RB 250 24MM CARB ECONOMY CARB/POWER TEST 38BHP

[Darrell Taylor]

Had a 24mm carb hanging about and kept looking at it and wondered what a hot rb250 would do with it fitted,now the rb isnt best suited to the 24mm carb with its 35mm inlet it needed a rubber reducer sleeve and its huge reed not helping it would have been better tried on a ts1 or monza or imola inlet/reed but still gave a good result,it was a rushed test with no jetting or air fuel tests done so no idea how the jetting was although it ran well so cant be too far out

[victor]

That's impressive

Would be interesting seeing what the same engine would do with a range of different carb sizes (20, 22, 24 - 38mm)...

[corrado]

It's the future. I've got a spare 22mm Jetex you could try.

[Darrell Taylor]

it did 41.6 on a 38mm that obviously suits the inlet and reed very well
i figured a 24 carb with matching inlet/reed and a pipe with a lower rpm peak would start to be a great basis for an economical super tourer
i have some cylinder design concepts ive been testing for a few years now that would add really well to the combo too

[corrado]

I've got a half built Rapido 250 in the garage that was going to be fitted with an LTH reed and a Mikuni TM24 along with an ADS3 touring exhaust.. It's on hold with other projects at the minute and the fact that I was struggling to jet the TM24 correctly on my Mugello 225.
I see a trip to Doncaster on the cards.
What carb did you use Darrell? OKO, BGM, Mikuni?

[victor]

41bhp on 38mm correctly jetted and fitting carb, and 38bhp on 24mm incorrectly and ill-fitting carb... (-7%) Seems like there's room for improvement to match the higher power outtake but with the smaller carb (26% smaller)...

[drunkmunkey6969]

Try a 25mm PHBL if you get chance Darrell....and make a note of the main jet size, you'll be gob-smacked at how small the main jet is when you run one of those.....it's possibly the most economical carb I’ve ever dynoed/set up. I tried it on a few engines, nothing as fruity as the RB you tested, but on all others it performed OK, and the fuel economy was astonishing. Worth a test if you get chance.....

Also...can you post an 'over-lay' graph of the 38mm vs 24mm you tested, to see if/how the bottom end benefited from the smaller carb, looks like its coming on pipe a lot earlier?

[Darrell Taylor]

the carb used was 24mm vhst dellorto,piston ports like the larger carbs but not always because there "piston port" more that the inlet manifolds in use are too big and the corresponding port on the cylinder matched to them is too big ,pretty much on every ported lambretta cylinder ive seen


the 25mm phbl works very well on the auto,s i tune around 22bhp @ 77cc or 26bhp @ 172cc (small inlet and very small reed) so can make good numbers,only tried a 25 on a rapido 225 and it worked well but went better enough on the 30mm delly to warrant the change

corrado your rapido 250 sounds like a nice build,ive done 1 on an ads3 that pulls 4.2 with ease but on a big reed conversion and 35mm so i think yours will be nice

[1469leach]

What exhaust did you use on this test Darrel

[Darrell Taylor]

was franspeed race,but for economy and rideability/touring use different pipes could be better but give less up top in exchange for more down low