is a 102 main jet too small for a Rapido 225?

[mick1]

that's a hole the piston with a 102..... it's 120 and over to be safe (think 1 ran a 122 on mine after holing it with a 112 !!)

[Adam_Winstone]

If we are talking about a 25mm PHBL, rather than a PHBH, these tend to have much smaller main jets, often around the 98 mark (!). Cambridge Lambretta have published (as has Tino) a fair bit of info on jetting 25 PHBLs for Mugello kits, so it might well be worth your while to do some online searches.

Adam

[a.lo.v2]

does seem small, my rapido never ran less than 125 on a 30mm. a standard vespa 150 sprint has a 102 on a 20mm carb, i know not a proper comparison just a bit of perspective.aren.t the "standard" 22mm 118

[psychedelicropcircle]

That must be a typo.... :fb: tell us you meant 120

[Eibar]

opinions sought please

rapido225 classic, with 48mm clubman, 25mm dellorto, rubber bellows and air box fitted, but no filter inside. carb is jetted as follows - 50 pilot - 40 slide - AQ264 atomiser - D36 needle on 3rd clip - and a 102 main jet. this carb has only just been fitted, and it all goes ok, but i haven't done any big miles yet or plug chops, so i wanted some opinion on the main jet. 102 just sounds too small, even though the scooter feels great. what do other people think?

Scooters do feel great with small main jets in cos the leaner they are the better they feel
(Whoooarrr - Bang). Are you still runnin it in. You need to do W.O.T plug chops.

For the record my 225 rapido classic with 28 and expansion runs a 118 main and AFR is fine , 120 is too rich though, all engines differ. Go bigger on the main get some full bore high speed plug chops done and then decide whether to come up or down on main. GO BIG AND COME DOWN then have loads of magic !!

[gtwildcat]

I run virtually the same set up as yours apart from it runs a breathe sweet filter and an innocenti cast 225 barrel. My main jet is a 104 and I'm having to go down to a 103 as its running a bit rich with the 104 so I wouldn't be worried by running a 102 with your set up. When I used the air box with no filter I ran a 101 and it was spot on.

[Adam_Winstone]

Looking back through my little black book...

Mate's MB205 was dyno'ed at Taffspeed (approx 1997) and set up to run a 90 main jet.

Stoffi's old PHBL details list a 90 main jet for a standard SX200 running a 24mm PHBL.

Scooter Centre's old PHBL jetting details list 2 needle/atomiser set-ups, 1 with 95 main and the other with 98 (no motor details, just basic starting point).

My mate Bruski did Brittany this year on his MB iron 175 with PHBL.... but I can't put my hands on his jetting!

Staffy Hatton on the LCGB forum listed that his Mugello 186 ran a 25mm PHBL with 98 main jet.
... and craziest of all, a mate ran back from Euro Lambretta in Germany with me. I was riding a TS1 225 and he was riding an iron barrelled Suzuki 225 conversion with 28mm PHBH... ON a 108 main jet! I doubted what he was saying so much that I actually stripped his carb down and listed it all in my black book! I should point out that we left the rally after breakfast and then I was back in Abingdon for teatime the same day, with non-stop motorway miles and a channel crossing too so it's not like we were hanging about!

All of the above show that main jet alone is not the issue, with the choice of needle and atomiser having a huge influence on main jet size. You can quite easily hole your piston in with a 200 main jet if you stick the wrong needle and atomiser in too!

Remember that ALL quoted carb set-ups are only ever a starting point when tried on a different motor. As such, 102 may be too weak... or too rich, only testing will prove it eiter way. Jetting 'errors' are also regularly quoted as a cause of seizure, piston holing, etc. when the real cause may well be wrong ignition timing, compression, wrong pipe for the motor spec, etc.

Adam

[corrado]

I'd have said it's probably fine with a filter but no filter could throw it out completely. I always start way rich with main jets and slowly work my way down via plug chops. I'd have built it with a 110 particularly if running in and done plenty of plug checks and flicking the choke on and off to see if it splutters, if it runs fine with choke on then it's obviously too weak at that point. Jetting isn't just about main jet though, you're just as likely to seize it with the wrong combination of needle, atomiser and slide. Some would say stay off the main jet for at least 500 miles when running in.There's even a case for leaving the main jet out when setting up the carb until you've done "X" amount of running in miles.
MB produced a jetting guide but they offered a range of usable jetting internals that are suitable, on a PHBL25 running through airbox and filter they reckoned on around 92 main and with holes drilled in the airbox around 105, so very difficult to say given that you're not using a filter. Although the main jet affects most throttle positions it's primary use is between 3/4 and full throttle an area that most people don't ride at, so plug checks at half throttle don't tell you much about the main jet. Rich = safe, lean = trouble.

[Eibar]

I like this thread, I would like to add that even though the dyno runs, plug colour and feel of the motor tells me my 118 main is fine, prolonged W.O.T runs do send cht gauge above the 220c mark and i can bring this temp down with a bigger main jet despite the other tests implying the motor was rich.

So CHT gauge removed because "ignorance is bliss"

[gpnick2004]

I ran a 25 mm on mine a could not get it to jet ultimately F)$king it. I put a 30 PHBH on and bib your uncle. Go to the Bristol Lambretta Club page and look at their jetting guide.