I was actually about to mention something about the intake, but the power company was knocking on my door to turn the power off in order to change out a faulty meter...
I was wondering if the intake manifold might have a crack in it somewhere, if it is sealed up well at the sealing areas, a crack could cause all kinds of strange things. Putting the factory iron intake back on might be your next course of action.

 

pull all the spark plugs and see which ones look rich. Those will be the ones with the problems.

 

Ever done a leak down on it ? Comp. gage on it ?

 

My 1979 has the 351M 2BBL with california emmissions and get this 1,400 original miles. Under the hood is 100% stock and intact. I'm breaking in a "new" 29 year old car. My problem is, it runs great cold. When it warms up it surges. A vacuum test shows erratic needle movement with the surge.
Start cold it fires right up, runs smooth pulls about 21 inches of vacuum @ 1,500 RPM.
Kick down the choke and it settles down to 18 inches @ 800 RPM.
Fully warms up and the RPM drops to 600 yet still holds at 18 inches of vacuum.
Let car idle and in about 30 seconds it starts to surge: RPM surges from 300 to 800 and the vacuum needle fluctuates from 18 to about 15 back to 18 back to 15 over a 20/30 second period.
Here's the weird part, it smooths back out on it's own for about 20/30 seconds running smooth at 600 RPM and holding @ 18 inches of vacuum.
Then it starts to surge again, then it smooths out, and it'll continue to do this all day.

If I hold the pedal at 1,500 RPM or higher for a few seconds and release slowly, it'll run smooth for about 30 seconds and then the surging starts again.

in park holding the RPMs at 1,500 2,000 2,500 3,000 the vacuum essentially stays dead locked at 21 inches. At 3,000 rpm there is a faint whistle; possible vacuum leak?

It'll drive all day smooth at any constant speed.

Fuel pump is new, fuel filter is new, dropped the tank-inside looks brand spankin' new.

All the vacuum lines, canisters, charcoal filters etc are in place and connected correctly like I said, it's a 1979 unmolested original 1,400 mile time warp.

Any ideas on what this coud be?

 

Stuck EGR valve or other emissions component? First thing I'd check for is rotted vacuum
to all your devices. Then, with the engine warmed up, watch the EGR valve; it should open
as the engine RPMs increase and close at idle. Failing that, you might have things like a
Throttle Kicker and EGR vacuum amplifier (I think that's what it's called, it's a black box
mounted on the pass side rear of motor on top of the intake manifold and has 5 vacuum
lines attached to it) and various other components that are all vacuum- & temperature-
operated.

-ct

 

The float in the carb has absorbed fuel and is sinking in the fuel, letting too much fuel into the float bowl and flooding the carb.

Check the plugs. If they are black, it's because of the rich mixture from the flooding carb. If so, just R&R the float.

 

I will check the vacuum lines and all attached components. I thought it may be a problem with the floats in the carb, I'll check that too. Oh, there is black smoke at the exhaust only when surging and a raw gas vapor odor at times. We'll definitely check the EGR and the floats.

 

Raw gas and black smoke are definitly frrom flooding in the carb. In addition to the float, the Needle Valve could be sticking. I would change the Needle and Seat as well as the float..

 

I spent about 6 hours working in the engine compartment tonight. I replaced the float/valve and seat dince it's easy, fast and cheap. Still surges. One thing though. When I took the top of the carb off, the bowl was dry. It had just been run to full temp 24 hours earlier and you'd think it had been sitting for years. Is it normal for the bowl to drain out over night?

Another observation: There is a black, soft, 1/4 to 3/8 thick ?gasket? (carb to spacer gasket) between the bottom of the carb and the plate that has the egr valve. It is soaking wet with gas.

I checked about 1/2 of the vacuum hoses present and all looks pretty solid so far. Threw the vacuum gauge back on for good measure and she pulls a strong 18 in/Hg.

Do I put money into it and rebuild/replace the current carb (I like things as stock as possible) or do I do what I really want to do and replace the retarded cam/timing gears/chain, upgrade to a 4bbl and pull all the emission stuff and converters off? possibly upgrade to headers too. Of course I'll save all the original parts incase I need to, but it's over 25 yrs old, carries collector plates and will never get inside an emission facility.

 

One more thing, when I replaced the float etc. I was studying the inside of the carb. There appeared to be metal filings around the bowl (possible it was something that looked like metal filings) that the needle valve and seat are in -and- when idling, if I gun the gas while looking into the carb I see gas pouring down the two vents, the atomization is taking place but there are also trickles of gas running down the inside of the carb...........does this thing need to be replaced/rebuilt? Keep in mind this 1979 has less than 2,000 miles on it since new. The longest it sat without being turned over was 8 years (the period just before I bought it), before that it would get turned over and ran to a full warm up every few months.

 

It is flooding. There should not be trickles of gas. Is the float set right?

Also it shouldn't drain out overnight.

I would be looking for a replacement carb.

Here's an idea! Get a rebuilt 2 barrel carb and get it running right. Then, at a later date, replace the timing set with a good double roller set, straight up. Then do the exhaust with headers etc. With the high cost of fuel these days, the 4 barrel carb is something that you can do without.

 

My intake gets hot enough that the gas in the float bowl boils away. You can hear it bubbling
in there, and the vapor from the vent tube looks like smoke coming out of a chimney.

-ct

 

There is an exhaust crossover in the Intake Manifold. It's purpose is to allow hot exhaust gasses to warm the manifold and carb for fast warmup in cold weather. There should be a heat riser valve in one exhaust manifold, that is closed when the motor is cold, and open when warm. The heat riser valve forces hot exhaust through the crossover, when the motor is cold, and allows the exhaust to exit the exhaust manifold normally when the motor is warm.

If the heat riser valve is stuck closed, or the exhaust manifold on one side blocked in some way, then the hot exhaust will pass through the crossover all the time, and the intake manifold will be too hot, and fuel in the carb will boil.

 

It's fixed.

I put a rebuilt carb on and it's fixed. The clue was the puddles of raw gas in my intake manifold every time I took the carb off to inspect. Something is wrong internally and it was literally flooding the engine. The cold engine was running ok since a cold engine needs more gas and the fully warmed engine was flooding out. I'll rebuild my original carb over the winter.

 

I always appreciate those who come back to explain what resolved their issue