Carb fine tuning
Ed was getting needled by his Ninety V8’s erratic running but he’s pinned it down
Acouple of issues ago when my Ninety V8 was completed I mentioned that the engine was running fairly well, subject to getting it on a rolling road to fine tune the carburettor needles. Because it’s an unusual combination of a 3.9-litre with carburettors, there is no standard needle to suit, so it was a case of fitting bestguess needles for the running-in period.
I intended to bring the Ninety back home from Britpart a few weeks ago, but another test run convinced me the fuel/air mixture was too weak to risk the 180 miles journey, given that a weak mixture can cause internal overheating and engine damage – and this is a re-manufactured engine that isn’t even run in.
So I decided to try alternative needles in the carburettors and consulted the SU needle charts and ordered two pairs that would give richer running at around 50-60 mph. For anyone not familiar with the SU carbs’ needle set-up, the needles are tapered and, as the airflow increases through the carbs, the needles rise out of the jet nozzle. As they rise, the thinner end of the taper moves into position in the jet, allowing more fuel to come
through. Simple enough, but getting the right taper on the needle is something of an art. The taper diameter is measured at every eighth of an inch along the needle, so that’s sixteen positions, each of which affects the mixture at a particular engine speed/load. There’s a choice of around 180 needles, all minutely different at various measuring points.
So, with two alternative pairs of needles, and back at Britpart where the Ninety still languishes, we first checked the earlier settings on the carbs. That’s when I found one of the original carb needles was incorrectly positioned – maybe it had moved, or probably I’d fitted it incorrectly when rebuilding the carbs. We checked the datum position of the jets in the carburettor bodies and found one was lower than the other – that was because we’d unknowingly compensated for the incorrectly positioned needle when tuning them, so it wasn’t having an adverse effect on engine running. We also spotted that the choke wasn’t coming fully off, but that’s just due to the linkages settling in.
It was Paul Myers who suggested the carbs were running out of fuel, rather than running lean. This fuel system employs a return line from the carbs back to the tank. By clamping off the return hose the engine suddenly ran perfectly. The problem had been an incorrectly-sized restrictor in the return line that prevents too much fuel going back to the tank so the carbs don’t run dry. Steve Grant welded the end of the restrictor, then drilled it out to a smaller diameter, ensuring the carb float chambers stayed full. The resulting test drive was a revelation: perfect acceleration through the range, smooth cruising and great tractability in high gears. It all feels absolutely right now, but it will still eventually be tuned on the rolling road to squeeze that last bit of performance out.
While all this had been going on, a mysterious transmission noise on overrun
had been getting progressively worse, so we weren’t finished yet. It sounded like a propshaft, but the front one was new, and we’d carefully checked the rear shaft during the vehicle’s rebuild. Nevertheless, we fitted a new rear propshaft and the noise was gone. We still couldn’t find anything wrong with the old shaft, and had to assume the sliding spline was vibrating under driving load – a load we couldn’t replicate by hand testing.
So the Ninety is now perfect, apart from a tad of paintwork being attended to by Marshbrook Garage who did the respray. By the time you’re reading this, I expect to be on the train to Shropshire to drive the beast home.