General

There are a few general things about carbs and fuel systems worth considering;

· Use a throttle return spring directly off the carb spindle. The standard A7 return is a spring on the cross shaft, if this becomes disconnected from the carb it could run away and over rev. I believe racing scrutineering requires two separate springs from the carb spindle. On twins if the linkage between them isn’t solid think about having a spring for each one.

· Fuel tanks have had lots of years to corrode inside, no matter how much you fill and slosh about and drain - you’ll never get that bit out that sticks your float needle! An in line fuel filter will look after the pump and carb. I learnt recently that the best way to stop a fuel tank corroding when the car is laid up is to have it full to the cap. Simple really, just excluding the oxygen.

· Talking about laying up cars, if you do as above modern fuel will go ‘off’ or ‘duff’ very quickly and the car will be a sod to start. New fuel is the colour of a good Chenin Blanc while old tends to discolour to an oakey Chardonnay colour, it doesn’t smell as nice either. Ensure you refill the float chamber with some fresh to get you away. As long as it’s not years old, the duff will still work on a running engine.

· Yet another aside, modern fuel can attack old rubber pipes etc in the fuel system. A nasty fire could result. Better to be on the safe side and replace all the parts with new. Don’t use that clear braided plastic pipe, not only does it look like a bodge job but it goes hard very quickly.

 

Throttle linkages are a subject in themselves! Many specials use the A7 cross shaft. That’s fine if the pedal is level with the brake pedal and the lever at the carb end moves the spindle the right way. Cut and weld to suit, make sure the pedal position is comfortable. If the space for the throttle pedal isn’t very wide rather than put a bulge in the body side try cutting the right hand side off the brake pedal and get another inch or so for your dainty right boot. If you haven’t got a cross shaft you could try a cable, one from the old Mini is about the right length. The hard thing with this is to make a pedal and pivot. Anchoring the outer end at the engine end of the cable can be a bit chewey as well. A heat shield or ‘goal post’ behind twin SU’s makes a good place to fasten this—also any return springs. With all linkages make sure the pedal hits a solid stop before the carb is fully open otherwise the full power of your right foot goes through onto the throttle spindle—something will break. Along a similar line, even with the stop, ensure you’re getting full opening.

The little 1” side draught SU with the horizontal flange fits straight onto the later A7 cast iron manifold. If you can find one they work well with the standard M9 needle on any tune of engine and improve the response. The linkage connects up easily too.

 

A single 1 1/4” side or semi down draught is currently a popular carb. I’ve seen a lot on the old down draught log type manifold with an adaptor, either mounted at the side or facing forwards to keep it all under the bonnet. If space is a real problem and you don’t want the carb sticking out of the bonnet (not really a good idea if it spits back) try one of Raeburn’s new ones that have the mounting flange inclined straight off the log manifold, or alternatively one of John Barlow’s for a horizontal carb like the Speedex one shown here. We’ve experienced icing problems with the adapted type manifold, in winter (it’s a trials car!) it would run fine on the road with everything warmed up under the bonnet but wouldn’t rev cleanly after standing for a few minutes. It seemed like the general temperature under the bonnet affected the induction system. We got over it by fitting a heat shield between the adaptor and the log manifold—to keep the heat around the manifold. ‘60’s SU’s from Mini’s, Minor’s or 1100’s are just the ticket or if you want to keep it more ‘in A7 period’ go for an earlier one with the jet gland and choke lever. An E2 needle seems to work with this set up and I’ve also seen V2 quoted.

Bigger than 1 1/4” isn’t a good idea, it won’t make it go any faster, the larger carb will struggle to give an even mixture at low revs and it’ll be even harder to squeeze it under the bonnet.

 

There’s lots of information around on tuning SU carbs and choosing needles, I like the old ‘Speedsport’ books, the little one on SU’s is good if you can find a second hand copy. All I would suggest is that you draw the profile of the various suggested needles large scale on graph paper so the subtle size differences are more apparent. The greater the diameter, the less fuel flows so a weaker mixture results. The only definite way to get it spot on at all operating revs is on a dynamometer with an exhaust gas analyser. This isn’t the sort of gear that we all have in our garage nor is it cheap to hire. If you’re patient and can read plug colours etc, you should be able to get the mixture near all the way through. Burlens of Salisbury are agents for SU’s now and have remanufactured a lot of the old components.