Modern Vespa

whats the deciding factor when upgrading to a phpb,phpl,vsht style carb?

25mm,28mm,30mm,35mm,flat,round?

if you was tuning a px177,depending on your setup how would you make a good decision when choosing the carb?

does reed or rotary change anything when choosing a carb?

not interested in keeping the standard si carb,,always wanted a fancy looking dellorto after seeing one sticking out a lambretta panel,,and I'm not bothered one bit about using more fuel as long as it runs well,sounds nice and I get to drool over a shiny carb

I've given a small example why I'm asking this question but if any of you have fitted large carbs to any vespa I would like to hear about it

cheers

I use PHBH28, no need for anything bigger.
Pain in th arse to get tunedin, so book a dyno.

I've seen 20/20 and 24/24 si carbs used on 125cc,177cc,200cc and the malossi/pinasco 210s,setup right they seem to do the job,the benefits of fitting a phpb style carb seem to be better throttle response and bucket loads of fuel

if a 20/20 carb can run a 177cc,what made you choose a 28mm instead of a 25mm?

Mine came with 20/17.
I did not know there was a 25mm option, but did a stack of research & the Polini 28 came up for sale at the right time.
To easy & common to do a 24/24... But reverted back to it for a few years due to difficulties with tuning.

Ohh yeah, they sure use lots of fuel.

More info on my motor...
modify VBC1. 2-port -got fuel, need tim(ing)? Only 67mph

Cheers,so did you buy the 28mm because it was best suited to your engine or because it was the smallest one available? If you knew about the 25mm would you choose it over a 28mm?

Jennings gives this basic formula for determining suitable carb venturi size:

D= K x Square Root(C x n)

D=Carb diameter
K= Volumetric efficiency of engine - depends on build -between 0.65 (stock) and 1.0 (race)
C=Cylinder displacement
n=RPM at peak power/1.000

cheers for that SoCalGuy

K might have something to do with it that I'm not understanding but my mate has a gp200 fitted with a 28mm dellorto,SubEtherBASS has a polini 28mm carb fitted to his polini 177cc,there is the fact that its a worb5 jobbie,not sure on his mazz crank,60mm,57mm? but if setup correctly on a dyno both will run with good performance? or would they

can someone help me decide what carb would be best suited to a 177cc using the formula given above?

cheers

honestly, it depends on the rest of the setup. while in theory you can calculate it from jennings formula, it's really just a calculated guess due to the huge fudge factor guessing the Volumetric efficiency turns out to be.

while a side draft carb setup on a scooter can perform phenomenally well, it's probably the last modification i'd recommend.

it's one of the most expensive modifications to do right, and it won't perform well unless the rest of the engine is modified to a similar level first.

full cylinder kit, pipe, variable spark ignition, gearing reconfiguration, etc, should really be done first.

that being said, if you're going to street it, a 28-32mm carb is about where you want to go. some go as large as 35, but it's overkill for most bikes.

I recommend staying around 28-30. it's the smaller end of the spectrum, but it is easier to dial in and better for most uses, except full throttle drag racing.

even when i ran a full race setup in MASS and took 2nd place in 2001, i ran a 28.

also, asa side note- i ran that bike on the street and due to the reed valve it got significantly better fuel efficiency than a stock 200 a friend of mine had.

rotary large carbs are another matter, and i don't really recommend them.

rob's right as usual. Volumetric efficiency is more or less a guess. But the formula does get you in the ballpark.

Using our lowly Super with the bolted on Polini as an example:

177cc running @ 6500 rpm (my guess where it currently maxes out):

177 x 6.5 (for 6500 rpm) = 1150.5

Square root of 1150.5 = 33.92

33.92 x 65% efficiency = 22

So according to the formula, a 22mm carb is about right. Turns out the 20/20 works fine. A 24/24 would probably be too big for our set up.

When you add an expansion pipe, a cut crank, porting and case matching, etc. you not only increase RPMs but (hopefully) also improve the efficiency of the engine. Plug those numbers in and you'll see you start needing a bigger carb.

I got it coz it was available & after a lot of research decided that i wanted it... I saw something about "stage 5(or7) tuning once, & aimed for that as a start.

But i would go for a 25mm, as it would be far easier to set-up & more fuel efficient... Jus don't tell Ginchi I said that.
There has been many occasion where i got 64km out of a tank! Yip, its thirsty!

This chart will help.

Cheers,starting to get a better understanding,that charts good,thats why i was asking for setups on the forum so i could ask a few more questions on porting,carbs etc

A popular upgrade for modern gilera runners fitted with malossi 172cc kits is a 25mm dellorto,would the standard carb which i think from memory is a 20.5 or just 21mm work well on a standard px125? Its even jetted for a 125cc,maybe a little rich being reedvalve

I've found two carbs I think would work well on a standard px125 and a tuned px177 giving better throttle response without chucking buckets of fuel out

the gilera runner/italjet dragster PHVB 20.5 dellorto and the stage6 PHBG 21mm or 22mm dellorto

the gilera carb comes with an electronic choke but can be swapped out a manual for a few quid

whats your thoughts?

honestly, unless you are going larger than 26 it really isn't worth going away from the SI carb.

the SI carb is specifically designed and optimized for this engine. to get an aftermarket carb to run as smoothly will require alot of swapping of jets, needles, needle jets, and slides. most carbs smaller than a 28 also have much less options for reconfiguration of the fuel curve complicating such a swap, although there are some exceptions.

in addition, a side draft carb is at a significant disadvantage to the SI carb in that it has a much longer inlet tract.

to be blunt, unless you are outside of the range of available sizes you are much better off staying with the carb specifically designed for this application.