I should also add that the two carbs have the connecting fuel line between them (that is how it came).
This is normal as there is only one electric fuel tap as standard, the second tap is a reserve tap,
do you mean the vacuum balance pipe on the vacuum side of the carbs?
In any case the vacuum balance pipe, if present, should be removed and the vacuum ports blanked or used to connect to the vacuum gauges/manometer during carb balance.
I am now stumped, how could 50 idle jets and 43 idle jets need the same adjustment at the mixture screw to obtain a blue burn? Also why do I have to adjust the mixture screw to almost full in before I get a blue flame?
Because as designed the diameter of the idle jet is not a controlling factor in the idle mixture circuit,
selection of the idle jet is based on opening (and closing) the throttle, the idle jet is important in progression and overrun.
So what does control the idle mixture when the carburettor is in good conditions, i.e no wear or blockages and all seals etc good.
1. The size of the progression hole
2. The vacuum, or rather lack of, this needs to be weak
3. The type of taper on the mixture screw, various models with HS carbs had a longer taper.
BS carbs usually have the short taper.
4. The position of the screw.
This is important to understand as its not obvious.
During idling the progression hole draws air in to weaken the mixture passing through the idle circuit (from the idle jet)
This is intended to weaken the idle mixture, actually I would say it provides a correction to the fuel air ratio but I will not argue with the Dell Orto manual.
How much of this weakened mixture is fed to the cylinder is controlled by the mixture screw,
The mixture will mix with a small amount of additional air coming under the tiny gap below the slide.
Often I have found that worn carburetors will never satisfactorily idle on the idle circuit.
simply too much air is leaking, the weak vacuum required by the idle circuit is destroyed, the circuit needs the air through the progression hole
so its too rich, we try to wind in the screw but it bottoms out, our only recourse is to weaken down the idle jet, drastically,
but this is a bodge.
What we should do is to figure out if the slide is leaking too much or is far to open. I can tell if a cylinder is idling on the slide as I call it.
The commonest thing is the mixture screw has no effect at all.
Typically as Paul says the cylinder is actually idling on the slide and
On the other hand if retarded, the slides may end up too far open to
compensate and you'll be on the progression circuit, not idle.
So you must check that the ignition is right. Not only the 6000 rpm full advance but also that it backs off at idle. There are
so many possible combinations of pickup and transducers that could affect the advance. The standard system
generally goes from about 22 degrees to 34 degrees, but often there is less advance range
If happy with the ignition timing.
The selection of a smaller idle jet is beneficial in some cases to improve the progression particularly
on worn carburettors but it wont fix the idle properly. A new slide would be a superior fix.
The conclusion I have come to (maybe wrongly) is that the carb is getting the additional fuel in some other way, possibly the float bowl is too full and the other jets are picking it up irrespective of the idle jet size?
To establish fuel level is extremely common place in other manufacturers carbs
and the special modified bowl connection and sight tube is readily available.
Excessive staining at the vent hole would indicate badly awry float level, but since this is not a two stroke
I don't think the fuel level to be super critical. The fuel level is the same for all type of floats suitable for VHBZ
carbs i.e round solid, more squarish solid, black round and hollow and the beastly white hollow ones which were
standard fit on Darts.
Another source could be via a poorly seated choke valve, the small rubber seal of which there are two types, can be perished
I think new seals can be obtained, but certainly new choke plungers can be.
Often overlooked is this:
The atomiser is 2.60mm and the standard needle is 2.45mm, therefore only (2.60-2.45)/2 = 0.075mm of wear on the
slide face or the carb body itself is enough to cause the needle to come in contact with the atomiser wall in this
case there is a similar effect to capilliary action causing unatomised fuel to draw up the needle. Though this tends to affect
progression very badly due to poor fuel atomisation at least until we are on the taper of the needle after 1/4 throttle.
Now to compensate the slide needs to be a little more open to get more air.
Then you get that typical coughing off idle of Morinis with shagged carbs these days.
There are some small compensations to jetting for modern fuel and many report that progression is improved with a smaller idle jet,
with 45 being a standard.
The question arises why the 260BD for A sports, with the additional higher holes in the atomiser,
which should be above the fuel level were placed to weaken the mixture in the midrange.
My understanding is the atomiser solution was developed for the Sport which was running too rich in the mid range.
Too rich? Yes indeed! Someone can ask Ing. Lambertini if we ever get to Morano!
Whilst the 260BD is no longer available it is easy to drill the holes needed by modification of a 260K
full details available to club members via ATG a few issues back.
Tuning guide is available from Eurocarb for very reasonable price.
Mark