Jetting

February 14, 2010

As promised, our jetting for twin Dellorto DHLA 40, emissions version.

NOTE: This will probably NOT WORK well for non-emission spec Dellorto or Weber sidedrafts on a similar engine.

Idle: 65 (this must be around the 45 mark if you run non-emissions series)

Air Corrector: 150

Emulsion tube: 7772.11

Main jet: 130 (probably needs to be 135 or so. Gets a little lean on high RPM)

Engine: SOHC 1300cc head worked with larger valves and mystery cam

1 1/2″ headers, 4 into 1 exhaust, 3″ secondary, high flow muffler

At the moment, got 35 degree of advance dialled in. The dizzy-less Megajolt ignition system is currently being perused as an alternative.

Industrial espionage is probably not an issue for 30 year old technology. If anyone has thoughts or comments on these figures, we’d love to hear from you.

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Research, then trial and error.

Research was teh crucial part, and the reason why it has taken us nearly a year to progress to the point where the car will start and idle successfully. I haven’t written on this blog for many months because it has been just too embarrassingly disheartening. For a while I lost faith, thought we’d spent too much on some crappy leaky sidedrafts which didn’t bloody work. Research shed some light. Here’s the saga as it unfolded.

Synch Carbs

Juice Synching Carbs

Possibly as far back as 12 months ago, some reading on Guy Croft’s excellent forum turned up the holy grail of 40mm Weber sidedraft jetting for our particular application. Some careful fudging of these figures to allow for a smaller displacement went on in Julian’s head, and we came up with a theoretically perfect jet combination. The carbs were already rebuilt, the world’s sexiest airbox was welded up so all that was left was to insert textbook jets and race.

Not quite.

To cut a year-long story short, we found out that there are many different series of Dellorto DHLA carburetors (See this article, possibly originating from the Sideways Technologies forum). Our particular ones came off an Alfa, so were probably emissions, meaning that the progression circuit was designed to meet the US emissions laws, and thus less tunable than earlier non-emissions (and most Weber) carbs. Thus the textbook idle jets we forked out for were way too lean (we’d planned on 47, currently running 60) and air correctors were too small (details to follow soon!).

The Interrogator

Pross, 'The Interrogator'

This much we now knew, so we booked in to see Pross at Enhanced Motorworks in Fairfield. After 5 or so dyno runs, and a consultation by a talking computer called the Interrogator, we had eliminated the lean idle, synch’d the throats, discovered the rich spots and dialed the advance. It took a morning, but Bianca is finally dialled for race. The final figure was about 60kW / 80 HP at the wheels. I guesstimate that the whole package weighs less than 800 kg wringing wet, which brings us close to 100HP/ton. I’m off to the Alfa club website to book us in for 14 March. Watch this space!

P.S. Thanks go to Woppy for his thoughts on why it wouldn’t hold idle, and Peter at Thornbury Carbureters for being so conveniently located.