Engine running despite the state of the SU

[Mark McKibbin]

When I fitted my SU (which is 1" OM-type), I tried a number of different needles and most gave rich running, I have used a GG (there are two different lengths....???), plus a 4, 6, and I set the idle mixture using Gunson's Colortune - the engine can then be revved across the range and the colour of the flame observed, I eventually settled on MO and the burn has a nice bunsen-blue colour, however, to get this, the adjustment is at the end of its travel, i.e. at its weakest point, fully up, so it is at its limit.

The car performs well and spark plug colour is ok, I would suggest that notwithstanding the bunsen-blue colour seen using with a Colortune it is running perhaps a tad rich, but I have nowhere to go with this, as I had a collection of other needles,  I decided last week to see if I could effect an improvement, ones that were definitely too rich I ignored, but I had several un-used that I thought I would give a go.

I tried the 6, but found out on the road, hill climbing appeared to be less strong, but at idle the engine sounded slightly sweeter, I then ran with the long GG (it is 5mm longer than the short ones, short GG were too rich) but could not detect any real difference on the road, but starting was not as easy, 2 or 3 tugs to get it fire up, 1 is only needed normally. All adjustment was still practically at the limit, so no change there.

So, I went back to the MO, and this seems to be the best compromise, it would be nice to put the car on an analysis machine to properly determine what gives the best performance throughout the rev range, but I am satisfied with the view from the Colortune window, so, if you can get a selection of needles and try what is best for your car, I have used the Colortune on all the cars I have ever owned (all my cars have had SU's) and I think this is a good tool for home use.

As for the spring, if the carburettor was designed for this, then you should stick with this, the piston is lighter than those without the spring and if you take this out, the carburettor will still work, but you may find that it stalls at inconvenient moments as the mixture will be messed about (years ago, I inadvertently forgot to re-fit the spring on a Rover 2000SC and drove it like that for a day and it kept stalling on the overrun, once home, I opened the bonnet and there was the spring sitting on the inner wing...).

Thanks for that, so a GG may be OK then or at least close, I think my problem was the open hole where the vacuum nipple would have gone caused me the richen it to make it idle but at the same time it would have made the rest of the range rich also I will toss the spring as the damper is weighted.

---

When I was putting the special together I put a H1 (1 1/8") SU on the engine that I found on my shelves with the intention of sorting it later.  I have now travelled about 500Km and it ran very strongly but for other reasons the engine is out again and as I am waiting for some parts, I thought it a good time to have a look at the SU.  First off it has a damper with a weight but also has a spring also it has a 3/16" hole on the top between the Damper and the butterfly I assume for a vacuum line ...but it was running with OK apart from the plugs being very black and lastly the needle is a GG also I have now blocked the hole.

The engine is standard other than a 37 head that somewhere in the past has had about 1/16" shaved off it and a set of extractors 4 into 2 into 1

Lastly I had a 18mm bung put in the exhaust so I can put my air fuel gauge in to get it right but I do need a starting point with needle and spring or no spring recommendations.

This site might offer some help. It has a lot of information on SU carbs: http://sucarb.co.uk/h-type-carburetters

Thanks I will bookmark that site although they do not mention weighted dampers.