Hi I’m sure I have posted about this before , when I take my car for a drive the idle speed creeps up from 850 to 1500 ish
Next time I start up from cold it seems to be back to its 850.
All the previous comments point towards the fuel system , carbs , air leaks etc.
Is it possible the problem could be the centrifugal advance sticking ? which is something I have not heard much about before .
Thanks Steve
You and me both. Car idles to a stall when cold but progressively speeds up to 1200rpm when hot. I’ve had the centrifugal advance to bits in mine to change the bob weights - new distributor came with 7 degree weights instead of the 12 degree ones needed. I made sure nothing was sticking and it made no difference to the idle speed.
It is kind of annoying - I’ve just come back from a 50 mile drive and it’s not that big an issue but I would like to work out why it’s doing it. The carburation is fine, it’s not showing any signs of weakness, hesitation or anything and the timing is correct by my strobe light.
Knowing which carbs you have would be used useful.
The advance should be easy to check with timing lamp when the fault is occurring.
Does the car return straight to an incorrect idle speed and hold steady or does it creep up or down over time?
what would you call “cold”? I set idle (1000 rpm rather than 900, to be easy on the bearings) when the engine is at operating temperature (i.e. radiator hot, confirmed by the temp gauge). When the engine is completely cold it does not even keep idling (weber, choke/enriching device left closed), so has to be maintained by the throttle till it warms up.
Which carbs? Weber, Dellorto, Zenith-Strombergs?
It’s normal for a carb engine to run slower when cold, and to pick up rpm as it warms up. ALWAYS set the idle speed when the engine is hot… up to full operating temperature. If it then idles too slow and/or dies when cold, use the ‘choke’/ enrichment device to keep the idle speed up and the engine alive until it gets up to temperature. Also use your right foot. If the ‘choke’ isn’t hooked up or operational, then it’s up to you and your right foot to keep the engine going until it warms up. User involvement is required.
If the carbs are Zenith-Strombergs, then the Temperature Compensator will also help with cold running. Not enough to eliminate the need for the ‘choke’, but it helps. If it is stuck or has been defeated/ disabled, then the engine will have to rely more on the ‘choke’/ enrichment device.
A Weber or Dellorto ‘choke’/ enrichment device doesn’t include a fast idle function, just a richer mixture. The Zenith-Stromberg ‘choke’ also includes a Fast Idle Screw that must be adjusted separate from the normal Idle Speed Screw.
Personally, I think you’re barking up the wrong tree by going after the centrifugal advance.
Regards,
Tim Engel
If Webers, fuel level in bowl too low at start? After a couple of pumps on the accelerator to start the car the level would be lower, rising after a run.
Just a thought.
Jeremy
I used to experience this with my Weber?s. As Tim advised above, once you get the hot idle sorted, it becomes more manageable.
Doesn?t take long for the cold lumpiness to subside (to coin a phrase…)
Regards
Richard
Thanks for the replies, for completeness the cars are weber 40 dcoe
Type 31s
Steve
Steve - I replaced my type 31s with 151s and they are so much easier to balance at idle.
It does sound a lot like air leaks. In particular whilst the engine is running run some oil over the carburetor to inlet tract flange joints. If there is a leak the oil will get sucked in and/or you will get some blue smoke at the exhaust. Check the brake booster for leaks too. Disconnect the hose to it temporarily and plug the opening to see if that makes any difference.
I’m assuming of course you’ve also done the mechanical check to ensure the linkage is not sticking. Rotate the throttle spindles by hand to see if the idle speed drops.
Oil on the flanges sounds like a good idea I will give it a go
Thanks steve
An old British car mechanic I knew used to spray gasoline on the intakes to isolate air leaks. Very effective as the engine sped up immediately. Of course I always worried about the car and the shop going up in flames but he was never the least bit concerned 
Chris 
You can do the same thing by spraying aerosol brake cleaner or carb cleaner… anything flammable, a butane lighter without igniting the flame, even hair spray. With all those sprays, the engine’s speed will pick-up when you spray a leak area, and they’re all a lot more safe than gasolinie/ petrol.
Regards,
Tim Engel
Check those headlamps for vacuum leaks too - fortunately something I’ve never had to worry about!
Is it something that changes after it warms up?
Vacuum hoses expanding, jets?
I have checked the vacuum hoses , no leaks as far as I can tell , the lights stay up for ages when you switch off the engine so a good sign.
I have run oil around the inlet o rings , no signs of smoke or engine speed change when I tried it with carb cleaner.
The jets are new , and driving / acceleration is really good.
The balance from rear to front at 950 rpm idle using a synchrometer is 5 , 4 , 4 , 4
I don’t think there is much I can do to improve the rear carb.
below is a picture of the idle screw. Probably a daft question but is there any benefit in fitting a second idle screw on the from carb ?
thanks Steve
I would also check the throttle butterfly versus the progression holes at idle as a partially exposed progression hole can result in higher idle
cheers
Rohan
On my dellortos the screw you show in the picture is the balance screw not the idle adjust.
If weber are the same twiddling the pictured screw will adjust the idle, but also ruin the balance,
Mick
Could it possibly be leaking air at the butterfly spindles and leaning off the mixture?
As I understand it there is an upgraded bearing with a better sealing arrangement. Have seen them on the -bay but have no experience of them… I’ve got a bit more work to do before I get to the Weber fiddling point
I think your thackery washers look a bit too tight.
