When carburation is out Recently we did a good days testing with our new Super Rok Tony EVRR chassis on a very hot day at the Pista dOro in Rome. This is a medium-slow track with lots of curves and was ideal for comparing our Super Roks performance against that of the Rok outfit we used last year.
We started with a transmission ratio of 11:87 and with the Bridgestone tyres set at 0.5 bar. Set-up on our Tony chassis was 140cm rear carriage and minimum front width. We used the standard camber and caster settings, and the front and rear heights were also set at average values.
Our initial laps immediately displayed the fantastic push of the engine, much more evident this time than at our first test at Lonato since the transmission ratio then was much shorter (11×80). Set-up however was not really ideal, most probably because the seat position is not yet set at its best and also the steering wheel has surely to be adjusted. This we will fine tune for future tests.
Still, our first laps were quite fast on a very slippery track with no rubber. Our fastest lap of 48.85s was already interesting as in the same track conditions we would have been running close to 50s with the Rok. However, our best maximum revs as indicated on our AiM telemetry system were just 15860rpm, compared to the limited maximum indicated by Vortex for the Super Rok of 16700rpm. We felt the engine had great push at low revs, but on the tracks only straight we felt that at mid revs the acceleration and push were weak and the engine had difficulty increasing revs. Since everything seemed to be working properly the first thing to check was the carburation. I stopped and we unscrewed the spark plug to check the situation inside the cylinder. With a flexible stick-light it is possible to illuminate the combustion chamber and verify if there is too much or too little oil covering the piston head, indicating respectively rich or weak carburation. This adds more precision to a simple check of spark plug appearance.
Our check confirmed our thoughts as the piston and spark plug were very wet with mixture. We then opened the carburettor and checked that out. We changed the carburation setting by moving the ring along the needle to reduce richness. The ring setting was on medium so we altered it to the weakest setting, away from the tip of the needle. The jet was left unchanged.
Back on track and now the engine was behaving better at mid and high revs, while at low revs the push was unchanged. Lap times improved immediately and, on lap 2, after the tyres had warmed up, I stopped the watch at 48.53s and went on improving with a best time on lap 5 of 48.40s. Maximum revs reached 16010rpm so we were still missing up to 700 revs.
We were now at the limit of carburation and were still missing something at mid revs that was the cause of the limited maximum revs at the end of the straight. Since we had no suitable alternative jet with us to further adjust the carburation we had to find other solutions, or at least try.
We wanted to increase maximum revs somehow so we shortened the transmission ratio to 11×89, still far from our objective of reaching at least 16500rpm, knowing anyway that at Pista DOro it is not really necessary and efficient lap time-wise to reach the engines real maximum revs.
We tried the new set-up and lap times did not really change. On the one hand the shorter transmission ratio should have brought around 370rpm more with the same maximum speed at the end of the straight, but on the other hand the fact that mid revs came at a lower speed slowed the kart down earlier along the straight with no real final benefit. Even some tuning of the exhaust valve to help torque at high revs did not really work out. Lap times remained almost unchanged at a best of 48.50s and there was also some slight deterioration of the tyres.
Generally an engine can work only well with the correct carburation. The Super Rok has shown itself to have much more performance than the Rok, but also more sensitive to carburation, which has to be looked at with great care. Changes to the transmission ratio, exhaust valve or other alternative ways to correct engine performance do not really work. Such fine tunings can only be done after the carburation is correctly set.