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DCOE idle jet ‘plenum’


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I’ve been chasing down my off idle lean, started with a 50f8 which gave me a massive off idle lean hole (17 to 20 AFRs), bought some 50f9 which made a small difference, rather than spending $$$ on idle jets I experimented using solder and drills, drilling out fuel orifice of the 50f8  to .55 again helped but only by a small amount. I then decided to attack the air orifice, soldered it up and drilled to f6 size (0.70mm) AFRs dropped to 11-12. I then gradually enlarged air orifice size gradually and now with the holes at 0.90mm I’m happily with the AFRs in the 13s.


Now to my question! The f12 jet has a air orifice of 0.90 but as shown on this chart the diameter of A is 1.5mm and the F8 I have been modifying has a diameter of 2mm, will this difference in ‘plenum’ size make any difference, I assume it is done for a reason but can anyone shed some light on why?



The plan is to buy some 55f12 jets as I don’t like the idea of roughly soldered and drilled jets staying in there too long!

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Yeeps- you're farther down that rabbit hole than I've ever gotten.


In THEORY, no, the size of the back drilling shouldn't matter, right?  Just 

the metering holes.


In PRACTICE, it probably does, as almost everything Weber Italy did

had a purpose.  I suspect it's acting a little like an emulsion chamber- giving

the fuel and air more volume to combine.  So the bigger the volume after the

air correction port and before the metering jet, the more volume for air/ fuel emulsion.

And thus, perhaps, the slower  the process, as both air and fuel will have a lower

velocity.  Or maybe the more complete  the process, as it has more time?

Or both- at lower velocity, less emulsion, but at higher flows, more?

Lots of what Weber did so successfully was to control the 'curve' of the

a/f ratio as flows increased...


Thinking about this a little, it would be tempting to play with number of air corrector holes,

placement in the jet body. yadda yadda.  Weber must have had some pretty neat flow

machines to come up with all of these...


A question of my own, if you don't mind- what did you use to close up the progression holes?




"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Thanks for the replies

Hans - Spanish Webers, the last post in my blog details the issues I had with them, now resolved.

Toby - yes I think I might extricate myself from the rabbit hole, buy some 55f12s and suck it and see. Did not touch progression holes just normal adjustment of throttle plates using idle/balance screws.

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13AFR seems a bit too lean to start out at idle and attack the lean hole. You  may want to adjust  your throttle plates so that they are in the middle of the first progression holes, shooting for an idle mixture in the mid to low 12s at an idle speed of ~1K RPMs.  This is presuming you have balanced the carbs.

Chris B.

'73 ex-Malaga

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Lean hole is not there with my current settings but out of interest  I will fatten up the idle a bit and see what happens.

I would ideally like the throttle plate to sit over the first progression hole as the Weber manual suggests but gives me a too high idle even after playing with the ignition advance at idle.

This is the throttle plate position at idle of around 950rpm



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