Dellorto Help Required
Discussion
Spydaman said:
Thanks for that. I had a read through it and believe it refers to the progression from idle mixture to the progression drillings/idle jet whereas my problem is the progression drillings/idle jet to main jet circuit.
The progression drillings section "is" the transition phase to main jet.http://www.webercarburatori.com/?p=handbook
Figure 11.
This diagram will help explain your issue much better than the confusing one you linked to with the terrible advice about main air corrector jets.
Your problem is the transition phase is winding down before the main circuit has really kicked in. This is partly because the carbs are rather big for the engine power and there isn't enough airflow to get the fuel moving at small throttle openings / low rpm. However on a more sophisticated carb like a DCOE there are ways to overcome this. As the diagram indicates it is primarily the auxilliary venturi size that determines the rpm at which the mains kick in but you appear to have no options there. Going smaller on the chokes should help a bit by increasing the overall air speed but it's still the aux that does most of the work.
Increasing main jet size will richen up the start of the main jet circuit but also richen it up everywhere else and that's no good. Changing the main air correctors is just flat wrong advice. That just wrecks the top end fueling.
The other alternative is to extend the progression phase until it properly overlaps the main jet phase. However even the idle jet air correctors don't actually extend the length of this phase much. They just richen or weaken whatever is there.
So as I said previously it's the location and size of the progression holes that determine the length of the progression phase. One more hole in each barrel drilled a bit further away from the idle jet would extend this phase as the butterfly opens as the Weber link I gave you earlier shows. Not easy to do though or even know what size and location hole to aim for.
36mm carbs would no doubt be a much better option or even a single twin choke carb on a different manifold. However your original jetting looked very close everywhere except this glitch which you said you didn't really even notice until you got AFR metering on it. Maybe just live with it. At least you now know a bit more about how carbs work.
Your problem is the transition phase is winding down before the main circuit has really kicked in. This is partly because the carbs are rather big for the engine power and there isn't enough airflow to get the fuel moving at small throttle openings / low rpm. However on a more sophisticated carb like a DCOE there are ways to overcome this. As the diagram indicates it is primarily the auxilliary venturi size that determines the rpm at which the mains kick in but you appear to have no options there. Going smaller on the chokes should help a bit by increasing the overall air speed but it's still the aux that does most of the work.
Increasing main jet size will richen up the start of the main jet circuit but also richen it up everywhere else and that's no good. Changing the main air correctors is just flat wrong advice. That just wrecks the top end fueling.
The other alternative is to extend the progression phase until it properly overlaps the main jet phase. However even the idle jet air correctors don't actually extend the length of this phase much. They just richen or weaken whatever is there.
So as I said previously it's the location and size of the progression holes that determine the length of the progression phase. One more hole in each barrel drilled a bit further away from the idle jet would extend this phase as the butterfly opens as the Weber link I gave you earlier shows. Not easy to do though or even know what size and location hole to aim for.
36mm carbs would no doubt be a much better option or even a single twin choke carb on a different manifold. However your original jetting looked very close everywhere except this glitch which you said you didn't really even notice until you got AFR metering on it. Maybe just live with it. At least you now know a bit more about how carbs work.
I've got a pair of 36 DRLA's but only have 30mm chokes for them. I've also got single 40 DRLA on a manifold but they're not supposed to be very good due to the length of the inlet tract. I guess the next step will be 28mm chokes in the 40 DRLA's with jetting to suit. If that doesn't work try the 36's. If they don't work I'll have to live with it. I won't be able to try anything for a couple of weeks as I'm holiday at the weekend. Thanks for all the advise and education.
It was fairly common practice to increase the size of the progression delivery drillings in the barrel on 48 and 50 DCOEs to increase fuelling over the first transient (progression) and prevent leaning off on very light throttle. it is a bit of a blunt instrument and a problem if you get it wrong, but it most definitely helped.
Dave
Dave
I believe my problem is the transion from the idle circuit to main rather than idle mixture to idle cicuit. Looking at the progression holes it looks like it has the full compliment already and as you say it's a bit difficult to make holes samller if it doesn't work. I think in the first instance I need to get a bit more suck on main circuit a mid throttle to get it to come in earlier. Or extend the range of the idle circuit so it overlaps the main circuit. As Pumaracing says by reducing the choke size.
I'll try and restate all that with the correct terminology.
Spydaman said:
I believe my problem is the transition from the PROGRESSION circuit to main rather than from idle mixture to PROGRESSION circuit. Looking at the progression holes it looks like it has the full complEment already and as you say it's a bit difficult to make holes smaller if it doesn't work. I think in the first instance I need to get a bit more suck on the main circuit at mid throttle to get it to come in earlier. Or extend the range of the PROGRESSION circuit so it overlaps the main circuit. As Pumaracing says by reducing the choke size.
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