How can I pass the test now?

QThe emissions computer at my MOT station is now linked to VOSA. If my 1988 Cavalier fails, gone are the days of twiddling the mixture screw. It needs a formal partial retest, and if it fails that, a full MOT again. How can I avoid this extra cost? David Hughes, Bangor

Ed says

AThis is a (rare) example of how the test doesn't cater for older vehicles. More modern cars monitor and adjust their own mixture. If it is awry, there's probably an actual defect. Anything with a carburettor is set by hand, with a certain amount of guesswork.

Perhaps the best bet is to ask the garage to adjust the mixture before testing the car. But this will be done as a separate job MOT scheduling doesn't allow for tinkering time. Or buy a DIY gas tester (from £125), which will pay for itself if it prevents two failed tests.

The traditional method of setting idle mixture is to adjust the mixture screw or nut to give the highest, smoothest idling speed, then readjust the idle speed screw. This should put the emissions in the correct ballpark.

We'd recommend erring on the weak side (lower carbon monoxide or CO) before a test. If too weak, though, the engine idles very unevenly. This increases the hydrocarbon (HC) emissions, although the HC limit is more generous.

The usual prerequisites to setting the mixture are now essential rather than desirable.

Make sure the oil's clean-ish and the air filter's in good order. Gap the plugs – and points if fitted. Set the timing, preferably using a strobe light at idle with the distributor vacuum advance disconnected and plugged. Make sure the valve clearances are spot on and, especially on belt-drive OHC engines, make sure the cam timing mark aligns with the crank pulley TDC mark (once every two crank revolutions). Make sure the carb float level is correct and that the needle valve doesn't leak due to wear or excessive fuel pump pressure (3 to 5 psi). Set the mixture with the engine hot.

The CO limit for vehicles from January 1st 1975 to August 1, 1986 is 4.5 per cent and from August 1, 1986 to August 1, 1992 (or in some cases, 1995) is 3.5 per cent, with an HC limit of 1200ppm for both.