Zenith 48RB Carburettor
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Folklore suggests that Vauxhall originally chose the 48RA carb as there was a ready, inexpensive supply of surplus aero engine carbs after the war. Whatever the truth, they are as rare as hen's teeth these days. Almost as rare is the 48RB (as fitted to the Gipsy 1 engine & others). The principal difference is that it does not feature the filter chamber under the float bowl. There are several 30-98's in Oz which have these fitted. If you find one, it will most likely require the choke and jets changed to suit the 30-98.
Images & information: John Kent
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Update Dec '21, John Kent reports: Almost all of what is in the RB manual (left) applies to the 48RA as well and is easy to follow. The one major variance is the main jet cover which features a conical diffuser rather than the plain open one of the RA. The jets and choke are also different sizes to the RA (48RB choke is 36mm, 48RA 30-98 is 32mm).
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Zenith 48RA Carburettor
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Slow running mixture adjustment - 48RA (by John Kent)
Zenith carb float level tool
Firstly, the float level needs to be set correctly. The level of the fuel in the float chamber needs to be 3 mm or 1/8" below the main jet cover with jet removed for clarity.
Assuming this is correct, the richest mixture for the slow running jet is when it is fully screwed down (decreased airflow) clockwise onto its seat - do not force it as otherwise you may split the end of the tube. This should result in a degree of black exhaust smoke on idle. Then turn mixture screw anti clockwise (increased airflow) until the black smoke just disappears at operating temperature. You may need to give the throttle a blip to clear the 'throat'. In practice, a small amount of black smoke, or slightly rich mixture, at idle will mean that the engine will be easier to start. If screwing the slow running jet to its richest position does not result in clouds of black exhaust smoke, then obviously the slow running jet is too small. Originally this jet was 1mm in size, but with modern fuel around 0.75mm works well. This can vary a little on individual cars. If you have the jet correctly sized, the slow running mixture screw should nominally be 1.5 turns from being fully seated (or wound in). Images & information: John Kent
Assuming this is correct, the richest mixture for the slow running jet is when it is fully screwed down (decreased airflow) clockwise onto its seat - do not force it as otherwise you may split the end of the tube. This should result in a degree of black exhaust smoke on idle. Then turn mixture screw anti clockwise (increased airflow) until the black smoke just disappears at operating temperature. You may need to give the throttle a blip to clear the 'throat'. In practice, a small amount of black smoke, or slightly rich mixture, at idle will mean that the engine will be easier to start. If screwing the slow running jet to its richest position does not result in clouds of black exhaust smoke, then obviously the slow running jet is too small. Originally this jet was 1mm in size, but with modern fuel around 0.75mm works well. This can vary a little on individual cars. If you have the jet correctly sized, the slow running mixture screw should nominally be 1.5 turns from being fully seated (or wound in). Images & information: John Kent
Autovac Instructions
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Zenith 48RB Carburettor
| zenith_48rb.pdf |
