Fuel system fettling
No surprise, the TR7 has had the lion’s share of my workshop time again. The electrical refresh is complete and, whisper it quietly, everything works. Lights all illuminate, warning lights go on and off as intended, gauges work and the clock ticks faintly in the dashboard! It’s anyone’s guess how long this happy state of affairs will last. I’ve also serviced the GT6, which will spend the summer months in the role of daily driver.
Recently, one of the TR7’s twin SU carbs started to dribble fuel from its overflow. Removing the float chamber lid to investigate showed multiple problems, including a missing gasket and several millimetres of rusty sediment at the bottom of the float bowl, no doubt originating from the old, corroded fuel tank. The only proper answer was to remove and rebuild the carbs.
Overhauling SU carbs is easy, as Burlen Fuel Systems offers an extensive range of parts kits
(www.burlen.co.uk). They soon tracked down which kit was needed for my Sprintengined TR7 and the parts arrived promptly. The kit is certainly comprehensive, with main jets, float needle valves, throttle discs, spindles, bushes and gaskets. The tired old carbs were removed from the manifold and cleaned in paraffin before stripping. On stripdown, more rusty sludge was found in the float chambers, shifted with liberal amounts of brake cleaner.
The overhaul kit doesn’t contain metering needles or dashpot springs, so these were inspected carefully and judged fit for reuse. Rebuilding the carbs separately one at a time avoided the danger of parts being accidentally swapped around. With float lid and dashpot removed, I replaced the main jet, followed by the throttle discs and spindles, using the locking
There’s now the surge of acceleration thats typical
of the 16V ‘Sprint’
screws supplied to fix the new discs. Finally, the float chamber needle valve was replaced and the carb reassembled.
With both carbs looking almost new, they were reattached to the manifold, taking care to engage the throttle and choke linkages. Each jet adjusting nut was turned down two complete turns from its highest position to give a rough mixture setting. While installing the carbs I replaced all the engine bay petrol pipes and clips and made up a bracket to secure the steel bridge pipe that takes fuel to both carbs. We were ready to test. The engine was spun over with plugs removed to give the mechanical fuel pump a chance to refill the empty float chambers. Plugs back in, choke out, turn the key and the engine fired up almost immediately. Sometimes things do go as planned! I turned the idle speed screws to give a fast tickover and left the engine to warm up.
Once fully warm, I could start the process of setting mixture, balancing the carb airflows and setting the idle speed. Airflow was set using a Crypton Synchro Check; the mixture proved to be a little weak and was adjusted on a gas analyser to around 3% CO. Getting the ideal carb settings is an iterative process, so I went round one more time checking balance and mixture. To finish off, the K&N air filter elements were cleaned and re-oiled. By now tickover was smooth and even, with crisp throttle response.
It was time for a test drive. As well as a smooth idle, the engine now pulls cleanly through the rev range. And there’s now the surge of acceleration above 3500rpm that’s typical of the 16-valve ‘Sprint’ configuration.
With electrics all working and fuel system sorted literally from stem to stern, at last this car is delivering the driving pleasure it always promised, and just in time for summer. Which just leaves the question of what to do next with the rolling improvement project that is my TR7… Watch this space.