Sachs SA 340 - Carburettor

Carburettor
The typ e of carburettor, the jet size and the j et adjustm~nt are d~termin ed by means
of t es ts in the fa ctory . Th e adj u stment determin ed in th1s way g1ves the bes t results,
and it is therefore not advisable to alter it.
No a l teration of t he carburett or setting should be made as long as the eng ine turr~s
over quie tly and smoo thly when idling, accelerates steadily when the_ throt tle IS
opened, and gives its full output at full rev s without choking. )f the eng1ne p~unds
or runs unevenly, or the exhaust gas is black, the mixture is too nch. Repeat ed sp 1tt1n g
or cough i ng, or backfiring with a blue flame stabb ing from the carburettor coupled
with d ifficult starti n g, indica t es that the mixture is too w eak.
When running with the cor re ct carb ur ettor adjustment , a clean a ir filter and a suitable
spark ing plug wit h t he co rrect ther mal val u e, the insula tor of the sparking plug should
be brown in colour . Soo ty or wet plugs indicate rich mixture, whereas white ins ulator
po ints to the mixture being too we ak.
Special care sho ul d be taken to ens ure that the carburettor setting is never too low,
as at low throttle ins ufficient lubricant w ill be conveyed to the moving parts of the
e ng ine. W ith a two-stroke en gine the carbur et tor setting should a l wa ys be sufficient
to provide adequate lubrication.
Cor rect c arb ur ettor se tting will h elp to ensur e economic and trouble-free r unning of
t he engine.
Adjusting the carburettor for special climatic conditions
When using the engine at a l titudes above 1000 metres (3280 ft.), adjustments of the
carburettor setting will be required. As the air density is reduced at high altitudes, the
s upply of gasoli ne to the c arburetto r shou ld be restricted by changing the main jet.
Starting
On Tillotson diaphragm carbure tt ors the choke is i ncorporated in the carburettor
itself.
The choke should be u sed to facilitate starting from cold. It s hould not be used for
s ta rting when t he engine is warm and it should be par tially or fu ll y c losed only for
s tarting the cold e ngine.
Wh en the choke is cl osed, a higher vacuum is caused in the mixing chamber , so that a
richer fuel/air mixture is obtained. Once the engine has started, the choke should be
s lowly opened .
Th e Tillotson diap hragm carb u rettor can function in any position. The HR-8 A type a nd
the HR- 18 A type have two diaphragms , the HD-7 A type and the HD-13 A type have
three diaphragms : a contr ol diaphragm for proportioning the fuel feed to the engi ne
and a pump diaphragm (HD-7 A a nd HD-13 A have two pump diaphragms) for
drawing the fuel from the tank.
Carburettor adjustment
The Tillot son carburettor is the first carburettor u sed for SACHS engines which has
adjustable jets. If engine troubles occur and the jets have to be readjusted, the fo llow-
ing points sho uld be no ted:
1. To adjust the ca rb ur ettor correctly for · maximum output (full load ), the en gine
sho uld be thoroughly warmed up.
2. W ith
adjustable jets, car e sho uld be taken not to use too lean a fue l/ air mixture
(see a l so page 28) .
3.
The adj usting screws for t he jets mu st n ev er be screwed too ti ghtly to the right
(i.e. clos ed) in the jet s eat, as this wi ll deform the sea t.
4.
The main jet is marked "H" and the idling jet is marked "L" on the carb u rettor
housing.
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