Amazon: B20b Engine Run-On

[Derek UK]

The Colourtune kit is only as good as the way it's interpreted and you really need two with twin carbs. It also came out when fuel was a lot different to what it is today. Better to get the MOT man to give you some proper figures and maybe spend a few minutes tweaking the mixture to get the numbers about right. I'd guess they aren't very close to optimal. Err on the side of a few % rich for safety's sake. Older engines don't like extended idle when running a bit lean. If the cooling system is good, there should be little or no difference to the gauge between normal running and a lot of idling unless the weather is very hot. Coming back from Sywell a few weeks ago I had about 20 miles of stop/start traffic at the bottom end of the M1 and had no change on the gauge. Helped that it was also raining!

[swedishandgerman]

Yes, I think a Colortune is good enough to get a near enough setting as a starting point, so to speak, as follows: set the mixture screw to the point where it just goes from yellow to blue on one carb, then re-insert it in the hole for the other carb and repeat. Afterwards, just incrementally turn the screw so that you achieve the most smooth running and fastest revs.

Sticking a lambda sond up the pipe is good enough on one hand that you can tell if the mixture is about right on average on the 2 carbs, but on the other hand, not great to set the carbs individually: it is possible to have a very weak mixture on 1 carb, but good on the other. All said, a Lambda sensor will tell straight away if 1 of the carbs is too strong because the readings go from lambda to off the scale if there's a drop in oxygen (strong mixture) but the readings don't change much if you weaken the mixture. As such, if you're using a lambda sond to adjust the mixture, the best method is to go from strong to weak and set it just where the readings go from crazy to good. Repeat that a couple of times and you can be sure both carbs are set right.

Me personally, I don't set it a bit on the strong side to err on the side of caution; it's either right or it's wrong and too strong isn't efficient! But strong mixture is better than weak mixture, for sure.

[arcturus]

Personally I use the lift pin method after the initial setting plus a touch on the rich side.

[Volvorama]

B20Bs usually have that awful manifold with the second set of flaps installed for some emission control idea. These wear and cause air leaks. Options are to remove the secondary spindles and block the holes (I seen them brazed) or better still throw the whole thing away and replace it with a late B18B manifold which doesn't have them at all.