Ignition advance

Deeper considerations on the distributor on VW 1302 Super Beetle led me to some conclusion that the double diaphragm one has complicated mechanical control system and also difficult and requires some skill to set firmly, some trial experiments would solve these problems.

Adjustment with importance for Legal Inspection on carburetor is followings(Carbon Oxide below 4,5% and Hydro carbon below 1,200ppm):

34PICT/3 has two adjusting screws, big one for bypass screw and smaller one for mixture control. Bypass screw is used for adjusting idle speed, so never use throttle valve position screw for adjusting idle speed, fixed never touched. Smaller mixture control may be fixed with some resin plastic preventing touching by the outsiders except initial factory assembling.

Dwell angle, contact point effective interval on cam construction, 44-50 degree with 400/1000 mm thickness.

Idle speed must be at 850 RPM for manual transmission models.

Distributor setting is ATDC 5 degree, After Top Dead Center 5 for double vacuum diaphragm advancer equipped distributor.

Generally 7,5 to 15 degree advance BTDC (Before Top Dead Center) is applied in case of single diaphragm distributor, one of double diaphragm retards the timing while idling so ATDC standard is correct for this construction.

Reading from the exhibited graph, advance by centrifugal advancer at 750 RPM is between 4-7 degree and that of 1500 RPM is 9-11 degree, at that time that of vacuum one is 4-6 degree, so the width would be 13-17 degree advance in total.

Some interferance was found out on the 34PICT/3 carburator to adjust firmly, hatered "Dashpot" mechanism is set behind the idle stopper screw. So the function must be killed for idle adjusting, that is making the rotation down from 1,200 to 850 rpm slow and gradual intending emission control.

Actual adjusting simulation:

Setting the valves, all in and out clearances 0,15mm in cold cold condition.

Setting the ignition timing at 5degree ATDC by rotating distributor body with dwell angle 44-50 degree and with point gap 0,400mm.

Setting 34PICT/3 carburetor with the choke valve fully opened.

Screw in 1/2 turn idle stopper screw from the pint of 0,003" feeler gage touched, keeping untouchable on idle stopper screw for any future procedures.

On 34PICT/3, only the air bypass valve play a part of idle air adjuster in stead of opening and closing the throttle valve as the former types.

The mixture control is performed by air volume screw(smaller one) with initial setting of 2-1/2 turns opened from the closest position. And the finally settled sweetest position exists within a half turn screwing in or out. So turning in this smaller screw mean the reduction of air mass leading to richer condition.

But remember here that the idle speed must not be adjusted by this smaller air volume adjuster but be done by bypass screw(bigger one) anyway. Surely idle speed adjustment is the job of bigger bypass screw.

After adjusting idling rotation at 850 rpm, very slight screwing in of volume screw until rotation down 20-30 rpm with 1/8 turn, another bypass screw adjustment, opening to 850 rpm, would be performed.

The burning condition of spark plugs would show the mixture,lean or rich, further adjustment on mixture control would be the task on volume screw.