Throttle plates closed, but still nozzle drip

[Marx3]

I just put together a 17081282 for a completely stock 170 hp 350 in a '79 Caprice.
Car had been sitting for 10 years when I bought it. I drove it home and it was running allright.
Since I am milling the heads a little and putting in a 214/224@.50 cam, I made a few changes.

Bypass .080
Tubes .036
Dcr .047
Upper iab .075
Lower iab .075

Mab's resized to .070
72 jets and 44 rods

Installed new throttle bushings.

I can't get the idle down below ~800 and this is with the idle stop backed all the way out: throttle blades are closed.
I am getting a steady stream of nozzle drip this way. Pulling a vacum hose does nothing to idle rpm, opening up the throttle a little will increase idle speed, but does not change the steam of drip.
Timing is around 15 initial and is getting ported vacum. Applying manifold vacum will increase the idle.
I have checked and adjusted all four throttle plates to ensure minimum to no light can be seen around them.
All hoses are checked and/or replaced and all but the necesary vacum ports are capped off.
I have even been spraying carb cleaner pretty meticulously everywhere to look for leaks.

What am I missing?

[77cruiser]

Float level?

[Marx3]

Thought so too. Checkes it, found it to be at the usual 1/4"
Lowered it a bit, just to rule it out.

I just don't get how the nozzles can flow fuel in idle, when the throttle is actually closed.
I have the rear of the car on jack stands, because I have been doing brake work, so the car is not level. Could this cause the drip?

[von]

Check your choke fast idle adjustment to be sure it's not holding the primaries open a little even when the choke blade is fully open. If that's not it, there's probably a vacuum leak somewhere.

[Marx3]

Already checked :-) This is far from my first Qjet, that's why I'm puzzled.
I am 100% positive the plates are completely closed.
I agree, the engine is getting air from somewhere, I just can't figure out how. And HOW is it sucking fuel from the nozzles with the plates closed...
Typical nozzle drip occurs when the plates have to be opened too far, to make the engine idle...

[Frank400]

The air could be coming from the secondary throttle plates not being 100% closed ?  Like being held a little by the linkage ?

[Marx3]

Already double checked that :-)

[von]

Just FWIW I once had a big Q jet vacuum leak caused by simply having the carb to intake gasket flipped over.

[von]

Also, I've seen bad (leaking vacuum) vacuum advances causing vacuum leaks. I'd plug every vacuum line and the PCV line and try it.

[Marx3]

I was thinking there must be a major vacum leak somewhere, but I just can't find it.
I am replacing the intake to carb gasket.

Other than that, I have had everything unplugged, from advance to brakebooster.
With everything unplugged and capped off I tried spraying each vacum stud and cap with brake cleaner. Found nothing.

Car runs great, it just have a slight off idle stumble from all the fuel.
It seems the fuel is being pushed out of the nozzles, rather than sucked ( it is not really physically possible with the throttle completely closed, right... )
So exsessive fuel pressure and/or to high a float level could cause it.
The fuel pump is the original good old mechanical one, tried swapping out the needle/seat assembly and checked the gasket and the sealing area in the carb body. I even tried two different used floats I had lying around, cause I thought the float might be saturated.
None of this helps.

[Marx3]

Found a nice 17057253 in the addict and rebuild it to the same specs. Nozzle drip is gone. Idle is a little unsteady, it tends to rise and fall about 150 rpm now and then, but I suppose that is on account of the idle system being calibrated too generously for the stock smog 350
.083 bypass
.051 dcr
.037 tubes
Both IAB's at .068
Mixture holes at .080

I gets plenty of fuel and air :-)
Cant wait to get the 214/224@050 / 112 lsa cam in there

[Cliff Ruggles]

Good news.  It would be nice to know why the other carb was acting up?

I did run into a main casting one time that had a pressure leak between the incoming fuel and bowl, probably porosity and it allowed the fuel level to rise at idle and flood if it sat there long enough idling.  That casting simply may not be any good.....Cliff

[Marx3]

I will see if I can find the cause. Like you mention, it must be some sort of main casting defect, since I used the exact same parts to rebuild the 253.
I'll post it in here when I have investigated some.