351C-4V very low vacuum...

Hey Fellas,

I picked up a 351C-4v / FMX Eliminator back in June with the intent of cleaning it up, sorting it out, and reselling it over the summer. Summer is nearly over and I’m still trying to resolve a few issues. Fix two things and one more crops up, but at least the list is getting shorter.

When I first picked it up it ran fine. Over the summer it has been slowly degrading, and now it will intermittantly stall when I put it in either drive or reverse. So far it has always started right up fairly easily. Once it is warmed up it doesn’t like to sit still in traffic, and now sputters coming off idle.

I was going through the basics this weekend;
Carburetor = Edelbrock 1406 (600cfm)
Idle is high ~ about 800-1000rpm. If I dial it down it bogs and dies.
Timing = 6deg btdc at the above rpm, with no vaccuum advance. Reving causes the timing to track upward over 20deg, as expected.

I put a vacuum guage on it: 8" vacuum at idle. Horribly low. blip the throttle and it drops to zero and then jumps to about 15", before settling back to 8".

It has a dual vacuum advance unit, with the distributor side being bad (leaks). I thought that was likely my culprit, being a vacuum leak, but even capping it off did not improve things at all. Have a new unit on order now, but I don’t think this is the root cause.

I’ve capped the ports on the carb as well as removed and capped the connection to the vacuum tree, but no change in vacuum.

I did a cursory leak check with carb cleaner around the base of the carb but the engine was hot so I didn’t want to get to aggressive with flamable sprays. I plan to break out the propane torch tonight and see what I can find.

Another item to note is that the car bogs way down and threatens to stall when I make a sharp left turn around a corner. I thought it might be the float level in the Edelbrock (a PITA to adjust), but some further research indicates that it may be an internal manifold gasket leak causing a bog when oil moves over the leak, throwing off the air/fuel ratio.

No apparent coolant loss. No apparent coolant in the oil, or vice versa.

Is it possible that I simply have the carb adjusted so far out of whack that I’ve induced this issue myself? I tried to follow the Edelbrock instructions, but at this point the main idle seems to be open too far to actually see any effect from the mixture screws. I’ve tried to find something online that says what the “baseline” adjustments would be for the mix screws and main idle screw (y’know, like Holley has), but I’m not finding squat to “zero out” the carb and start from scratch.

So… I’m interested to hear some other possibilities. Thanks in advance!

I should add that the choke is working perfectly. Mostly closed when cold, fast idle comes up to about 1200rpm as the choke comes fully open after a few minutes of running.

Tonight I decided to check / retorque the intake bolts. Found several of them that responded to the torque wrench when it was on the first torque step (10#)! Yikes >.< So went through all the steps in the proper sequence. Quite a few on the passenger side that were not up to spec, including the little choke pipe fitting in the center of the passenger side. I’m hoping this will at least get me a few more inches of vacuum.

Plan for tomorrow is to fire it up again and see what vacuum is at. Also plan to put the propane to use and see if I can find a leaking area or fitting.
If there is time I’ll fiddle with the timing a bit and see how it responds to bumping it up a few degrees. Then with the engine hot I’ll go through and verify the torques again, I think (the manual says torque it cold, then retorque it after bringing it up to temp).

If the intake bolts are indeed that loose, i would suspect a leak around the intake like you said. But checking that with propan or soap or brakeclean should show a difference in idlespeed.
Almost stalling in a sharp corner could be related, but I think that might be a different issue (like float level for ex).

Not sure on the Cleveland, but a windsor likes around 11-12 btdc timing wise on idle without vacuumadvance.
My idle is around 800-1000 aswell, so I don’t believe it’s that high (fast idle is at 1300).

Does your vacuum gauge fluctuate around 8" or is it stable?

Timing it at 12 degrees is a good starting point depending on the tune of the motor.

I neglected to mention that the cam is supposed to be a stock grind, so nothing radical there to justify the low vac.

Vacuum is pretty stable around 8".

Thanks for the tips on bumping up the timing. Book says 6deg, but from what I’ve been reading most folks are running 10-12 with modern fuels.
So I’ll check it tonight with the retorqued bolts then bump up the timing some and see how that goes.

Did you block off power brake vacuum line? trans modulator line? But since normal vacuum should be at least 15" and 8 is really low, I would suspect intake gaskets. You may not detect it by spraying on top of the engine. They may be deteriorated on the bottom over the galley. How is the compression?

If there was any sealant around the exhaust crossover holes in the intake manifold gasket it can burn and cause a big vacuum leak (metal bathtub intake gasket)

I did. I disconnected and capped off the main vac line where it comes off the manifold and feeds into the vacuum tree on the firewall.

That’s good to know, thanks!

Well, turns out the timing was NOT at 6. More like zero. In my defense, I was out in the sunshine, so the marks were kinda hard to see.

Anyhow, dialed the timing up and vacuum followed right along! Sitting at 15" at about 8 degrees. I called that a “win” for the night. Have a new vacuum advance unit coming tomorrow, so I’ll fiddle some more once I get that installed.

Very happy!

Thanks for your help and input!