Tuning carb

[IanHood]

Does anybody on here run a Dellorto PHBH30 AS Dellorto on a 450 and know the best combination of jets / needle position etc. I’m having a hell of a job getting mine to run properly. Main issue is that it hesitates badly on rapid throttle opening. Any advice would be greatly appreciated before I tear out what little hair I have remaining

Ian

[Duccout]

Hi Ian,

I would guess that your best bet (if you have not done it already) is to copy the settings from the 750 GT.


Cheers,

Colin

[IanHood]

Well I have one of those so that’s worth a try but the 750 carbs are different (PHF 30). The PHBH 30 AS is sold by Eurocarb as a suitable replacement for VHB 27-30 carbs for 4 stroke Ducati / Guzzi engines up to 500cc. It comes with 116,120 & 125 main jets and 48, 50 & 52 idle jets. I’ve tried all sorts of combinations with no real success so far.

Ian

[ranton_rambler]

Have you read the Dellorto tuning manual? It’s easily found online for free download.
Assuming it starts and idles ok, it sounds like your issue is what they call “progression” and from memory you need to look at atomiser, slide cutaway or needle profile.
Dellorto needles are available as knock-off versions on eBay, with a set of 10 different ones for about the cost of one genuine one! The profiles are available on line so you can see which ones give you richer or leaner mixture at different throttle positions.

[IanHood]

Yep, I do think it’s a progression issue in some way - I’m talking to Eurocarb at the moment to see what they suggest.

Ian

[ranton_rambler]

I’ll see if I kept my notes from setting the PHB24 carb on my bike. I think I went to a bigger atomiser which makes the mixture richer at small throttle openings by the bigger annular area around the parallel section of the needle. After that it’s the needle profile for the mid range. I made a spreadsheet of the annular area vs slide position so I could compare based on road tests.
Main jet is only really effective at full throttle. Luckily my 175 is slow enough to be able to hold full throttle without travelling at lunatic speeds.

[Eldert]

atomiser 265AB , needle one with a 2.45 mm shaft . K1 or K6

Eldert

[ranton_rambler]

atomiser 265AB , needle one with a 2.45 mm shaft . K1 or K6

Eldert

My carb is PHBL24 which uses D needles.

I'm not an expert but I think the PHBH30 uses X needles and K needles go with the PHF carbs?

Maybe they can be swapped around anyway.

Useful needle chart - https://www.lambrettaspares.com/pages/d ... EEDLES.pdf

[IanHood]

Yep, PHBH use X needles.

Ian

[IanHood]

Well, I have solved the problem and it had nothing to do with fuel delivery.

The bloody crank tab washer had sheared which resulted in the Electrex flywheel coming loose. It remained tight enough to generate a spark to start the bike but not tight enough to maintain a spark under load.

Not too much damage fortunately and I should be able to get away with lapping the flywheel to the crank and a new tab washer. I may use some loctite on the flywheel taper this time - any thoughts on doing that?

Ian