Weird idle characteristics troubleshooting

For reference: 3.8 E engine, modified head by Bill Terry circa 2002, 3x HD8 SU, UO needles.

Recently rebuilt my carburetors, old jets were frozen in place and diaphragms were shot.

First puzzle is that with the car fully warm, choke off it’s idling around 1000-1200rpm with the slow idle screws all the way in against their seats.

2nd puzzle is the manual says adjust the mixture screw until you get the highest rpm, and that if lifting the piston results in rpm drop it’s lean. Increasing the mixture (screw down) raises the rpm, and piston lift results in stumble indicating lean mixture.
However, removing the manifold vacuum hose (that goes to brake booster) results in higher rpm also, indicating rich mixture. Further, it started very easily this morning with no choke.

The high idle seems to indicate a vacuum/air leak. I have checked for tight connections, and have dripped some oil around possible sources (throttle shafts, etc) to no effect. Also tried the carb cleaner spray method around other areas, also to no effect.

Vacuum at rear manifold port (to booster) is a bit low ~13-15”. Vacuum at front port (to distributor) isn’t really measuring which seems odd…

Timing as about 10d BTDC @1,000rpm

Obviously I’m missing something… but what?

Additional point… took it for a 13mi drive and it ran beautifully. Smooth, good power etc, only indication anything was amiss was the high idle.

In my experience this is normally the butterfly’s not seating properly. Have you tried/ensured that each one is fully closed before connecting the operating levers?

I had the same problem after fitting HS8’s ,it was the Butterfly’s, I did add a extra spring on the bottom , that did not work , I had a Y bracket laying about , so bolted that to the top , and added a spring , Butterfly’s shut 100% now ,

I have visually checked the butterflies, and they look closed, have a spring on each carb to close them, and there is some slack in the linkage to allow them to close.

If the slow screws are all the way in, would that fully block the bypass passage, or does it still allow air through?

yes they should shut off all idle air…i have had the issue…like others, with HD8, always been a butterfly…or linkages out…try twisting or pressing all the linkages firmly in the shut direction with the engine running and see if it makes any difference…the tiniest amount of butterfly makes an amazing difference

Before I fitted the other return springs , I found if I dabbed the Accelerator pedal , quick , the rev’s would drop !
May be worth trying .

Thanks, this is a big clue then. I did note the slow idle screws looked a bit pitted at the tip, though I would be surprised that’s the main issue, I am planning to order some new screws and seals (I did not replace the screws as part of rebuild).

I did go back and fiddle with all 3 carbs throttle shaft, found the front one had lower spring tension than the other 2, so I adjusted all 3, and that did make some difference, but it still idles now right at 1krpm with slow screws full in and about 1500 with them out one turn. Before adjusting the butterfliy spring tension the first 1.5-2 turns on the idle screw had no effect.

Drove another 9 miles yesterday, and it’s still driving very nicely.

I have a smoke machine on order so hopefully that will help it solve quickly.