The view from the garage….. pt.4

About those carbs….

As I’ve previously mentioned, the Spitfire turned out to be a ‘bitsa’ and my initial thoughts were that this probably extended to the carbs too.  When I bought it there was a single carb setup which was apparently liberated from a Herald to try to bring down the fuel consumption according to the seller – The carbs I’m trying to rebuild / recommission haven’t been used in anger for at least 20 years and certainly not by me!

I decided the best course of action was to choose one and dismantle it, identify it, clean it and then buy a servicing kit before starting the rebuilding process (before repeating on its twin)

Predictably, the dismantling didn’t happen without issues.  Screws that haven’t moved in a minimum of two decades and in all likelihood a further two on top of that, didn’t take kindly to being turned and despite the best efforts of WD40 and a little heat, not all of them survived my efforts…..

Some while later it was time to identify what I had and to look into ordering the parts I needed to bring them back to life.

I recalled somewhere in the distant past being told how the naming conventions operated with SU carbs and that I’d need to measure the aperture on the butterfly side but couldn’t remember how the measurement related to the numbering.  I had to rely on a quick Google search which returned this helpful little video from Moss that not only refreshed my memory but also revealed what the prefix stood for, mine being HS – the H for Horizontal and the S denoting a separated bowl.  The aperture measured up at 1 ½ inches (4/8ths over the minimum one inch aperture) thus HS-4.

I wasn’t so lucky with the tags that denote the jet size as these didn’t exist.  No matter, I guess I’ll be working from a blank canvass then 😉

Having always wanted to strip down a carb I intend to use this as an in depth learning exercise so when I’ve done these, I can apply the learnings to the MG. I spotted a book online whilst searching for servicing kits called SU carburettors tuning tips & techniques (including SU fuel pumps) which I promptly ordered.  I’ll hold off ordering the  parts till I read this just in case it has some gems of wisdom that might influence what I need.

The next stage then became cleaning – As the photos show, there was quite a build up of crud to contend with.
I’m lucky enough to have a small parts washer to assist with the job which will be put to good use.

Something I noticed that differentiated the carbs on the MGB from there (apart from the physical size) was the addition of a poppet valve on the lower part of the butterfly and I had no idea what it did – once more to Google !
It turned out that it was an addition that was produced to act as an anti run-on device, the theory being that if the throttle closed and the engine was still on high revs, the valve would open to equalise the pressure on both sides and stop excess fuel being drawn in (which also could be viewed as a form of emissions control).

It seems that weakening springs make these prone to causing tuning issues and the first few sites I visited were pretty unanimous in their advice – replace them with a simple, solid butterfly.  I’ll wait for the book to arrive before deciding but it seems they may not be the way forward.
Feel free to comment below this post if you have any experience of or opinions on these little devices and their merits or otherwise!

Something I intend to do is polish the carbs & assembly to a mirror finish having seen the work done by Lee Overy on his ‘Refurbished Offshore’ page.  If I can get them to look half as good I’ll be delighted.  If I can’t then I daresay the MGB carbs will be going to him 😉

I’ll update you on my progress next time !





  1. Site Testing Account June 4, 2018
  2. Tony Penman June 4, 2018

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