Metal detector
Nigel’s ill-fitting carbs mean getting busy on the workbench
Afew months ago (PC, June 2019) I detailed how I intended to make the SU carbs from a Dolomite Sprint fit under the GT6’S low bonnet line. The bigger carbs are needed to feed a Triumph 2500 saloon engine that will soon be shoehorned into the little GT6, replacing the standard two-litre lump.
I thought I’d found the ideal solution, by asking a friend to cut and weld the 2500S saloon manifold faces parallel, allowing the SU carbs to clear the underside of the bonnet. I’d considered making adaptors to fit the new carbs to the original GT6 manifold but the gentler curves of the saloon manifold provide superior gas flow. I’d missed a vital difference between the different inlet manifolds. Not only does the saloon manifold have an eight degree downdraft angle, it also carries the carb mounting flanges higher than the GT6. The cutting and welding had taken care of the angle, but I hadn’t spotted the extra height of the carb flanges, meaning the bonnet still couldn’t close.
I’m indebted to PC readers for getting in touch and offering their experience. Father and son team Mike and Kris Grimwood have built a 2.5-litre GT6 using the big saloon manifold but did even more extreme surgery. By cutting the manifold to slope down by two degrees from the cylinderhead face, the SU carbs just clear the
bonnet on their GT6. Rich Simpson went a different route on his 2.5-litre Spitfire, making alloy adaptors to fit the larger carbs to a GT6 inlet manifold. While this works, he admits he’s not entirely happy with the set-up yet, as the adaptors only allow each carb to be mounted by two studs, not four as intended. As a result, there’s a risk of air leaks.
Time pressure to get my new engine in and running meant that a quick answer was needed for a problem I thought had already been solved. Adaptors would be more straightforward than trying to cut and weld the manifold again. The problem with adaptors is crowding: one of the two bolts attaching the adaptor to the old manifold would partially obstruct a mounting stud needed for the new SU carbs. Then came a lightbulb moment. On the Dolomite Sprint, the four-stud HS6 carbs are mounted on rubber connectors, which attach to the inlet manifold via three bolts.
‘A quick answer was needed for a problem I thought had been solved’
Gathering together manifold, Sprint rubber mountings and the SU carbs, I tried designing an adaptor. Though it’s tight, I found an adaptor could connect to the original two-stud Stromberg face of the manifold with countersunk screws, leaving just enough space to drill and tap all three holes necessary for the rubber mount. I bought 10mm alloy plate and set about transferring the design from paper to aluminium.
I’m not used to this kind of metal work; it turned into an interesting challenge. I started by drilling and countersinking the pairs of holes to attach each adaptor to the manifold. A trial fit using 5/16in UNC countersunk screws on a spare manifold worked fine. Next the three holes for each rubber mounting were drilled and tapped before another trial fit.
In for the drill
Finally, I needed to cut a large hole in each adaptor for the fuel/air mixture to flow. This was done by chain drilling at a diameter of 1.5in to match the GT6 inlet manifold. Following this drillathon, each hole was tidied and blended to fit the 1.75in carb throats using a half-round file. I was pleased with the look of the adaptors but needed a trial fit, at this stage on the old two-litre engine.
The original twin Strombergs were removed, then each adaptor bolted up nicely to the old manifold with new gaskets. As a precaution, I’ve used thread-locking compound on the countersunk attachment screws. The rubber mountings were bolted onto the adaptors and sealed with silicone, and each carb was bolted in with a gasket. Closing the bonnet with a blob of Blu Tack on the front carb showed there at least 13mm clearance, similar to the old Strombergs.
A little more improvisation was needed for the throttle linkage, with brackets made to support the accelerator cable and an extra return spring. Now throttle and choke both work smoothly. I connected up fuel lines and breather hoses then it was time for a test fire. After a little cranking to fill the carbs with fuel, the engine fired and ran smoothly. There’s no doubt the two-litre engine is ‘over carbed’ but the fuel system works and it drives nicely, giving plenty of confidence for when the bigger engine is installed. It won’t be long now! practicalclassics@bauermedia.co.uk