Carburettor air regulator screw setting

[Bryan Walker]

A small thought,
In the Maintenance manual P.14 and the club notes/handbook it suggests to turn the screw out 2 turns for the approx. slow running position.
While the Instruction book for the PC blue book (& the latter brown) suggest only three quarters of a turn on P.38.

I've found my car to be closer the the latter.


Bryan

[Keith Andrews]

The idle circuit in the carb has an adjustable mixture screws because every engine is different , and because of the low rpm any slight changes in the fuel/air ratos are very noticable.
Not only are the variables in the engine (bearing clearances, wear, a 1/100- in tappets etc) but atmospheric conditions like high altitude also make a differnce to idle quality.

The position of the screws are a working guidle line
On the other hand a diff of 3/4 to 2 turns is is rather a large anomoly, and considering the general rule of thumb for idle mixture screws is generally around the 1 1/2 turns (1 to 2 turns) to me both are rather strange???

If the screws need to be set outside The factory given settings (say 1 1/2 +- 1/2 turns for example)then this signifies there is something wrong, like worn butterfly bushes, inlet air leak or (where exist) the sec butterflys stops are not set correct.

Knowing that there are no air leacks, and everything is in good condition and specs, plugs, pionts, tappets, HT leads etc....where the mixture screws and what intial timing u have doesnt realy matter, they are set to have a smooth comfortable idle at a low as rpm as possible it is only a convieniance thing. Keep in mind thu changing the idle advance can effect the performace under power...hence it best to set timing at spec max rpm and advance (vac disconnected) then work backwards to a smooth idle.

A too high intial advance (when set by ear, never do that) can load startermotors and burn them out over time, cause hard cranking, and if the curve is not adjusted have the engine firing at cruise and high rpm, when the piston is still coming up, causing bearing wear, huge crank loading and failure. This old school tuning idle by ear has been responsable for many a crank fail, burnt valves, valve resenction etc and blaming on poor enginering/oiling in these areas

An exteme example:
It is this over advanced curve that has given LPG as a fuel a falsly bad reputation on engines...Due to lpg needing a very fast advance, conversion ppl instead of re curving the dizzy simply up the intial to compensate to get the power. But LPG needs a much lower total. The ave engine will have around 40 to 50 deg (intial+ cent+ vac adv)all in around 3000 to 4500 rpm. But only LPg 27 to 36 total all in about 2200 to 2500 rpm. using a petrol dizy curve plg as a fuel is a slug...upping the intial 10 or 15 deg so there is 30 deg at 2500 rpm makes the car perform very well, but cruising down the highway it needs about 30...using a petrol curve it is runing 20 to 30 degrees over advanced on a feul with a huge BTU rating and octane between 100 and 115!!!!!

I will note here the idle curcuit mixtues do not have any effect on the power curuit mixture jets or pumps...it is like have 2 separate carbs, one for idle and another for under power. It is a very common misconception that mixture screws change fuel to air ratios under power 'power' in this post is anything where the throttle is open and car moving, rpm above idle.

Appling the priciples above, where do the mixture scews actually adjust best to at or close sea level?