Carb Jets

[Martin Prior]

I've never taken a lot of interest in jet sizes, but I've just been cleaning the 26VA carb on my bog-stock RP and have noticed that the main jet is 55 and the compensating jet 72.  I gather that the standard sizes are 57 and 50 respectively, and these are the sizes in the other seven or eight carbs I have on the shelf.

The car has always been quite lively and is particularly good at keeping up with stop-start urban traffic.  Is the oversize compensating jet likely to be making a useful contribution?

[Ian McGowan]

Hi Martin,
              Quoting from the Zenith Standard Carburettor Instruction Manual 1929:

"Compensating Jet too Large will cause sluggish acceleration and
woolly running at low speed. 

Compensating Jet too small will cause missing on acceleration
and possibly knocking when pulling hard at low speed."

It doesn't sound to me that the "too large" symptoms are applicable to your case.

[Martin Prior]

That's very interesting, Ian. I certainly don't have the "too large" symptoms, although on the face of it, my jet appears to be very much larger than standard. Although the car runs very well, I may try experimenting with a standard set of jets for comparison.

I fitted the current carb in an emergency about five years ago, but didn't look at it closely.

For around forty years I ran the car on a 26VEHC with standard jets and it went very well indeed until the Mazak started to disintegrate. Was the 26VEHC ever a standard fitment on A7s?

[JonE]

wasn't the VEHC the more expensive forerunner, which was more tweakable than the VA? And updated by the accountants.

I was going to suggest reverting to stock jets to see what the difference would be, but I also wonder if it is wise to tempt altering anything with a seemingly perfect car!?

[Ian Williams]

Sounds like you may have an Austin 10 carb with the larger choke Martin, what number is stamped on the inlet flange, 7 or 10?
What is the mixture like according to plug readings?