DIRTY STUFF

INEEDED a special carburettor, for I have a truckload of spare parts from a pre-war German car, and figured I could use these to build a rear-engined sprint and hillclimb race machine. Something like the 1950s English Coopers, powered by Norton and Vincent motorcycle engines, which were so quick they dominated all the Oz hillclimb tracks.

So I had this two-cylinder, two-stroke engine of 700cc, or 42 cubic inches, and I knew I could screw lots of grunt out of it as it was an uncomplicated two-stroke. Two strokes are like that – no camshafts, no valves, just ports and pistons and no barrier to extreme rpm. There was a bloke called Boet who lived in South Africa and built a front-engined one of these. I don’t know exactly what he did to the mill of his low-budget open-wheeler race car, but it had such blistering straightline acceleration that he could blow off Formula One machinery. All I had to do was raise the compression, heavily port the barrel and fit a proper carburettor.

So that was where the need for a larger fuel dispenser came in. As the intake throat would face the rear of my race car, and I wanted to dig into my store of grotty SU carburettor bits, which dictated picking a 32mm (1¼-inch) sidedraught assembly, with its bolt-on float bowl on one side. And that created a problem. The SU’S main bit is a casting with a straight-through hole, opened and closed by a butterfly valve and topped by a suction chamber with a rising and falling piston, holding a mixture-controlling tapered brass needle. The side-mount float bowl feeds fuel to the chamber, which holds

the single vertical jet that is in contact with the needle, and I needed two float bowls. As this carb was going to face backwards, if it only had one float bowl on the left side, screaming around a tight right-hand corner would take all the fuel away from the jet due to centrifugal force, and the engine would stop. Same again with a right-hand bowl and left-hand corners, so the solution was to have two float bowls, left- and right-hand sides.

The last twin-bowl SU I bought from SU Midel cost me a grand, as this 50mm brandnew carby was specially made to fit a GM 3/53 blower. But I didn’t want to spend that many dollar notes this time, so I figured I could make my own by welding a left bowl into a right bowl. Voila! The job would be done. But I only had one 30-degree-angled bowl, while the other had no angle and would have to be bent.

I had done this before with SU alloy bowls, holding the bit that bolts to the carb in a vice, gently heating the alloy to near-melting, and applying force with a short piece of tube inside the bowl until the right angle is reached. Did that, but now I had to join the two. I cut through the banjo of one bowl until it was U-shaped, then carefully drilled out the original fuel feed holes until I could fit a short piece of 6mm steel pipe, necessary to keep the bowls in line as I welded them together. But I had this bundle of aluminium solder that somebody sold me years ago, and I reckoned I would use that.

So I cleaned up the joint to remove oxidisation, and with a small welding flame I succeeded in soldering the bottom bit. Turned it over to repeat the process on top, and as soon as I waved the torch near the joint, all the previous solder melted and fell on the floor!

Damn. That wasn’t going to work. Had a think, and figured I could alloy-weld the sides, which is more difficult, but that would hold the two bowls together while I soldered the rest. Went to my pile of special welding rods and picked out what I thought was an alloy welding one, got the torch going and started proper welding.

But this rod wouldn’t melt, until the alloy of the bowls did and tried to make a new hot puddle on the floor. Then I realised I had picked up a nickel bronze rod, which is a similar silver colour. Bugger!

Cleaned up the joint, found a proper aluminium rod and began this tricky welding bit again. Succeeded in tacking both sides, then soldered the bottom bit again. Went to solder the top, and yet again, the bottom stuff made a puddle on the floor. Mongrel!

So I had another think, and decided to solder the bottom of the joint again. As the vertical load of these dual bowls where they hang off the body of the SU wasn’t a helluva lot, I would fill the top joint in with Loctite’s epoxy ribbon, which is okay against fuel. Found a Webermade intake trumpet, bolted that on, and my home-made, race-ready carburettor for this no-valves, screaming fury of an engine was finally finished!

THE LAST TWIN-BOWL SU I BOUGHT FROM SU MIDEL COST ME A GRAND. I DIDN’T WANT TO SPEND THAT MANY DOLLAR NOTES THIS TIME, SO I FIGURED I COULD MAKE MY OWN