4 stroking between 1/4 and 1/2 throttle

[CPH Lambretta]

I have a stage 4 175 kit fitted with a standard 22mm dellorto carb and clubman thats proving hard to set up.
Its 4 stroking between 1/4 and 1/2 throttle, what do I need to adjust to make it stop? Its crap when pulling away and tickover isn't great either.Pilot jet is a 47 and mixture screw is only a half turn out.
I'm only used to PHBH carbs and this standard carb is a pain.

[bristolmod]

need to know what jets; slide and atomiser you've got fitted.

As a starter I'd be inclined to jet/ slide / atomise as per GP150 ( 118;45;50/ 7895-2/ 5899-4)

Chris

[CPH Lambretta]

My full setup is Harry Barlow mild tuned stage 4 175, clubman/22mm dellorto with a Malchard filter on a GP bellows.
47 pilot jet/5899.2 atomiser drilled to 2mm (had fuel starvation problems when not drilled) GP 200 slide and a 145 main jet.
Harry advised a 135-140 main jet but with that the plug was white ,so I upped it to a 145 and now the running low down is s**t.
Even with the mixture screw on half a turn its spluttering at 1/4 throttle. I was thinking of going back to the standard 45 pilot jet to see if it cleans up the running low down/pulling away/tickover.
Like I say I've never had a standard carb on a tuned cylinder before, I usually run PHBH carbs.

[bristolmod]

theres something wrong there- the main jet doesn't come into operation until 1/2 throttle at least, so your low down bogging is something else.

That said, even a 135 on a mild 175 tune does seem excessive- but if thats what Harry Barlow recommends.

Has the carb been stripped and thoroughly cleaned? sounds like a blocked drilling or something somewhere?

Can you try another 22mm carb jetted for a standard machine to see if there is any difference?

Is it a Dellorto or a Jetex you are using?

[mick1]

Sounds like the choke could be sticking and not closing fully. Worth a look. Also try without the filter on, may be strangling it low down (some filters work better than others).

[CPH Lambretta]

Its a dellorto carb and I checked the choke and its closing fine.Thanks to you both for your input.
I think maybe the remote filter is flowing too much air and was wondering about trying it through an air box?
The low down running was fine until I upped the main to 145 from 135 because of the lean running above half throttle.
Like I say I'm used to a carb that has a needle and atomiser where the main jet has no affect until well over half throttle, but upping the main jet on this standard carb seems to have richened the jetting through the whole range?
I tried putting the standard (weaker) 7895-2 slide in but it is nowhere near ticking over with the tickover screw as far in as it will go.
Think I might go back to the 7895-1 slide/drilled atomiser/135 main jet setup which ran quite well and then slowly increase the main jet to try to get a balance between boggy low down and weak high up.

[OAP]

Personally I would run it through the air box without a filter. Is the carb a genuine 22/2 Dellorto? Go smaller on the pilot jet, keep the main jet high.
Also air screw at 1.5 turns out. give me a call if you get stuck. RM

[fastfrog]

dello carbs are based on Weber mixture tube principe. As a non needle carb mixture tube does what needle is usually doing on a standard carb. In this very case main jet controls from 1/4 to complete opening of the throttle. If your engine is 4stroking between 1/4 and 1/2 and going great at WOT try to find an atomizer with more and bigger holes in its upper section. Anyway 145 sounds big to me...

[CPH Lambretta]

@ Ron, thanks for the advice and offer, I might give you a call if I can't get it sorted, and yes it is a genuine Dellorto not a jetex.

@ french frog, cheers for the info, what you said makes alot of sense, thats why its bogging as soon as the main jet comes into play at around 1/4 throttle, runs great at WOT but crap lower down.

[fastfrog]

If you cannot find the right atomizer. Modify yours. Take inspiration from a vespa BE2 mixer tube. the leanest at 1/4 to 1/2 throttle opening. On its narrower section are 8 holes. Compare with your atomizer and see if you can drill bigger (same diam as BE2) or higher holes or both... Begin by bigger ones....