LCGB Forums

Thought I'd start a new topic/continuation of the PWK manifold thread

Recently fitted a 30mm PWK style carb (actually a Polini CP)
with no filter I had the carb setup and running very sweetly in about an hour. No mean fit for a amateur like me
text book plug colour, no bog in any throttle position

All went to rat s**t when I fitted a breath sweet filter, using a 50mm BGM hose with a drain
It refused to hot start without a bump, despite dropping the pilot to 30!!

Trawling some US site's I found one comment 'PWK's don't like boot's on them'. I don't think this statement applies to 24mm PWK's as I have seen images and real life running standard airbox
I PM'd with someone on the LCGB forum who told me they had difficulties also, and I spoke with someone whose setup a few on his dyno and confirmed the same

I think the issue is the 30mm needs so much air, with panels fitted there isn't the space to fit a hose big enough
interesting that a PHBH can run a remote with panels

I assume this is down to slide shape difference and efficiencies

anyone offer any opinion/advise?

Well, you've already had my twopenny worth!

IMHO, PWK's lend themselves to filtration better than the very, very common choice because of the float chamber overflow.

Just to clarify, for ANY carburettor, if the filtration includes an elbow, you have to go for something as big as will fit so that the carburettor is not stifled.

I have experimented with differing makes, & found that JUST a 40 mm bore elbow (NO FILTER) on a 30 mm carburettor was enough to cause issues. As said previously, I have serious doubts to the state of tune on 'kitted' lambrettas that use a SH2/22 elbow & are supposed to run well. Just think how big those bellows are for just a 22 mm choke size & ask yourselves how can that work well on, say, a 30 mm choke which is 86% greater surface area! With any natural increase in RPM, the volume required would be even more, so look toward doubling a standard bellows size @ least.....

There must be a market for substantially sized Cobra Head elbows, but the nearest I have got is to acquire a reducing silicone elbow/bend off ebay & even use packing rings on the bellmouth to achieve the volume required in the 'plenum' area immediately upstream of the carburettor. BTW, the bigger end of any reducing elbow needs to be away from the carburettor, so it is a bit like a funnel effect.

EDIT

Worth a look:

viewtopic.php?f=10&t=8987

viewtopic.php?f=7&t=8624

viewtopic.php?f=10&t=8427

Is your engine piston ported by any chance? Bellows and even the breathsweet cause problems with PHBH as well. CASSA (I think) do a large bore filter rubber that is flater and wider in the mid section so that it doesnt get squashed and deformed by the panel.

Yes. 225 Indian factory stage 4

I did look at the Casa bellows, but wanted real life experience rather than potentially waste more money on something that wouldn’t work