Mixture and timing

I drove down to Gillingham in Dorset on Tuesday morning last week to get the engine set up at Sigma Engineering.

What a great place - some superb old cars being worked on, including a huge 1910 Chrysler, Triumphs, Bugatti...... The Elan was the youngest cars there. They have 2 sets of rollers coupled to some really high-tech diagnostic equipment, and drawers full of components for Weber carburettors. The owner, Pete Lander, is a real technical guy of the old school.

As the alert amongst you will know, carburettors basically work by drawing air through a choke, basically a tube of a defined (tuned) length and a specific diameter. The main fuel jet protrudes into the airflow and fuel is drawn in. The amount of fuel is proportional to the velocity of the airflow. There are other jets for acceleration, cold running and idle, but the main jet is key for steady running.

First run on the rollers, it was clear that the engine was running very lean (insufficient fuel / too much air) mid range. One idle setting was slightly wrong too, easily corrected. Distributor timing was a couple of degrees low at tickover and was adjusted up to give more advance over the rev range and higher up in particular. Main jets were standard, so Pete measured the chokes which were oversize. It seems that the previous owner had crudely machined out the chokes from 30mm to 33. Not only does that mean 20% more area and hence 20% lower average air velocity and therefore 20% less fuel for the volume of air, but also "crudely" means turbulent air and hence inconsistencies.

Sigma had 30mm chokes in the magic Weber box, these were fitted and a second run on the rollers showed a massive improvement. A test drive confirmed this, very tractable, mid-range misfire has gone. CO2, I quote "nearly good enough for a MOT".

So, happy owner drives home. Until the exhaust fell off, again, just by Fleet services on the M3.