LCGB Forums

I have a 175 gori kit on my series 3 with a clubman exhaust… the current 22 mm Scootopia carb works as it pleases and needs replacing…. what would you recommend as a reliable replacement?

A PHBL should be fine and I run mine through the original air box.

Tractorman wrote:A PHBL should be fine and I run mine through the original air box.

Agreed, 24 or 25mm

Rich'

I would need a bigger manifold presumably….also would it offer anything performance wise ?
And who supplies them?
Thanks

Yes you need a new manifold and rubber coupling but the air hose should fit.

My interpretation of 'best' would be a flatslide or D-slide carb such a Mikuni TM24 or a quality PWK24. Your interpretation might be easily accessible jetting information.

I would look at the commonly known issues with the Scootopia SH2/22 before replacing it, such as the sticking floats.

vegansydney wrote:My interpretation of 'best' would be a flatslide or D-slide carb such a Mikuni TM24 or a quality PWK24. Your interpretation might be easily accessible jetting information.

I would look at the commonly known issues with the Scootopia SH2/22 before replacing it, such as the sticking floats.

Very wise words.

It's because of the fact that I have set up a few engines without specialist equipment, that I by now know my way around a few carburettors of various manufacture & type. Without doubt, the 'flat' slide carburettor comprises several variations, whether, guillotine, oval or 'D', they are more suited to the two stroke engine than a round slide. The science of the issue concerns the flow within the venturi, specifically the flow characteristics created by the slide.

depends on your budget I'd go for a jetex 26mm thru a drilled airbox & breathe sweet filter, worked well on my extra transfer ported Mick Abbey tuned 175, with an Ancillotti
or a 25/26mm PHBH, STILL thru the airbox, which worked well on my tuned Gori 200 (on a TV175 block), narrower carb rubber so that side can fits, now have a 26 PHBH bored out to 27.5mm with an Armadillo airhose & BGM

The original Post asks "Best carb" & upon reflection, my belief is that the OEM air filter & box will be the limiting factor. The frame hole size is not the problem @ 50 mm, but the envelope of space in which filtration occurs is restrictive by modern standards. Drilling holes in the outer is not the solution that a larger neck connection in the air box will allow. Back in the 70's I bought a scooter so inherited a modified airbox. Plugging the DIY holes with grommets made no perceivable difference. The filter/box/bellows are restrictive & that TV200 with it's Amal carb would regularly heat seize when given the berries, two up. Ever ridden pillion on your own bike & had to reach forward to de-clutch, the rider oblivious of the seize? Twice, in separate journies.

Even with a modified box, a standard bellows will then be a concern, especially if remade & manufactured from Black-Snot or Collapsium.

If you do the maths, a 26 mm venturi is 40 % greater in cross section than a 22 mm. Given a likely increase in RPM that a freer flowing exhaust allows, then perhaps my doubts can be understood. If an engine suddenly can rev freely on to 8500, than that alone will result in about 24 % more volume required over 6900 RPM.

Even Innocenti themselves changed filtration process with the use of the SH carburettor on the Luna.

Of the other two strokes that I am aware of, the Dell'Orto PH was OEM to the Honda NSR 125 which would deliver about 33 BHP unrestricted. I doubt the choice of carburation was a free choice as the Honda was assembled in Italy.....Nonetheless, with the large, well designed airbox that the Honda used, the PH's worked adequately enough with no tendency to flood a hot engine. That is the only accolade I can bestow on the PH carburettor as Dell'Orto have within their range far superior products better suited for the likes of 'us'

So, to sum up:

Probably the best thing in the Poster's scenario is to blueprint the carburettor that is fitted, obviously the priority being checking of the float.

Without some serious consideration of the OEM air filtration set up, fitting anything much bigger than perhaps a 24 mm venturi is not the solution that is sought.

Please do not misunderstand my point in all of this. It would be wonderful & convenient if the OEM air filtration could function well with bigger carburettors on even a slightly tweaked engine, but in my experience, it will not work very well.

My favourite carbs are chips

bsso78 wrote:My favourite carbs are chips

Have you moved 'Darn Sarth' & become slack?

Surely, your favourite carbs should be beer?

bsso78 wrote:My favourite carbs are chips

We know

Many thanks for the help and suggestions gents.l now have an electric start on my series three….needed rather than wanted due to damaged ankles before it went in there was a problem with starting …after the initial engine rebuild by Harry Barlow she started and ran fine then after a winter of her being laid up and only occasionally being kicked over by the time she went in for the electric start to be fitted she would not start. The guy who fitted the electric boot got her back running again and indeed when I got the bike back she was firing up with choke and petrol on from cold and minus choke for a warm start spot on… if I leave it for a couple of hours and try and start it up she will run for a minute or two and then just stop.If I-try and start her again she will catch , run for a couple of seconds if that and stop again….it sounds to me with my limited knowledge that fuel is just not getting through?

Would a sticking float cause this issue?

Could be, but have you checked fuel flow from the tank to the carb? I would start with the basics before you change the carb, simple things like the breather in the petrol cap, look in the float bowl for any signs of rubbing, air bubbles in the fuel line. Its probably a simple fix but its just a case of being methodical and looking at one thing at a time. To my mind there's no point in changing the carb until you are confident that's the problem

Agreed …got a day spare next week will be going through the list then thanks

gazzar15 wrote:Many thanks for the help and suggestions gents.l now have an electric start on my series three….needed rather than wanted due to damaged ankles before it went in there was a problem with starting …after the initial engine rebuild by Harry Barlow she started and ran fine then after a winter of her being laid up and only occasionally being kicked over by the time she went in for the electric start to be fitted she would not start. The guy who fitted the electric boot got her back running again and indeed when I got the bike back she was firing up with choke and petrol on from cold and minus choke for a warm start spot on… if I leave it for a couple of hours and try and start it up she will run for a minute or two and then just stop.If I-try and start her again she will catch , run for a couple of seconds if that and stop again….it sounds to me with my limited knowledge that fuel is just not getting through?

I think the general consensus of opinions will now be that it would be best to sort out what is wrong with the fuelling, including the carburettor, to get things functioning again.

As missing Lynx said, being methodical will enable you to fix it. It's worth mentioning that a good fast flowing tap is essential & although Scootopia's have had some issues, I can recomend them. They need dismantling & checking that they have been assembled correctly, just like any tap IMHO.

See how you get on, but if you want to borrow a genuine SH2/22, I will lend you one of mine. I can send it via Herpes if need be from Northamptonshire.

Thanks….working until Tuesday ..will relate back what happens

As it turned out I only got the chance to check things out Thurday…fuel level ok ,tap working….took the carb off and tested the float was possibly sticking but not now….reassembled and recharged the battery…Bingo !Third go on the key and off she went..chuffed next up a bit o a ride later on.
Thanks for all your help gents