Si carb modifications considerations and celebrations

Good info for the memory banks; thanks for taking the time to post it.
Are the fuel ones are only of benefit with a fast-flow tap as well, or good improvements anyway?

Addendum to edification:
(much like yours, but with a few extra handy photos)

modernvespa.com/forum/topic135335

saw a thing in scootering yesterday n thought of you , a guys using watchmakers tools to produce dellorto jets , after cuttin dell n spaco jets in half n discoverin how bad theve become , he's only doing 5-6mm screw in dell's as yet but is going to produce push in mains soon , at 1.65 for 5-6mm thats half of what dell want
says he's up for specials/one-offs , contacts at "info@kmtproducts@co.uk " an his website when launched will be "www.kmtproducts.co.uk" , worth keeping a eye on vespaco ,even if its to send atomisers to be re-sized ,or get mains bigger than 135 eh , H

Cheers H

Although I've bought SIPs' jets upto 162? I think. Quite large. And as long as i stick to one brand i should be ok.
But youre right, the quality of all jets is not so good. But dellortos, or at least the older ones, were top quality.
Im not sure of the Dellorto equivalent to 162 but i think the SIP ones are bigger than Dellortos' equivalent.


Whats more I've been experimenting some more, trying to improve fuel flow by drilling out the carb and float top.. BUT ...
Although by drilling out the fuel supply holes it did actually improve fuel flow some what BUT ....
I also found the problems ive had recently with trying to dial it all in. It was near impossible as the carb was now soooo rich, especially between 4000-6000rpm i couldn't do much to get it running smooth.
The other day I 'restored' an old warped si24. Spending a few hours filing and sanding out the slide groove so that slide will not stick.
I didnt drill out any supply passageways either, left it totally stock.
First i tried the stock carb with the modified cosa top.... Still way too rich at 4000rpm+
So i tried a stock float top ,unmodified, with smaller stock needle....
Smooth, very smooth, from idle to wot......so i had a look at the plug @ 1/2 throttle/6000rpm/60mph...showing lean
So i upped the main jet to 148.,Still smooth, still lean.
Upto to a 158mj ... Still smooth, still lean at 1/2 throttle
So i 'upped' the atomizer to a BE1, aiming to richen the mid range/1/2throttle, as thats where I'll be riding it most.
Still lean, still smooth
So upped to a BE5,,and it just started to stutter a little. I could either go from the 55/160 idle to a 50/160, or a 52/140, which would soon sort out the little stuttering at around 4000rpm.
I have a BE4 i may try but think i wont need it as ill have to lean the idle too much to compensate.
I think the BE1 and the 52/140 idle combi should be the one??!

It got late and i was on reserve (yes, i was riding on reserve with no fuel starvation issues whatsoever!! YiPpIe!!!) So i didnt get a chance to look at the plug.

I dont want to lean out the idle range as I'll also likely be riding at good speed at 1/8 - 1/4 throttle....or in the overlap period of the idle and main, especially if there is a back wind...that can be quite scary, riding at 54mph with the throttle barely open.

First is to get it WOT and check the plug
Then work back from there to get it running as rich and as smooth as possible in every rev range.

Before, I spent a lot of time swapping jets, atomizers, slides and have learnt that you CAN drill out si carb and it WILL Supply more fuel to the sysytem...BUT.....!
If you drill too large...(from my guesstimations that would be anything bigger than the needle valve hole, which is around 2.3mm) if you drill too large it WILL cause problems!

Ive ran the carb for years, running smoothly but with the main jet feed hole drilled out to only 2mm...this seemed to work ok.
What ive found in the past few days is that drilling it out to 3mm causes very rich mid range.
And i also drilled out the cosa top to 2mm (or was it 2.5mm?) and it ran ok back then.

But i have to say the stock si24, uncustardized, runs very smooth.
I have stock jetting except the main at 158 at the moment.

Now its possible to jet the carb, i just need to find the time to get it dialled in properly.
It wont take so long this time !!

_-----------------------


I also spent some time measuring fuel flow...
Heres what i found, using my limited equipemt;

With 1litre of fuel in the tank, draining through 530mm of fuel pipe with banjo fitted, straight down into a container
SIP fast flow - 19litres per hour
Stock Rally tap - 12l/hr

With the tank and float approx the same distance/angle/height from each other as it would have been in the chassis...,


Edit: i started writing this yesterday!!
Ive been to the seaside today, on the vespa,
No issues at all.
But i are mostly been riding at no more than 1/2 throttle/6000rpm/60mph-ish
Im using a 155 jet atm, deciding 158 was taking too long to pick up the revs, sticking with the BE5 as its as rich as in can get in the mid range with just an ocassional stutter which i can get over if need be by drilling out the BE4 or BE1
I basically need that quite rich quite low down the rev range as i noticed today,,cruising behind a lorry and a car at 56mph i was barely on 1/4 throttle! And temps went higher too!
There was only one thing to do... Wait for a clear spot to overtake
Then back at half throttle at 60mph
Its running well so far
Still need to check the plug again tho'

sorry me ,but i think your wrong on atomisers , there is a be2 , from rich to lean ive had it as be4,be2,be1,be3,be5 , with the cosa be6 on end, vivo scooters posted as , be2,be3,be1,be6,be5,be4 , here , lmlocgb.co.uk/phpBB3/viewtopic.php?f=5&t=1707, i havent found anymore reliable info on the net ,an weve discussed it on here before , dell do list a be2 atomiser but its always out of stock/unavailable , it was only fitted as stock to rally's , an is often replaced with a E2 or even a E1 ,
your way of numbering/coding atomisers from 1 to 5 sequentially would make more sense/logic , but where dealing with scooters/piaggio here ,logics got nuthin to do with it ,
not trying to pick a fight ,but all this is old ground thats been well covered , H

Well thats what i thought me, this is why i went confidently on and drilled the carb passageways even further...but in practice it effectively makes for an over rich fuel supply.
My theory is, drilling out the supply holes to anything higger than the needle valve affects the flow into the venturi.
Try it for yourself. Start with an unmodified carb/float top. Ride/dyno it.
Then drill the supply holes 0.5mm larger and ride/dyno again.
Drill another 0.5mm larger and you will likely notice rich like symptoms developing, particularly at 1/4 range.
Drill a further 0.5mm and youll likely find the stuttering gets beyond the joke.

In another theory, so long as all holes preceeding the main jet are larger than the main jet, then there should be adequate flow to feed the venturi.

I also use the 120 main jet = 1.20mm as a guide, although ive never actually seen dellorto state it themselves.
Ive also not seen any Dellorto literature on the si...please share


Although Im quietly confident with my limited knowledge of dellorto jets i am eager for more info.


Ive put these leaner to the left, richer to the right - at 1/4 throttle range!!!
The more holes/the larger the holes, AT the TOP of the mixer (in the blue area) nearest air corrector, the leaner it will be at this 1/4 throttle range.
The idle is still in use at this stage and could also be changed to compensate for a rich/lean 1/4 throttle range. (This would equate to around the 4000rpm range on my set up)



I also have an E2...this is similar to a BE2 but with even larger holes.

The BE1 has smaller diameter lower holes than all the others....this is the richest atomizer at mid/hi rpm.
At lo/mid range the BE1 would be richer than a BE3 and leaner than a BE5
The BE1 (and BE4) would therefore be a good choice of atomizer if you ever wanted to drill one out to get perfect jetting.


Atomizers can be richer at mid range and leaner at WOT, or richer/leaner in all ranges, so saying one atomiser is richer than another gets complicated and it ain't necessarily so.

Idles..i dont understand the, this ÷ that = ×mm
You can get the fuel/air mix ratio by doing this
But for me, i would 'tune' the idle the same as main stack. If i wanted a leaner idle, i would start by going down from, for e.g. a 55/160 to a 50/160. This is the same as lowering the main jet in the main stack.
The idle im currently using and will likely stick to is the 52/140.
52 being smaller/leaner than a 55.
But the air corrector oj the idle is a 140 not a 160. So this will richen it up overall.
Less air + more fuel (than a 50) , so will be richer all round...which is what i want for riding at 55mph at no more than 1/4 throttle.

The idle is just the same as the main stack, except as one component instead of three seperate components and i can only dangerously presume, that they are measured the same way as other dellorto jets.
I did actually ask Matt Cooper at Eurocarb/Dellorto about this andnhe wouldn't give a proper ajswer, just saying theirs are different to the lower quality ones available at the moment.

The main jet on my mildly ported and 'flowed' pinasco 225, may well come down a mark or two. I dont want it 'perfectly' tuned for max power thats correct. It will end up on the richer side in all rev ranges when ive finished fine tuning.

Ive a choice of two air filters, both drilled but one has only one layer of mesh on it.
Atm, im using a stock filter, holes drilled out, no mesh removed.
52/140
160 a/c
BE5 - which is clearly richer at this 4000rpm - 1/4 throttle range.
Take any well set up motor with a be3 in it, and chuck in the richest atomizer for the low/mid range - a BE4 (as it has no holes in the top section at all) and chances are i will stutter like crazy at around 1/4 throttle (+/- a little)

Depending on if the carb has been updated (with the lead shot in the previous 'air corrector' hole)
Would determine if you need an idle with a hole in the top or not.

Hi Vespasco

I like the way you are working - but i'm not sure how you are working out the atomisers - the BE6 (sorry H) is weak - which means lets less fuel into the carb - what it is actually doing is letting more air mix with the fuel which in return makes the mixture weak - its the same with the air corrector the larger the number means a larger hole in the end which lets more air into the stack making it weaker - less fuel....

The pilot jets are wokred out by dividing the smaller into the larger - a 52/140 is a 0.269mm - the 55/160 is 0.29mm which is only 0.03 mm larger but enough to let more fuel through, you'll be supprised to how little fuel is needed to make the combustion

In reality the IDEAL fuel to air ratio is about 13-1 - 13 parts air to one part fuel - i say IDEAL as its quite hard to achive -

Also your last comment about the old style carb and the lead shot - if your riunning the old style carb with the seperate air hole then you need the solid pilots if you block the hole you defo need the later style pilots -if no hole then same - need late pilot.

What exactly is your set up??? have done 20 odd Pinascos over the years and some still running 15 years on. i might be able to help get it spot on