>Now, the engine will run at idle but
>sputters and stalls when the accelerator is applied. What did I
>do?????
>
There are a number of things to re-check and I'm working from the generalities
of carbs not specifics.
Are the really tiny drillings, and some might be quite obscure, really clear ?
Is the main jet, emulsion tube and air corrector if fitted correctly assembled ?
Is the accelerator pump working - that is the linkage and springs in the correct
place, the pump jet and the discharge nozzle clear ? The biggest pitfall is the
likelihood of loosing or mis-assembling a single, small, ball bearing that acts
as a non-return valve so the pump discharges into the venturii not back into the
float chamber. This commonly found single item is the cause of more hassle than
you can shake a stick at owing to loss on strip-down and inadvertent
mis-assembly.
Maybe its worth taking a moment to review what a carburettor needs to do and how
this is achieved, leaving aside variable venturii instruments (SU, Stromberg
etc) for the moment although these have the same facilities, the fixed venturii
types are invariably found on MV's.
Petrol/gas engines are constant volume machines, the combustion volume is fixed
although the quantity of mixture admitted varies, note the mathematical CR is
only achieved at full throttle and then its less than theoretical owing to
dynamic losses, at idle the running CR is way below 1:1. A diesel is a constant
pressure machine, the cylinder pressure at the point of combustion is always the
same weather it be idling or at full power, there is no throttle a (nearly) full
cylinder of air is admitted every induction stroke.
The petrol gas engine carb needs to supply accurately metered fuel/air mixture
from a minuscule amount at idle to vast quantities at full power/revs, with some
arrangement for extra enrichening on cold start and for sudden power demands.
No single arrangement can accomplish this and separate circuits are provided to
meet the requirement.
The idle arrangement is normally a small metering jet, an adjustable volume
needle screw "valve" or jet and a tiny discharge hole on the engine side of the
throttle butterfly with the throttle closed where it sees a very high manifold
vacuum at idle. The tiny flow of fuel needed for idle is discharged from this
downstream drilling with a small quantity of air drawn past the throttle
butterfly held fractionally open by the idle stop screw, so small are the
quantities that the screw needle valve is provided to fine-tune the idle mixture
ratio. The rest of the carb plays no part at idle and the idle or pilot system
can only supply fuel up to a fast idle.
There is a range of throttle positions just off idle to about 1/8th or perhaps
1/5th open where the engine demand is way above the idle circuit capability but
insufficient to produce enough vacuum through the main venturii to operate the
main jet. This would produce a weak "hole" in the carburation and is covered by
the progression system that normally can be seen as a single or up to five
staggered tiny drillings _just_ on the atmosphere side of the throttle butterfly
with it in the idle position. Small movements of the throttle butterfly angle
effectively connect these progression drillings from the inoperative atmosphere
side to the high vacuum engine side of the throttle plate as the butterfly
rotates. Sequentially these supply additional fuel with the increasing air flow
either from the idle jet itself or sometimes from a separate progression
metering fuel jet.
Further throttle opening has enough air flow through the venturii, which is only
a circular aircraft wing in section to create a slight vacuum without impeding
flow excessively, to pull fuel through the larger main jet system and discharge
it into this high volume flow. With this level of throttle opening the idle and
progression systems have little vacuum applied to them and discharge virtually
nothing.
For best power and economy it is often the case that fuel and some air is
premixed before discharge as a froth or emulsion, this is done by the main and
air corrector jets and a perforated tube known as the emulsion tube, usually the
main jet is at one end in the fuel and the air corrector in the other.
The sudden throttle opening is a problem, with a relatively low volume of
mixture flow through the manifold/header pipes sudden openings of the throttle
create a large pressure gradient from the piston crown at almost a perfect
vacuum to the mouth of the carb at atmosphere. There is a finite time taken for
the main jet to begin flowing extra fuel, if at all from idle, and the column of
mixture in the manifold/header behaves like a rain cloud being forced over a
mountain and de-pressurised, it rains petrol/gas wetting the header/manifold
walls and the engine stumbles/backfires/dies owing to the weak mixture. The
throttle driven accelerator pump covers this by discharging a jet of neat, extra
fuel into the air flow to somewhat crudely ensure the engine gets a fireable
mixture momentarily. Being a pump it has to have some form of one-way valve
system to refill itself which is where the loose ball comes in, often the ball
valve arrangement is so organised that little or no discharge occurs with very
slow throttle opening rates, the fuel being simply pushed back to the float
chamber or not discharged but cycled back to the inlet, sudden increases of
pressure are needed to seat these check valves.
There only remains the start enrichment which is simply done with an upstream
flap valve so that the full engine vacuum is felt on the main jet at cranking
speed and there is normally an arrangement to allow the engine to partially open
the choke flap when it fires by using an off-centre sprung choke butterfly or
having a lightly sprung flap in the choke valve itself. Occasionally a small,
start only, extra rich, separate carburettor is built integral with the main
body and by-passes the throttle butterfly.
There may be another self-contained system commonly found using a sprung
diaphragm normally located on the outside of the carb body. This is variously
referred to as the power or economy valve and just by-passes a little extra fuel
around the main jet when the manifold/header vacuum is low and has the effect of
leaning the mixture slightly under conditions of fast cruise and providing a
slightly rich one at full power.
No matter who makes the carb or how the facilities are realised in metal these
elements will be apparent as a fundamental necessity to make the engine work.
Richard
(Southampton UK)
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