B40 SS90 alternative carb options?

[berniej]

I think I'm going to have to try an alternative carb. Since restoration it just ain't been right and I thnk I've exhausted other options.

The bike idles too fast when timed correctly and with the slide bottomed out. It also hunts up down a bit.

It idles nicely and pulls steadily at lowish revs but misses badly on accelerating and running at higher throttle openings. This seems to get worse as the engine warms up.
Adjusting timing has no effect and neither does the needle position. Dropping the air slide makes it worse.

When I bought the bike as a U.S. import project it was crusted in a dry, red, sandy dust and had no air filter fitted. I suspect that the carb has worn through ingestion of dust particles when it was last running.

Rather than trying a replacement slide or like-for-like monobloc I think I'd like to try a more modern carb option.
A Mk1 concentric would probably work well if I can get the model number, needle and jet options right but I suspect there are other contenders too.

Any opinions or experience on this idea please?

Thanks,
Bernie

[AdrianS]

The most important thing to consider when trying a different carb is to make sure the sizes of the intake and carb are the same. You also have to consider the mounting of the carb.
I know Bantam owners get on well with Mikuni carbs and Rex Caunt sells a mounting conversion kit and the carbs. I know of someone who bought a " pit bike" carb of Fleabay for about 20 quid for his Bantam and the bike runs really well.

One problem with running an old carb is wear within. The slide wears as do the jets - don't think that they don't wear, they do! So does the needle itself. I rebuilt my monobloc for my B40 but it still didn't run as well as I hoped. Ended up buying a new carb from Hitchcocks and hey presto - much better!
Another consideration is also the air filter arrangement. Is there sufficient room for a filter with a different carb? Do you want to run without one?

Things to consider, let us know how you get on!

[berniej]

Thanks Adrian. I fitted the BB ignition kit today and should get that set up over the next few days so will see if that makes any difference.

I guess I could go for a like-for-like replacement and get a new monobloc but was also interested as to opinions on whether a suitablre Mk1 conc might be a 'better' solution given that they superseded the monoblocs?

Cheers,
Bernie

[chaz]

according to RR, in his book, the monobloc is a perfectly good carb wheras the concentric needs more setting up, his quote.
from past experience  the concentric I think does slightly better fuel consumption but no great increase in power.
the concentric is better for short circuit style racing, the monobloc better on wide open road tracks like the TT, the advantage being in the response time when shutting to opening again, the concentric being quicker response than the monobloc (accounting for fuelling not physical open, close, open times)

[johnan]

If you go for a direct replacement Amal monoblock, before ordering check the part number stamped on the mounting flange...I ordered a carb from Amal to suit my "standard" C15 only to find when it arrived the carb inlet bore was much smaller than the bore in the head, as someone had fitted a Sports Star head to the engine in the distant past.
In my ignorance I did not know the part number was stamped on the mounting flange and assumed the engine was standard.
Anyway, Amal swapped it by return of post so no problem, but a bit of a downer at the time.