Carburetor throttle and choke control mechanism

Details Carburetor throttle and choke control mechanism Carburetor throttle and choke control mechanism

2001-03-05

2002-08-27

Chiesa, Richard L. (Department: 1724)

Gas and liquid contact apparatus

Fluid distribution

Valved

C123S179180, C261S064600

Reexamination Certificate

active

06439547

ABSTRACT:

FIELD OF INVENTION
The present invention relates to throttle and choke control mechanisms of carburetors for internal combustion engines, and more particularly to such a mechanism incorporating a choke-throttle, cold-start-setting latch mechanism that automatically positions the throttle valve slightly open when the choke valve is fully closed.
BACKGROUND OF THE INVENTION
In small carburetors designed for use with low displacement gasoline fueled engines, such as used on chain saws, weed whips, lawn mowers, garden tractors and other small lawn, garden, and forestry portable appliances, manually operated choke and throttle controls are typical provided and often hand cranking is employed for starting the engine. Prior to the late 1970s, chain saws equipped with such choke and throttle controls often involved a basic starting sequence which left much to be desired. First the choke valve was fully closed to its start position, and then the starter rope was pulled until the engine fired. The closed choke valve usually caused the engine to immediately die at this first firing due to over-enrichment of the air/fuel (A/F) mixture. This is commonly referred to as a false start. At this point the choke valve had to be opened. Then the starter rope was pulled again until the engine finally began running.
This starting sequence was subsequently improved by adding another start-up control to the chain saw whereby the throttle valve could be held at a partly opened position, known as fast idle position. This generally avoided false starts due to the increased air flow permitted past the throttle valve.
In order to avoid the need for three separate manually operated controls, namely, a throttle control, a choke control and fast idle start control Johansson, U.S. Pat. No. 4,123,480, issued Oct. 31, 1978 (which is incorporated herein by reference), disclosed an improved chain saw engine control mechanism. In the '480 patent a fast idle secondary lever
9
is pivoted on the choke valve shaft
11
and is operable to engage a latch arm of a throttle lever
4
fixed on the throttle valve shaft
2
to cause the throttle valve
1
to open to a predetermined angle corresponding to the fast idle position (FIG.
3
). With this arrangement, the operator need only operate a single start-up control, namely the choke valve control (not shown) coupled to the choke shaft control lever
12
in order to set the throttle
1
in fast idle condition. Thus, when the operator moves the choke control to swing the choke valve
10
from fully open position (
FIG. 1
) to its fully closed start position (FIG.
3
), the pivotal motion of choke shaft control lever
12
, via a push coupling tang
14
on the adjacent fast idle lever
9
, pivots fast idle lever
9
and causes its notch
8
to latch engage the throttle lever latch arm tang
7
, thereby automatically setting the fast idle latch mechanism. The normal biasing forces exerted by the respective fast idle lever spring and throttle shaft return spring (i.e., biasing the fast idle lever toward push coupling with the choke lever: biasing throttle valve
34
toward closed) and also used to provide the latch closing forces.
Then, due to this automatic latch up, if the chain saw engine experiences a false start, the choke lever
12
may be moved to the open position (
FIG. 4
) without thereby moving the fast idle lever i.e., because it remains engaged with the throttle lever to retain the throttle valve
1
in the fast idle position. Once the chain saw engine starts, the operator simply depresses the throttle control trigger
6
to open the throttle valve
1
. This pivots the throttle shaft lever
4
, thereby causing it to disengage the fast idle lever
9
and thus cause release of the latch. If the choke valve
10
was still in the closed position at this point, the choke biasing spring
15
, acting through the fast idle lever
9
and tang
14
coupling it to the choke lever, would automatically cause the choke valve
10
to be returned to full open position upon such unlatching of the fast idle lever
9
from the throttle lever
4
(FIG.
1
).
One of the disadvantages of this fast idle starting system (FISS) '480 patent design was its failure in practice when mass produced to insure complete and/or consistent closure of the choke valve
10
when setting the fast idle latch starting system. The specific problem has been found to be due to a pull-back or rock-back effect by the fast idle lever exerted on the choke lever resulting in the choke valve sometimes not being completely closed even though the operator has fully engaged the choke control to indicated start position. Further, it has been found that this problem is due to the need to provide an “over-travel” gap in the resting engagement of throttle lever tang in the fast idle lever notch to accommodate a stack up of normal manufacturing tolerances in the parts as manufactured for assembly into the fast idle latch mechanism.
Such manufacturing tolerances are, of course, necessary to set up minimum dimensional range limits or allowances to accommodate normal manufacturing equipment capabilities at acceptable manufacturing cost levels. This is a particular problem in producing carburetors for engines for chain saws, lawn mowers, clearing saws, weed whips, etc. that require very low manufacturing cost due to the low retail price of such consumer products. The problem is compounded due to the small size of the carburetors for such small engines, and the corresponding minuscule size of the choke and throttle parts involved in the carburetor mechanisms. These factors make it particularly difficult to reduce manufacturing tolerance allowances in order to reduce the adverse effects of unavoidable manufacturing dimensional variations in such tiny parts when assembled for operation in the mechanism.
Thus, in the case of the incomplete and/or inconsistent closure of the choke valve in the operation of the fast idle starting system of the '480 patent arrangement, it has been found that, without the aforementioned over-travel gap allowance, a shift in tolerances for all parts (tolerance stack-up) in the latch mechanism to one end limit will render the choke valve incapable of reaching the fully closed position. This prevents, or at least hinders engine starting. On the other hand, and without such gap allowance, a tolerance shift in all of these parts to the opposite end limit will cause the fast idle lever to fail to even engage with the throttle lever, so that no “latch up” action occurs. This results in a loss of function of the entire choke throttle fast idle system.
The culprit in this resultant choke valve pull-back or rock-back problem has been found to be the push coupling of choke lever
12
with the fast idle lever
9
(via tang
14
). This dictates that the actual latch-set position of choke valve
10
when initially swung to fully closed position will be controlled by the final latched-up position of fast idle lever
9
. The over-travel gap in the engaged tang and notch parts allows the fast idle lever and throttle lever (if indeed engaged) both to be swung slightly back by their biasing springs until latched into their spring held, stable, latched position after manipulating forces are removed from the manual controls of the appliance. This problem of the adverse “spring-back” or “pull-back” effect on the fast idle start settings of the choke and throttle valves when latched will be further explained and seen in more detail hereinafter. Another prior art solution to the problem of achieving automatic fast idle setting of the throttle valve is found in Hermle U.S. Pat. No. 5,200,118, issued Apr. 6, 1993 and assigned to Walbro Corporation of Cass City, Mich., assignee of record herein. (U.S. Pat. No. 5,200,118 also being incorporated herein by reference). The '480 patent is also described in the '118 patent. It will be seen from
FIGS. 1-5
of the '118 patent, and by reference to the specification and claims of the '118 patent, that the choke valve
10
is “divorced” as to its oper

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