Zenith Carburetor Model 29 Service Manual

BulJ«~tin

."' n o Z-2l!i- B

ZENITH 29-SERIES CARBURETORS OP:ERATION AND SERVICE The Zenit h 29 Series is a downdraft ca rb ur etor with a conce ntr ic float bowl design. This design assi st s in th e proper metering of ai r and f uel t o t he engine, withou t flooding, wh en the vehi cle is operated on extre me a ngles . It is a "sealed" and

good idling of the eng ine , it would indicate either an air leak or a r estriction in the flow of fuel for idl ing. Look f or air leaks at the manifold flange; at carburetor t h r ot t le body t o intake gasket, and at carburetor bowl to cove r gasket, du e to loosened assembly screws or damaged gaskets. A badly worn throttle shaf t will produce sufficient air leakage to affect t he idling mixture. Dirt or other foreign matter in the idli ng jet calibration will restrict the flow of f uel for idling and affect the mixture. If the idling jet becomes completely clogged, it will be impossible to run the engine at idling spe ed regardless of adjustment of the idling adjustment screw.

Fig ure 1

"balance d" ca rburetor in that all air fo r f uel bowl ch a m be r v en til a ti on a nd idling mus t come th r oug h the ai r cleaner. Air cleaner r estrictions hav e a minimum influen ce on f uel-ai r ratio. A double ve nturi is employe d to aid in t he complet e vapor ization of th e f uel. The pow er jet and accelerating pu mp system s are operated by en gi ne vacuum and are complet ely enclosed and pr ot ect ed f r om dirt. Th es e a uxiliary jet systems are t o provide t he extr a f uel needed f or cert ai n ope rat ions. INSTRUCTIONS FOR ADJ USTING ADJUSTMENTS: 1. The thr ot tl e stop scre w (1 ), F ig . 1, should be screwed in (clockwise) against the stop pin (2) t o hold t he throttl e j ust slightly open. Ad just th e t h r ottle stop scre w to obtain t he desired idling speed of t he engine . 2. Th e idling adjust ing scre w (3) should be f r om 1 to 1112 f ull turns off it s seat. Ad just the idling adj usting screw to obt ain smooth idling whe n en gine has becom e thoroughly warmed up . Turning t he screw in (clockwise) cut s off air, ma king th e idling mix ture ri ch er; while t urning it out (counter - clockwise) admits more air, making t he mixture leaner. If i t becom es neces sa r y t o tu rn t he screw in to less t han 1/2 turn off the seat t o obtain

3. Some models of this series are supplied with a main jet adjustmen t which is installed in place of plug (4) , Fig. 1. Turning th e needle clockwise cuts off fuel making the medium and high speed mixtures leaner. The ne edle should be adjusted to give highest manifold vacuum (or highest R. P. IV!. on a tachometer) for a set-th r ott le position. (This is usually about 11/2 f ull turns (counter-clockwise) from t he seat .) If engi ne is equipped with speed governor, set the throttle to hold the eng ine speed j ust below the go verned speed while ad justing th e main jet ad justmen t. If a djustme nt is set t oo lean, t he engine will lack pow er and t he fu el economy also will be poor. If se t too rich, t he eng ine will be sluggish and t he fuel econ omy poor. STARTING: Open t he throttl e ab out one -qu a r ter . Pull the choke control all t he way. Step on the starter. As soon as t he eng ine st a rts, p ush t he choke control in about one-third of th e way and as the en gi ne warms up , continue t o pu sh it in gradually unt il t he choke valve is wid e open. Note: If t he engine, after running sat is fact or ily, sudde nly ceases to perform properly, do not change the carburetor ad j us tment. Look ove r t he intake ma nif old, ca rburet or flang e gasket s, t h r ot t le, choke , and f uel connections. Make sure that t hr ot tle and choke open and close correctly and that f uel is reaching t he carburetor in a stea dy stream. Do not cha nge carburetor ad justments until oth er ca uses of trouble, including ignition, have be en inv estigat ed . Changes in adjustment should be necessary only with change in fuel or climate. MODEL IDENTIFICATION TYPE-DOWNDRAFT: STYLE 29- Throttle and choke shafts parallel with priming plug loca t ed under air in take.

STYLE 29-B-Throttle a nd choke shafts a t ri ght an gl es with priming plu g loca t ed on left hand side of t h r ottle body . STYLE 29-BB - Throttle and choke shafts a t righ t angles with priming plu g loca t ed on right hand side of t hr ottle body.

Air for ventilation of the fuel bowl is provided t h ro ugh a pitot tube. Th e pressu r es in t he f uel bowl and air entrance a re "bala nce d" by ve nting t he f uel bowl fro m t he air entrance . IDLE AD JU5TlN G NEEDLE

'DU NEEDl E VALVE SEAT

STYLE R-Built-in governor . STYLE W- Va cuum operat ed pu mp a nd powe r jet.

METERiNG WEll

STYLE X-Flange next size larger t han sta ndar d. Size Designation

11 12 14 16

Nominal Size

Throttle Bore Diameter

Plange Size S.A.E. Std.

1%1"

1.535- H I" 1.653-1 B" 1.889-1 H" 2.16 5-23\ "

1%,"

1112"

1%" 2"

11/:!"

IDl E .~!!l-!!~----t-----t:­

FEED HO t E

IDLE JET

1 3).1," 2"

OPERATION IDLE JET METERING W Ell

f UEl INtO

CALIBRATION

fU El VALVE 5EAT

AI R PASSAGE

IDLE DISCHARGE

PORT

Fig u re 3

Fig u re 2

FUEL SUP P LY SYSTE M : The function of t he float is to maintain t he correct level of f uel in t he f uel bow l un der all ope rating cond itions . Gasoline passes through th e fu el valve se at into the fuel chamber. When t he amount of fu el re ac hes a predetermined level it ca uses t he float to ri se and pu sh the fuel valve needl e against it s se a t, sto pp ing t he inflow of ga soline. When t he eng ine is ru nning and fuel flows f r om the f uel. cha mbe r to th e metering system j ets, t he fu el valve a ssumes a position wit h just eno ug h opening to s upply the requi re d am ount of f uel to keep t he f uel level constant. Th e fuel bowl is loca t ed in a central position directly above t he t hrottle pla t e. The mai n discha rge jet and met ering well are loca t ed in the center of the f ue l bowl.

IDLING SYSTEM : This system consists of an idle di scharg e port locat ed in the side of the throttle body, an idl e jet to meter fuel, a vacuum passage connecting wit h t he idle port, a n idle ad j usting needl e, an idl e air in t ake pa ss age, and a pe r ma ne nt idl e ai r bleed. At idling spee d the t h rottle pla t e is slig htly ad vanced f rom a compl et ely closed posi ti on lea ving about one- half the area of t he idle discharge port revealed to the suction in the engine manifo ld. Th is suction is transm itted to the idl in g jet through a passage running th ro ugh the air intake, fuel bowl, a nd fuel bow l cover. Fuel f rom t he fuel bow l chamber flows throug h t he main j et into the meter ing well. Fuel for idling flows fr om th is well t hro ugh the idle f eed hole at t he lower end of t he metering well t o the idle well to be metered through t he idle jet. As t he f uel lea ves the idle jet a nd enters the vac uum passage leading to the idl e discha rge port , it is mi xed with air admi t t ed f rom the air in tak e t hr ough t he idl e air passage in an am oun t m easured by t he permanen t idle ai r blee d a t t he upper end of t he idl e air intake cha nnel. An additional var iab le volume of air is ad m itt ed past t he. idli ng adj ust ing needle a nd its seat. Th is em ulsified mixture t he n passes t hrough t he idle vacu um passage t o be discharged into the eng ine ma nifo ld a t t he idle discharge port. T he permanent idle air bleed calib ration is to prevent f uel from being sy phone d in t o the intake manifold.

WEll VENT J ET

PERMAN£!,