HD8 carb balance

Hi All,I have a 1967 420 and have been giving it much needed servicing as it’s been out of service for several years. Rebuilt carbs. Tuning well but to get balanced and proper idle speed, the rear carb is 1/4 turn from seated on the bleed screw. The front is about one turn out. Should I have any concerns?

It’s important to ensure the butterflies are fully closed. You should disconnect the link between the 2 carbs and the throttle operating connection from the pedal and manually push the operating levers on each carb towards the closed position. If you do this when the engine is warm try starting it without the reinstating the connections and see if the idle is more easily adjusted.

Thanks. I have done that already. Also polished the seat on the plunger on a surface plate for the auxiliary carburetor . That helped a bunch getting the idle speed down. Just wondering if final adjustment could be split that much.

Thought I would update. I must have a induction leak. The rear carb is 1/4 turn from seated and front is about 1-1/2 out to get balanced . Now idle is back to 950 rpm. Ill try the propane trick to see if I can find a leak.

Will it stall if you close the volume screws?

No it will idle really slow 300-400 rpm

If you had an improvement when you lapped the disc on the starting carb try blocking that port with something like a cork to ensure it’s not allowing air in.
After that as far as I can remember it’s only the servo take off that can allow air in from an external (non carb) source on your model.

With best regards
Philip Dobson

Its entirely possible that the throttle shaft has free play in the carb body/bush

This lets air in

~ 50% of Jag carbs that I have fail the following test;

grasp the throttle shaft at it outer tip (everything else must be disconnected)…its about 1" outboard of the carb…if you can detect free play, that is undesirable

I havent put a dial gauge on one, but I am guessing my “detectable free play” might be in the order of 0.020", maybe less

edit…I just noticed you say carbs rebuilt, so maybe not that, but possibly worth checking

The dial indicator set up goes in damper locations for balance setting. I put new Teflon bushings in, what a pain! Shafts are tight. Also new shaft seals. I started a conversion from Auto to manual transmission, so now its unable to run for now. Plus we’re supposed to get 1-4 inches of snow, so it’s not going anywhere until April. I may remove the carbs again and pressurize the intake to make sure I don’t have a leak at the head. I do see some indications of weeping on intake manifold at the head when looking from below.