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An appeal for help
I haven't been able to use my ARR Ruby on the road since last year's Autokarna in June. It was running extremely rich and the intense heat generated while queuing to leave the grounds 'cooked' the spark plugs. With a change of plugs it got me the 50 miles home and has been in the garage ever since, apart from a couple of attempts at driving around the block. I was unable to sort it in time for Lincolnshire PWA7C 'Sevens to the Sea' run in April and this year's Autokarna is very close.

This is the current state of play:

SU carburettor dismantled, cleaned and needle changed from EB to weaker GG. The jet is set at 2 full turns and centred.
(I had refitted 2 different Zenith carbs, one newly refurbished, the other tried and tested, but no improvement)

Manual distributor DK4a fitted in place of electronic unit (now repaired and ready to reinstall at future date).

Timing statically set at TDC.

2 fresh gallons of petrol in tank.

Compression test shows 115 on all 4 cylinders.

The standard valves are new and were set correctly 500 miles ago.

The engine was completely rebuilt 500 miles ago - .060" oversize pistons, inlet manifold opened up to 1" and  sharp edges smoothed.

It ran very smoothly and with a noticeable difference in power, but the plugs became very black and sooty.


The engine will start on full choke, but begins to hesitate, fade and pick up as choke is slowly pushed in.
Pressing on accelerator either kills it or it catches the revs, but once released dies.
When choke is pushed in fully the engine dies.
Advancing or retarding the distributor doesn't seem to help - I think the best spot has been found, albeit with choke out.
Points are set at correct gap of .015" and plugs at .025".
Plugs have been swapped between L86C and hotter NGK BP5HS - now back to L86C.

I have now run out of ideas and experience. Can anybody suggest what else I could try, or better still, recommend someone who might be willing to help me get it running properly in the Doncaster, South Yorkshire area? ( I don't have a trailer or towbar).

I am a member of the Sheffield PWA7C, but haven't attended any local events recently and don't really know anybody well enough to bother them. Enjoyment of the car is fast draining away.

I have a video clip of the engine running but unable to attach it to this post. I'll post it on the facebook site.
Whilst BP5HS plugs are probably the correct heat range, they are extended nose and as such will soot/ coke up very readily. I ovrcame a similar problem with a set of B5HS plugs. Two full turns out of the main jet is the usual starting position but if your plugs are still sooting up it's set too rich. Try weakening the mixture a couple of flats at a time until it runs cleanly.
Hi Mick

Assuming you are using 14 mm plugs, try to get hold of a Colortune test plug. There should be dozens tucked way in the back of people's garages since their days of popularity in the 1970's. You can then "see" the flame colour to make sure it is blue and not orange.

Apart from all the obvious checks, make sure there is no inlet manifold air leak as this will weaken the idle mixture. An air leak also means that if you adjust the carb mixture to get the idle right the running mixture will end up too rich.

Good luck

Thanks Reckless. I chose these after one of the posts on the forum recommended the extended nose hotter plug to overcome rich running.

Thanks John. The facebook responses also seem to be hinting at air mixture/leak problems. I'll see if I can locate a colourtune kit.
Mick, I am quite happy to come over to Doncaster to have a look, but I must admit I hve no experience whatsoever with Austin Sevens running on SUs. If you can get to the next Sheffield meeting in ten days time there are friendly members who are running on SUs who may have a better idea of what's wrong.
Is the damper piston rising as it should? If it used to be ok then we can discount springs and piston weights, and I’d be looking at if the damper is sticking or if it’s for a blocked breather.
Alan Fairless
Hi Mick

I'm no expert on SU's but just reading your post and the main change in what you have done is the needle.  Have you tried putting the old needle back in?

did the Zenith carbs exhibit exactly the same characteristics when accelerating and using choke? In which case it must an air leak somewhere... have you tried spraying the gasket areas to see if the problems go?
Mick, my instinct here is to go back as far as possible to a known condition.

I think you've done the right thing returning to L86C plugs - unless there is something exotic you are not telling us about these should be fine. Personally I would gap them to 0.020". Thereafter leave them alone.

Make sure your points are clean, again personally I would set them 0.012 - 0.014". Then leave them alone. After re-setting timing for points just starting to open at TDC that is.

Fresh fuel - good. And bags of compression, also good.

The bit I'm wondering about is the queue getting out of Wollaton. It's hardly the most extreme set of circumstances - are you sure overheating was caused by the state of engine tune and not another reason? e.g. blockage in water passages / rad somewhere? A newly re-built engine is likely to run hot anyway if still in the 1st 500 miles.

Have you checked your tappet clearances?

Which leaves carburation. As Alan said, is the piston rising and falling freely? And is it damped? Check you have some thin oil in the dashpot (3 in 1). I would take steps to convince myself there are no air leaks between carb and manifold / manifold and block (or windscreen wipers) before mucking about with needles or jets. An air leak here is hard to spot and will throw you right off.

As for needles, go back to what was last running OK. After all that work on your engine it may need some adjustment, but the old condition should still run. Start again from scratch with the jet setting, as Reckless says start with 2 turns out and refine from there.
Mike, thanks for the offer of help. I'll try to get over to the Sheffield meet. I really should attend as a matter of course. I would like to sort the problem out before Autokarna. It would be great to get it running as well as it did when I last met you at Wentworth Woodhouse, its first run after the rebuild.

To be honest, I cannot tell the difference while it's behaving as it is. I don't think I'd be able to tell which is better until it's on the open road. I initially set it up with the GG and only later changed to the EB because it seemed to run better with it.

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