one carb drawing less air

[MarkRado]

Hi all,

after a complete engine rebuild of a friend's V12 I had a successful first startup.
First iteration of adjustment was ok,
now it starts instantly and idles good at around 850 at 12° BTD
all needle/jet heights are adjusted identical and idle is synchronised / carbs draw same amount of air when idling
there is no hesitation etc when accel-decelerating

Sounds good, but:

with more throttle opening the piston in left (B) front carb is almost not moving upwards.
This carb also needed much more idle adjustment than the others.
Looks like its drawing very little air (temp compensator adjusted and checked, bypass valve plugged)

plugs:
B1 black
B2 only lightly coloured
B3 colourless

B 4-6 (left rear carb): all medium dark
A1-6: dark / medium dark

Therefore I assume that
- there are either manifold leaks at B2 and B3
- valves at B2 and B3 are not working as they should

Next I'll check valve clearances again and replace the manifold gaskets.

I obviously miss something.
What else should I check?

Thank you for your input

Mark

[MarekH]

Are you suggesting that the left front carburettor doesn't perform because its piston is stiff or that the cylinders B1-B3 are "different"?

You can differentiate the two from each other by swapping the left front and right rear carburettors over with each other and seeing whether the "fault" follows the cylinders or the carburettor.

kind regards
Marek

[Jack the lad]

First thing to check would be the Vacuum Diaphragm in the top of the carb. they wear very slightly and you get a very small crack which gives the exact symptoms that you describe. Often the cracks are almost invisible to the naked eye, well worth changing all of them. Also its easy to fit them incorrectly, if you have already fitted new ones, then its worth looking closely at the one causing your problem.

[MarkRado]

Jack,

thank you for your hint;
I would have thought this impossible, since I had rebuilt the carbs,
but: indeed the 4 screws holding the membrane to the piston had not been tightened!
the threads were not clean giving resistance, which obviously had prevented me from tightening the screws back then.
I cleaned and tightend them and heureka!

I'll check all the carbs and ports again for leaks, which obviously has a very big effect on this engine.

Thanks again to you and Marek

Mark