Engine tune-up problems - make sure everything is correct for the age of the engine

This week I had an interesting tuning job. Using my Air/Fuel setup & Timing light, I was asked to assist with a stubborn MGB that refused to idle at low enough specified rpms, misfired randomly, and was impossible to balance the carb airflow. Firstly the usual checks were done with the ignition system (distributor cap removed, points gap set, timing checked, vacuum advance checked etc). The carburettors were an SU HIF twin setup, however from the outset there was something obviously wrong. The mixture was first set to 1 Lambda which is the optimum mixture for combustion (or 14.7:1 air/fuel ratio). It had been running at about 50:1, and so was way too lean on the standard 2 turns in setting of the mixture screws. The engine idle now was even faster (up to 1400rpm), and the throttle idle adjustment screws were undone so that the butterflies should have been fully shut. The balance between the carbs was out by a couple of kg/m on the synchrometer reading, so I set about balancing. I couldn't get closer reading than about 1.5, so checked the butterfly of the highest carb. Taking the carb off, I could see the butterfly wasn't quite aligned (using a light to see the "halo" around it, thus the throttle was jamming very slightly open. This was adjusted so as tight as possible, allowing the throttle to close slightly more than previously. The carb was refitted, and tuning continued. The engine was now able to idle around 1300rpm. Balancing was slightly better, but now the other carb was unable to drop enough to match the one removed/adjusted. This carb was then removed, but the butterfly appeared to be shutting properly. However, there was a slight area of damage to the edge of the butterfly, looking like something had been used to jam the throttle open at some point in the past. The owner decided to replace this and the spindle. The result allowed the carbs to be fully balanced. However the irregular misfire persisted, along with lack of power if driven. The mixture remained as set at approx. 1 lambda. Moving on to the timing, there were a couple of things that had to be proven before continuing, and this is the reason for this blog entry. The car was a late model, but the carbs fitted were found to be from a mid production range a few years previous. The important difference with these earlier carbs was a blessing rather than a problem, however piecing the data together to find this out was very difficult. With a factory manual & Haynes manual to hand, there was no specific info on the position of the vacuum port for the distributor, at least for this year of vehicle. The Haynes manual mentioned the vacuum take-off was on one of the carbs. This engine had the vacuum take off on the inlet manifold (and was connected to the distributor). However the rear carb had a vac take-off port as well. I had spotted this earlier in the tune up, as the pipe was missing a blanking cap. However, there was absolutely no data available to say whether the distributor had to be matched to the vacuum port location. I had read online that later vehicles had no carb vac port, and used the manifold port instead. The owner had therefore connected this correctly for the age of car, being a late model. The vacuum signal from the inlet manifold allowed the timing to advance 20º when the pipe was connected at idle (1300rpm lowest idle possible). Trying the pipe on the carb vac port instead gave no noticeable ignition advance at the same idle speed. This was the key to the misfiring issue, & also the lack of power when driven. With no idle (or in this case fast idle) vacuum, the ignition was now 20 degrees retarded when compared to its setting at the start of the tuning. The advance in the factor manual stated 20º advance from vacuum, and a total advance could be worked out to be 34ºBTDC given the static timing and the centrifugal advance. The timing was adjusted down to the spec. which now helped drop the idle speed to something around 925rpm. The misfiring disappeared, and the max. advance was now checked. It was still too high at approx. 39ºBTDC, so the idle timing was reduced a further 5º to match the max advance in the manual. There was no noticeable change in engine smoothness, with certainly no misfiring. This is where factory data again was lacking, as I know various distributors would have been used on the engine over production time, with variations to both the vacuum & centrifugal advance curves. In vehicle manuals, you sometimes find distributor data for advance capsule types & curves per engine range, & can then usefully work out how to alter the timing to suit the different engine. (Modern fuel is also a big factor in this setting, and renders a lot of original data to be nothing more than rough guides anyhow, see my review of the book further down in this blog). The relevance of this info. is simply to point out that the distributor is possibly not the original, and also may well be incorrect for the carb vacuum take-off (as opposed to the manifold take-off), Even if the distributor is the original then who knows if the vacuum advance capsule has ever been replaced, and with the incorrect one? Anyhow, with the max. advance timing set, the dynamic timing at idle was now about 1ºBTDC. Too low! However, as the vac off idle snapped the vac to full, it jumped up to over 20 degrees immediately, and so for anything of idle was fine. The road test proved this to be the case, however the limit had been reached for the ignition tuning as far as vacuum advance was concerned with this vac capsule. Using an Aldon Amethyst would be the ideal way forward for this engine, to allow a proper vacuum & centrifugal simulated timing curve to be programmed in for optimum tuning. Combined with a knock sensor tuning tool & the Air/Fuel meter, this could be tuned as well as if done on a rolling road. If you are interested in any engine tuning work, whether on your TVR, or any other classic car, please feel free to contact me here.