Need help to set idle
I'm having trouble getting my 81A to idle properly.
My 81A has a crab distributor and a later model Holly 94 carb. We've had it a couple of years but the more I work on it the more I realise that it has never been set up properly and was more or less pieced together to run but not well, so I'm working through each system in turn to make it more reliable.
A couple of weeks ago I noticed a type of surging at idle. I have long suspected the carburettor needed some TLC so I have overhauled this myself with a kit from Daytona parts. I also got the carb casings vacuum blasted cause I like that sort of thing.
While that was away I also pulled out the ignition loom holders and painted them and replaced the loom with parts from the hotrod company. I particularly wanted the right angled plugs for the crab distributor because previously it had straight ones on it and that end of the ignition wiring was way too close to the water pump pulleys.
The distributor and coil are from Skip and have been working beautifully.
Putting it all back together resulted in a first turn firing, and it appears to run well with a bit of choke and a bit of throttle but when it warms up and I open the choke and start to lower the revs it starts to die. It often recovers and then starts to die again and this cycle repeats itself. It seems to be a bit similar to the surging before I rebuilt the carb.
I have attached a vacuum gauge to the mount for the carb (shown in photo). There is also a vacuum takeoff from the front of the intake manifold that is connected to the retard/advance mechanism at the front of the block for the leather plunger in the distributor (could someone let me know if it retards or advances on vacuum cause I'm confused about this also).
The engine runs well at about 1100 RPM. I am measuring this with an optical RPM meter with a piece of reflective tape on the end of the crankshaft. I have wondered if this could be reading double but I doubt it but just to throw all the facts in the mix. I have set the idle mixture screws at this point to just off stumbling and then I start the process of backing down the idle adjustment with the expectation that I will need to readjust the idle mixtures as the idle drops.
I can get the idle down to about 820rpm but its quite sensitive and doesn't seem to be stable at this speed. Intake manifold vacuum is also only about 14" at this point and I start to experience the dying. Winding the mixture screws in any further worsens the stability. Winding them further out seems to have minimum effect. At this point they are about 1 full turn from closed.
When the engine starts to die the vacuum drops away to almost zero then most of the time the engine will pick back up and the vacuum rises to 14" again and then the cycle repeats. Some times it dies completely.
Any further reduction on the idle screw results in it dying.
My first thoughts are that I have an inlet manifold leak and certainly one of the inlet manifold bolts is missing (cause I had to steal it for another inlet manifold socket which holds the rear of the RH ignition holder which I found had been partially stripped. I had thought that the gasket would likely hold vacuum with one bolt missing but of course the problem might be staring me in the face. In any case I thought that before I go down any further rabbit holes I'd better get some advice.
Sorry for the long winded post but I'm fairly new to this although I have reasonable mechanical skills and I'm very keen to learn.
Regards
David
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