Dellorto Progression Holes and Butterflies

[William2]

My car has Dellorto DHLA40Es' fitted which have 5 progression holes. My question to the panel is at tick over should these holes all be shut off by the butterflies or should one or two of them be open?

[Craven]

That?s the name of the game, enough air should pass the butterfly plate for tickover WITHOUT any contribution from the progression holes.
Angle of butterfly at which the progression starts is what it?s all about!!

[mbell]

Generally you should be able to see the rear edge of the plate thou the first person hole but none of them should be exposed. So all the fuel for idle cones via the idle screw and not the progression holes.

[Esprit2]

With everything properly jetted and tuned, enough air should pass around the perimeter of the butterflies to support normal idle without any involvement from the idle progression holes.

As tighter emissions started coming into play, the amount of ignition advance at idle was cut way back. All else being equal, less advance results in a slower idle... too slow to keep the engine running. To compensate, the idle speed screw has to be turned to open the butterflies a bit more than normal to and bring the idle speed back up. It is a symtom of 'minimal advance emissions tuning' that you start to see more of the first idle progression hole being exposed. That's not uncommon for later engines, but it's not desireable for good running or performance. A more open throttle at idle also lead to the problem of the engine running on when switched off that was so common to early, carbureted emissions engines.

If emissions aren't your first concern, then you can improve how well the engine idles and runs. As usual, the first step to tuning carbs is always an ignition tune-up, and that might involve re-curving the mechanical advance to something a little more aggressive, plus dialing in some more static advance. Doing that will bring the idle speed up, so the idle speed screw will have to be adjusted to close the butterflies a little. Emissions tuning may expose the first progression hole, while performance tuning probably won't.

40E & 45E carbs have idle mixture screws with fine threads and long-taper needles. If you adjust them for peak manifold vacuum, then they should end up 3.0 to 3.75 turns out from seated. Any more than that and the idle circuit is too lean and you're trying to compensate (incorrectly) by turning the mixture screws out further. Any less than that, and the idle circuit is too rich, and again you trying to compensate with the screws. (The "non-E" Dellortos more coarse, blunt idle mixture screws will end up around 2.0-2.75 turns out.)

Tune up the ignition, get the idle circuit mixture properly tuned (both idle jets and idle air correctors), set the idle mixture screws for peak manifold vacuum, then set the idle rpm to 900-1000 rpm. In that condition, the engine will, for the most part, be idling without the idle progression holes. The edge of the butterflies may be visible along one side of the first progression hole, but the hole will not be significantly open.

Progression hole exposure is not a tuning tool, it's a after the fact result of proper tuning. Don't focus on them. Instead, focus on tuning as noted above, and the progression holes will take care of themselves.

By the time Lotus was using Dellortos on the Twin Cam, early emissions standards were in effect. The jetting set-up given in the manuals and TSBs are pretty good as a starting point, but the idle circuits are on the emissions-lean side... they had to meet mandated standards.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[William2]

Many thanks Tim for your comprehensive advice. As a point of interest this thread relates to a problem I have with idling as described under "Dellorto Idling Frustration" posted 4th Dec. If you have a moment to spare maybe you check it out. I think I will consider taking the car to a Lotus dealer with tuning experience and get them to set the idle and timing. I will though firstly check the position of the butterflys with respect to the progression holes as Rohan has suggested in case a spindle is twisted.
Aside from the idle problem the engine seems to run well with no hesitation, no running on after switch off and plug colour looks fine.