Delly mixture screw and progression circuit

[davidblythe]

Trying to get my head round this, so please correct me

on a delly, there is a mixture screw as opposed to an air screw

the mixture screw controls the idle by allowing fuel from the pilot jet to pass through the idle circuits but doesnt affect the progression circuit, only the size of the pilot jet does this

therefore if 1/8 of a throttle is too lean, increase the size of the pilot jet, you can lean the idle back with the mixture screw but the progression circuit at 1/8 of a throttle will still be richer

is this correct

conversly an air screw affects both the idle and the progression circuit by bleeding more air to the fuel supply from the pilot jet

any good info on the internet for a numpty to understand

[rog60]

The mixture screw is situated on the engine side of the carb(after the slide) and the air screw on the intake side of(before) the slide.............

I have a copy of the guide to tuning Dellorto carbs which I'll try to copy onto here...........

Text is too small to copy and be readable so here's a synopsis of the text:-

Progression System.(For Dellorto carburettors)

By progression we mean the transition period between mixture delivery from the idle circuit and the beginning of mixture delivery from the main jet circuit.
On first opening the throttle, the air drawn into the engine increases and therefore, in order to have an inflammable mixture still, the fuel supply must also be increased.
As previously noted, the idle hole situated just in the front of the slide, only delivers sufficient fuel for engine idle operation and the main circuit still does not deliver any fuel because of insufficient vacuum upstream of the throttle. The progression hole, just under the front of the slide, is therefore necessary to deliver the fuel required during this transition period. The progression hole draws fuel from the idle circuit (underneath) and is positioned immediately upstream (in front of) of the closing edge of the throttle slide for the prompt response to fuel demand when the airflow suddenly increases.
It is interesting to note that the progression hole serves a duel purpose. When the engine is idling, air from the main barrel passes into the progression hole and weakens the mixture flowing through the idle circuit. When the throttle is opened slightly, the idle circuit mixture flows into the main barrel through the progression hole. The progression hole therefore first feeds air in one direction (during idling) and then feeds mixture in the opposite direction.

Finally, as I understand it, this phase is linked to mixture screw/idle jet size til at least 1/4 throttle opening and through to 3/4 throttle so what you are saying is essentially correct...

Hopes that others agree and it makes some sense to you.
A Guide to Tuning Dellorto carburettors is availble from Scooter dealers or Eurocarb Ltd, 256 Kentwood Road, Tilehurst, Reading, RG31 6DR. tel 0118 943 1180. http://www.dellorto.co.uk and is a worthwhile purchase to understand how Dellorto carbs work.

[J1MS]

If I remember rightly as I havnt got one to look at thats running. The Amal mk 2 has a petrol correction screw located after the slide but a carb with an air screw adjuster has this located before the slide... I thought both had progression tubes that relied on the size of the pilot jet governed by drillings in the carb that were revealed to the venturi as the slide is lifted up to about 1/8 to 1/4 throttle, which by this point the venturi is already drawing on the over lapping needle /needle jet.... so I think pilot jet size would effect both the same...

[davidblythe]

Hi JIMS

are you sure

as i understand it an air correction screw affects both progression and idle circuits together whereas a mixture screw only affects the idle

always willing to bow to superior knowledge thou

[davidblythe]

Cheers Rog60

the post makes sense after reading a few times

[J1MS]

Sorry Dave I was explaining what I thought it was. I am not reading anything with facts, just what I thought..... I could and probably am wrong... I think I need a refresher course on the subject to give a more definitive answer... It honestly is only what I thought, As I havnt done any reading up on Progression systems in probably 25-30 years... I will see what I can find out thats fact based... to Assume, is just a guess... its always best to know how, based on fact... And Im obviously guessing...

[rog60]

Cheers Rog60

the post makes sense after reading a few times

I had the same trouble with trying to take it all in..........when I finally got my head round it and studied my Dellorto it all started to make some kind of sense and I am now starting to understand what effect the carb has on the engine at any given throttle setting and then make the appropriate changes when necessary,but there are still plenty of combinations of solution to trip up the unwary