LCGB Forums

Need to open out my inlet valve, it’s 2mm currently, and only flows about 200ml of fuel a minute, where the tap flows the required 500ml. What size does it need to be? Getting fuel starvation at full revs when I’ve been full out for about 30 secs, and MB say it’s the carb not flowing enough. The float valve rubber is 4mm diameter so that’s the max, calculating the area opening out to 3mm will give me about half as much area, up to 9 mm squared from 6mm squared. So it will never flow the required 500ml through the float valve.

After anyone’s experience, cheers

Is it removable?

No, has to be drilled in place. Pretty easy, it’s only brass. I think I’m going to go for 3.5mm, it stays less than the 4mm rubber cone on the float valve.
I’ve seen other posts which says 4mm on Dellorto carbs. But the float valve itself is only 4mm, so there’s no room for error on tolerances.

I'm guessing the "500 ml per minute" comes from the excellent publication by Sticky, but that relates to the tap flow rate suggested as 300 to 500 ml per minute.

However, if you look @ the recommendations by Dell'Orto, they suggest the fuel inlet valve size should be 30% greater than the main jet size.

That is a good 'rule of thumb' in my experience & one that I try to adhere to. There is a tendency with a lot of people to go even bigger than necessary, but my belief is that the float level is better maintained with the smaller (safe!) sizes with less tendency to flood or run rich.

I have a couple of PWK carburettors knocking about & think only one has been modified, but I cannot remember the hole size.

Bearing in mind the Dell'Orto recommendation, I would suggest you drill in stages & test the improvement in flow with each drilling, starting off with 3 mm....

The miniriders link would be more helpful if his photo link worked

The most relevant thing he says though is checking the float height which is crucial on these carbs as the float bowl is quite small and doesn't take many seconds to be sucked dry

I have spent a LOT of time messing with PWK type carbs, persevering as they are quite a bit smaller than the PHBH equivalent and can make lot easier to fit a filter under the panel. BUT I have thrown the towel in mainly due to the fuel starvation issue.......Warkton's advice on the main jet to float jet ration has got me thinking though

Ever since I've tried to glean tuning information about two strokes from sources beyond Lambretta, time & again I have noticed the preference for carburettors specifically designed for purpose.

The PHBH round slide carburettor is not amongst the best products for two strokes & it's popularity is probably due to availability of knowledge & the wealth of parts to allow it to be set-up to something like satisfactory.

Keihin products are renowned within other two stroke fields & the copies that are available take license from the PWK range.

The slide, in particular, is crucial to the efficiency & is a proper guillotine shape that decreases the effect of eddying within the Venturi. Dell'Orto with the VHSA/B/C etc range, Mikuni, Lectron & EI have all designed carburettors specific to performance two strokes.

Sorry if this appears to be getting 'off subject' but I wanted to explain why I think it's worth persevering with the PWK, despite hitting a hurdle with fuel delivery.

Here's a couple of websites worth reading through. I only hope I can remember them when I eventually get around to trying my own PWK's! :

http://www.classictrial.co.uk/okokeihinsetup.html

http://www.mid-atlantictrials.com/Instal.html

That 2nd link seems very useful, cheers

Further to previous couple of posts, my experience is that you have to have the patience of a saint, and the wallet of Abramovitch, to set up copy Keihins (buying lots of needles and jets).

Dell’ortos are simple in comparison, albeit that the initial response may not be quite as good.

Give me a PHBh30 every time albeit that I do like the Mikuni TM24

Storkfoot wrote:Further to previous couple of posts, my experience is that you have to have the patience of a saint, and the wallet of Abramovitch, to set up copy Keihins (buying lots of needles and jets).

Dell’ortos are simple in comparison, albeit that the initial response may not be quite as good.

Give me a PHBh30 every time albeit that I do like the Mikuni TM24

Your experience seems somewhat different to what I have read on other non-Lambretta Forums, but there are copies & then again copies....

If anything, the ScootRS database would indicate that there is very little variation in the set-ups for a varied range of engines & I would judge from that the PWK to be no worse than the PHBH on comparable databases contributed to by end users.