On the road, finally!

[galfredus]

Evening all,

I can report that GC8609 is back on the road and making (mostly) all the right noises.  I'm being gentle with the newly-rebuilt engine till it's properly run in, and finding a Vintage AC6 a fun thing to drive.

This car was fitted with triple SUs in the 1950s.  The carbs are quite worn, and I'm getting a fair amount of fuel standoff from the air intakes.  In the long-term (and given the cost of a new set of SUs) I'm thinking about changing it back to the original Stromberg set-up.

Does anyone have any views on that, and does anyone have an old stromberg/manifold on the shelf?

Many thanks,

Geoffrey

[Old Crock]

I think I have both the correct inlet manifolds (two types) and Stromberg OE-1 carbs (correct for vintage AC's) but I would need to check at other premises. Before doing that, and making the journey, send me a p.m. if you decide you want to go back to original carburation....

[galfredus]

Thanks Old Crock - message sent.

I'd also be interested if anyone has experience of the different setups and their relative performance.  Rumour has it that the updraught arrangement gives better bottom end torque, but less power at the top end.  I'm more into touring on the road than racing on the track, so the former might even suit me better.  Fuel consumption is another thing that I would take into consideration.  If the updraught arrangement was more frugal that would be another significant factor for me.

Further thanks for all advice.

Geoffrey

[galfredus]

Ps I'd be interested to know what needles those running SUs are using. Mine are just marked L.

Thanks!

G

[Old Crock]

I think your summary, on one or three carbs, is pretty accurate for the vintage car. I was underwhelmed by the first vintage Six I drove with three carbs (I was accustomed to the single Stromberg), expecting more noticeable performance improvement, but the second car I drove had definitely sprightlier acceleration (maybe the first was simply not tuned well). Top speed on this second car was maybe five/six mph better but, remember these models are restricted by the gearbox and the r/axle worm and wheel ratio. That's why AC, late 20's, offered various packages - different camshaft, high comp. pistons, three carbs, different ratio gears and final drive. For example, the Monte Carlo car of Bruce was a 16/66 but still had one updraught carb, the extra h.p. coming from these engine mods. Also, for consideration, the standard vintage cars have gears designed to get into top quickly, so from 3mph to 60+ mph in top was possible (a selling point which was advertised to good effect). The 'three carbs' car may accelerate a little better and have a marginally higher top speed but there is a point where you have to say, with a standard VINTAGE vehicle, 'is this worth the effort'? The 'Sports/Montlhery' model got around this by having all the mods here, including a better diff ratio thus top speed was increased by up 15mph or so, and the car was capable of prolonged high speed driving.

A number of vintage AC cars that have been changed to three carbs do use SU's but another variable is which ones? I think AC, for example, used six variants of SU's over the years. Tuning, plus the carb types, making a difference to end result. For example, the needles on your car you say are marked 'L'. Your carbs therefore are not the same as AC offered from the vintage period. 'L' was the standard needle from AC's of the mid-30's, when also 'weak' and 'rich' alternatives were offered. Its size is .090in. There were carb sets before your type where the needles were different and another part number...I think 'WX', if I recall correctly? For further information you could try Burlen Fuel Services in Salisbury who are the experts on SU carburettors (and they do offer many needle types and sizes to suit your carburettors).       

[galfredus]

Old Crock - your detailed knowledge of the history and development of old ACs never ceases to amaze me. While I'm running the engine in I'll put up with the haze of petrol vapour from the SUs but I'm definitely interested in the possibility of an updraught future.

Incidentally, I have the invoice form the purchase of the SUs on my car, back in the 50s. Various bits have come down to me with the car, but no the original manifold and carb.

On a different but related matter (the other side of the coin you might say). I've got a bit of a lash-up for an exhaust at the moment, with flexi pipes going into a Skoda silencer under the floor. It's pretty loud! Did the vintage cars have full length systems? I know from old photos that this car had a pipe coming out the back near the fuel tank back in the 50s, but was that the original arrangement?

[Old Crock]

The vintage Six exhaust system was full length and exited at the back i.e. rear offside. With your car, at the right hand side of the fuel tank. I've attached a page from the 1924/5 parts book, the system was the same through the 1920's. You'll see there were pipes leading from the two exhaust manifolds that entered the silencer and one pipe left the silencer to continue out to the back. The silencer ends were cast metal, held in place by two long rods (plus nuts) going completely through the silencer. There was no packing inside but two simple baffles, each with single offset holes. A larger view of this is attached. I've made up these in the past and a rough drawing of measurements is attached also. The exhaust system is simple yet effective; it is not noisy and the exhaust sound is typically 'vintage' (= period).

The alignment of the front pipes to the silencer can be time consuming, specifically bending the pipes to suit, and it can be made a lot easier by using, and welding, flex for a section in the last 12-15''. The last one I made was stainless steel. One final point, the flanges may be two hole or three hole - the exhaust manifolds were changed by AC. As the cast iron manifolds can crack, particularly at the flanges, they are extremely rare and, if needed, do ensure you find the right flanges (or weld very carefully!) 

[musicman]

Hello, I read with interest your  problems with your carburetters. You mentioned that you have fuel misting at the entrance of the carbs. This normally happens when the valve timing is far advanced. For the vintage AC six the valve timing is inlet valve opens at 3.5 degrees after top dead centre and tappet clearance is .005 th.  Later engines have the inlet valve opening at 12 degrees before top dead centre and tappet clearance 20 th. It might be that if you adjust the timing chain sprocket one tooth it might help solve the fuel misting and the engine should run much smoother. These are only suggestions as I cannot know what way the engine has been set up re valve timing etc, but it's worth looking at to check all is well.

[galfredus]

Hello Musician,

I didn't know of those differences between earlier and later engines. It does occur to me though that the triple carb set up was intended to be used on the later engines, so if my cam is set up for earlier inlet opening it still doesn't really explain it. From memory I did check the valves opening and closing on the starting handle and there is no overlap to speak of. The car is temporary in storage, and the first thing I'll need to attend to is the nearside rear brake, which was a bit grabby on the last run.