Well, it appears it's not just the carb.
Truck is running but still not well. It's running rich still and idling kind of lumpy though it starts and idles easily. It's also stumbling or stalling if I open the accelerator quickly (It will rev up slowly). I have the carb fuel adjustment screw pretty far out on a weber right now (like 5-6 full turns). The rpms do not go up any further at idle if I keep turning it.

Thus far I've:
Tried two zenith carbs, both working when taken off, and currently have a working Weber on.
Replaced the later Lucas 25D (with reversed vacuum advance) with a 45D from a running engine.
Retimed multiple times. Currently set to TDC.
Replaced the coil with known good one (and even tried a new one).
Removed and reinstalled carb gaskets.
Removed and reinstalled pipe to brake servo in case it was leaking vacuum.
Confirmed camshaft timing. It's dead on EP mark on the flywheel when #1 exhaust valve is fully open.
Adjusted valves and rechecked.
Blocked off port at the base of the carburetor adapter. I also tried leaving it open, connecting to the oil filler tube vent and now it is connected to the air cleaner elbow at the carburetor intake via a T connector (also connected to the rocker cover vent).

I should note that I had it running okay with a different head all summer but it began idling poorly and misfiring. After a leak-down test showed the valves were not sealing I replaced the head with a newly rebuilt 8:1. It's still running poorly. Compression and leak down test with the new head show good consistent compression and minimal leakage.

I'm thinking of buying a smoke tester to see if/where I may have vacuum leakage and then the only thing after that is my initial hunch that it's worn camshaft or worn cam followers/guides.

lcdck said:

It's also stumbling or stalling if I open the accelerator quickly (It will rev up slowly). I have the carb fuel adjustment screw pretty far out on a weber right now (like 5-6 full turns). The rpms do not go up any further at idle if I keep turning it.

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could be the accelerator pump.. you should see it squirt if you look down the barrel with engine off and operate the pump by hand.
are you sure its 'turns out'for rich? on one carb, can't remember which, its 'turns in' (its actually an air screw not a fuel screw)
it could also be ignition timing.. have you checked the vacuum advance is connected and working? is the ignition set?

Accelerator pump is working. New fuel in the tank. Vacuum advance is connected and working. Ignition timing is set to tdc. I could try advancing it.

Firing at TDC is too late.
What does the manual say? So I would start there.

Wind that air screw in and start at 2 out.

Sounds like a basic tuning to me that an experienced ear would sort quick, so don't lose hope.

J

Manual says 8:1 head: static timing set to TDC for 90 octane. 3º ATDC using 85.

What fuel are you using?
Would still try a little advance on the timing personally.

J

I will experiment with timing.

try 3 degrees before TDC as a starting point, assuming 95unleaded. you might need a bit more

kermit_rr said:

try 3 degrees before TDC as a starting point, assuming 95unleaded. you might need a bit more

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Good starting point and would soon show up low octane fuel, which was why I asked.

J

You just changed that number while I was typing

Back to one of the Zenith carbs and it's working much better. I blocked off the PCV valve hose at the base of the carb adapter and that seemed to help as well. Didn't change the timing yet.

Running lean but running well. I determined it was suffering from 2, maybe 3 issues.

The coil was working intermittently so I replaced it.
Two of the three carburetors I tried were getting clogged at times.
The brake servo may be introducing a vacuum leak on the intake possibly due to have a cracked diaphragm.

It's very easy to suck air through the hose going to the servo check valve, nearly no resistance. On a parts vehicle I'm feeling more resistance when performing the same test. The engine is running lean but seems to run well with the pipe from the intake manifold to the brake servo blocked off.

lcdck said:

Running lean but running well. I determined it was suffering from 2, maybe 3 issues.

The coil was working intermittently so I replaced it.
Two of the three carburetors I tried were getting clogged at times.
The brake servo may be introducing a vacuum leak on the intake possibly due to have a cracked diaphragm.

It's very easy to suck air through the hose going to the servo check valve, nearly no resistance. On a parts vehicle I'm feeling more resistance when performing the same test. The engine is running lean but seems to run well with the pipe from the intake manifold to the brake servo blocked off.

Click to expand...

Have you drained the fuel tank?

Yes. New fuel, new lines, new filter.

Just the brake servo issue now.