Setting the idle mixture on a fixed jet Solex carburettor

I would appreciate the opinions of members on how to set the idle mixture on a fixed jet carburettor, such as a Solex. I can easily determine when the "beat" changes upon screwing the volume control screw(s)  in, but I am unsure by how much to then turn it/them out as I cannot readily identify the over rich situation that would require the opposite response. I have noticed that when it is running a bit lean  there is a popping sound in the exhaust when coasting down hill. My initial response was to make the mixture a little richer until the popping sound disappeared.   Richard Melbourne Posted by RichardWallach 13/01/2015 08:02:56

If you have TVAM, Chapter 15 158 (a), page 42 covers quite well.  If you don't have, let me know and I will type for you. Posted by johnlayzell 13/01/2015 23:38:02

John,  Thank you for the reply; no I don't have the reference so a typed version would be much appreciated.  Thanks,  Richard Melbourne Posted by RichardWallach 14/01/2015 09:58:30

Mixture - Adjustments for idling should be carried out with the engine only moderately warm; if adjusted when the engine is hot, there will be some difficulty in starting from cold.  Closed throttle running is affected to a considerable extent by the idling adjustment.  Too rich a mixture is recognised by a rhythmic surge or 'hunting' which the engine develops when warm, and by a heavy coating of soot being deposited on the plug points. A weak mixture setting is shown by an irregular misfire when idling and by difficulty in starting. Adjust the idling by turning the air regulator screw W.  Turning it outwards weakens the mixture and vice versa. To adjust the setting, start with the screw W fully home to give the greatest richness, and then gradually unscrew it until the best idling performance is obtained.  On HP 9877 I've found this to be about 1.5 turns anti-clockwise.  Good luck, and let us know your progress. Posted by johnlayzell 14/01/2015 23:46:52

Thanks John,  I have done as recommended and have found that about 1  1/8th turns out seems to give a good result. I get an occasional "spit" in the carburettor but this tends to happen mostly when the motor has not fully warmed. I say mostly as it can happen at any time when I start from rest. This aspect has plagued the carburettor no matter what the idle settings.  I might also add that the Solex on my TA21 does not change smoothly from the idle to main jets if I increase revs very slowly, it will often splutter as it switches over  to the "mains". This is not a problem if I give it a decent amount of throttle when  I take off from rest.   With respect to the carburettor it is in very good condition and is fitted with the jets recommended by Solex.   Any further thoughts?   Regards   Richard Posted by RichardWallach 15/01/2015 08:55:10

Are all the gaskets between carb & intake manifold good? Possible air leakage?  Following the statement that "all mechanical problems are electrical" are you 100% certain it is carburation and not a duff coil, points, condenser? Posted by johnlayzell 16/01/2015 21:59:00

John,  I have been down the "electrical" path and all is fine. I will further explore the gasket issue as well as to play with the timing.  Thanks  Richard Melbourne Posted by RichardWallach 17/01/2015 00:21:46

The last couple of times I've started the 12/50, it has taken longer to fire than is normal.  Yesterday was no exception.  Fresh in my mind from this exchange, I turned in the idle mixture screw about 2 1/2 turns before it was fully in.  I turned it back out the previously mentioned 1 1/2 turns and it fired immediately.  Thanks for starting this exchange! Posted by johnlayzell 19/01/2015 18:14:27

John,  That's the beauty of the BB, one learns so much.  Regards  Richard Melbourne Posted by RichardWallach 21/01/2015 01:26:21