I had this problem many years ago on a so called re-conditioned monobloc & it caused a lot of head scratching with exactly same syptoms as you describe. hope this might help you.  All the best Dave

[bsa- bill]

You say you have a pancake type filter, I take it this means an after market filter - round with chrome circular surround and felt/paper filter, maybe you should try the bike with this off as the original BSA filter was a lighty oiled gauze ( little restriction to airflow), not sure if thr Rockets had a filter at all ?

All the best - Bill

[roadrocket]

I have now tried to fit a bellmouth - much better, though I had to put the no 3 slide in, bigger main jet and raising the needle. Much quicker throttle response, and a helthy constant roar from the pipes. Plugs are now tan after some time at 60-75 mph, though a no 25 pilot is too lean. I'm, not sure how much the filter is needed, and what kind of dirt intake I'm now letting myself in for...

[Pollock]

This should give you some good background info on amals.

http://www.britbike.com/ubb/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=11;t=001129;p=0

Pollock