I will be going for a rolling road session at some point in the coming months but this has puzzled me for some time:

My twin webers on my 2.0L Zetec are currently jetted with Raceline defaults (the GEMS ECU is also Raceline programmed).

34mm chokes

120 main jets

125 air correctors

F16 emulsion tubes

idle jets 65F8

pump jet 50

spill valves 45

these settings are also recommended by Westfield (carbs supplied by them are apparently jetted like this)

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Dave Andrews’s recommendations look like this:

(performance/flexibility)

38/36mm chokes

155 / 145 main jets

190 air correctors

F9/F16/F2 emulsion tubes

idle jets 50F9

pump jet 40

spill valves ?

My car idles well pulls away without hesitation and accelerates very very quickly from 0 to just over 100 Mph (on the track obviously) without a hiccup at which point it runs out of steam (34mm chokes restricting the engine at this point this much I can understand). It pops and spits a little on light throttle (this may be because I have the air bleed screws opened a little to pass the SVA emissions test).

I have no complaints, I just wondered how the jettings could be so different and presumably end up with the same result ?

[Edited on 20/2/05 by givemethebighammer]

I am not a Zetec man so is mainly based on Lotus Twincam and Imp DCOE Weber experience -- there are guys around here with a lot more Zetec and Weber experience than me ----

Looking at your current jetting I strongly suspect your engines is running mainly on the idle jets; the main jets are small, the idle jets are big and the air correctors small this will mean the idle jets are supplying fuel to well up the speed range.

The Dave Andrews set up the change over from idle to main jets occurs lower down.

I don't think the 34mm venturis are affecting your maximum speed to any great degree but I suspect the small main jet size "might" just be restricting the top end.


As a rough guide for for getting initial setting I used to use these formula
Main jet size = venturi size x 4
Air corrector jet size = main jet size + 40
Also main jet size is roughly related to power output --- it should be in the roughly same ball park as the expected BHP.

Using these formula Dave Andrews jet sizes fit into normal practice Racelines don't.

So what determines when the carbs change over from the idle circuit to the main circuit ?

is there any problem with the engine running on the idle circuit further up the speed range ?

By my thinking the engine just sees petrol and air it doesn't care where it came from ?

I must read more into this carb theory it is looking to be quite interesting.

The change over is all to do with airflow, at high rpm & full throttle the full bore of the carb is flowing air this gives a very even fully developed velocity profile across the full dia of the carb, at low flow rates the velocity profile of the flow is entirely different it has a very distinct peak in the centre the air round the edges forms boundary layer which is largely stagnant this is why the the carb has an auxilary venturi right in the middle of the carb bore to give enough venturi effect to draw fuel through the idle jet. The idle jet was usually called the slow running jet in the UK and this is in fact a better description of its duty it can only provided anything like an approxomation of correct fuel up to say 2000 rpm above that it can't physically flow enough fuel because of the physical restriction of its metering orrifice size and the fact the velocity profile changes to fully developed. The main jet only comes into play when the air velocity close to the edges of the main venturi increases. In the main jet the hole through which fuel is drawn is much larger so it has the capacity to flow much more fuel than the idle jet.

At the change over point from idle to main jet the mixture will tend to be over rich so the air corrector jet is fitted which introduces air into the fuel flowing through the metering hole in the main jet.

At full throttle high rpm the main jet must be able to flow enough fuel to provide the heat engry to give peak power --- if the main jet is to small not enough fuel will be burned.

Interestingly the venturis and main jets you currently have fitted are roughly the same as would be fitted to a Lotus Elan Sprint 1558cc which developed a claimed 126 bhp at about 6000 rpm, your engine should be a fair bit morepowerful than that .