Carburetor throttle and choke control mechanism

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

1999-02-18

2001-03-20

Chiesa, Richard L. (Department: 1724)

Gas and liquid contact apparatus

Fluid distribution

Valved

C123S179180, C261S064600

Reexamination Certificate

active

06202989

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 1970's, 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. The automatic fast idle setting mechanism of the Johansson patent U.S. Pat. No. 4,123,480 is shown herein in
FIGS. 1
,
2
and
3
which correspond respectively to FIGS. 1, 3 and 4 of the '480 patent. The direction of air-flow through the carburetor throat is indicated by the arrow labeled “A” in these views, as well as in all other views in the drawings herein. For convenience, the reference numerals employed in
FIGS. 1
,
2
and
3
are those employed in '480 patent, to which further reference may be made for the details of the construction and operation of the same.
In the '480 patent a fast idle secondary lever
9
is pivoted on the choke valve shaft
11
and is operable to engage tang
7
of 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.
2
). 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
, via lever
12
, from fully open position (
FIG. 1
) to its fully closed start position (FIG.
2
), the pivotal motion of choke shaft control lever
12
, via a coupling tang
14
on the adjacent fast idle lever
9
, pivots fast idle lever
9
and causes its notch
8
to latch engage and hold the throttle lever latch arm tang
7
, thereby automatically setting the fast idle latch mechanism. The bias of the respective choke and throttle shaft return springs
15
and
3
also provide the yieldable latch closing forces.
Then, if the chain saw engine experiences a false start, the choke lever
12
may be moved to the open position (
FIG. 3
) without thereby moving the fast idle lever
9
so that it remains engaged with the throttle lever
4
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 '480 patent design is 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 the choke valve sometimes not completely closing 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 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 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, 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 problem has been found to be the push coupling, via tang
14
, between the choke lever
12
and fast idle lever
9
. This dictates that the actual position of choke valve
10
when swung toward closed position will be controlled by the latched up position of fast idle lever
9
when the engaged throttle lever latch tang
7
and fast idle lever notch
8
of the latch system (if indeed engaged) swing slightly back to their spring held, stable, latched position after manipulating forces are removed from the manual controls of the appliance, as will be explained and seen in more detail hereinafter in conjunction with
FIGS. 8-13
.
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. A fast idle throttle latch system with automatic release in accordance with the '118 patent is shown in
FIGS. 4
,
5
,
6
,
7
A and
7
B in the drawings herein, which correspond respectively to FIGS. 5, 3, 2, 1, and 4 of the '118 patent. Again, for convenience the reference numerals employed in
FIGS. 4-7B
herein are those appearing in such drawing figures of the '118 patent, to which reference may be had for further details of construction and opera

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