New engine won't stay runnin'

[ITSANSS]

1985 Chevrolet Silverado
Rebuilt 350 (355c.i.) 0 miles
Rochester Quadrajet carburetor
Automatic transmission
New everything just about...

Got the new engine in and broke in the other day.  After about 20 minutes of about 2,000 RPM to break in the camshaft, I let off the throttle and it idled normally for a minute or so before I shut it off. 

Then I changed the oil, filter, installed the wheel wells/inner fenders, battery tray, battery, and took off the cast iron exhaust manifolds and put on the metallic ceramic long-tube Flowtech headers and new gaskets.  Now I'm ready to time it and tighten down the distributor and take it for a drive, but it won't stay running.  The electric choke ain't hooked up, but even after pedaling it a bit to get to operating temperature, it won't stay running on its own for some reason.  It runs nice and strong at 1,500 RPM, but as it starts to dip below 1,500 it starts running kinda' rough and dies out around 600 RPM. Holding the accelerator and idling it at, like I said, about 1,500 RPM, it runs nice and strong and doesn't hesitate, spit, sputter or anything.

Nothing with the engine has changed since the break-in except for oil, filter, and headers...?  It has great throttle response and sounds real smooth.

What gives?   

[TexasRed]

Hook up the electric choke. Are you doing the pedaling in cab or underhood? I thought the choke kept closed unless hooked up and/or sufficient throttle was applied (you don't want choke going 60 mph).

[ITSANSS]

There's not a wire for the electric choke.  lol  That'll be another project in itself trying to find that wire somewhere and hook it back up.  It stays open unless I push the flap down with my finger.  We've been working the accelerator from both cab and from engine itself.

Found that I forgot one of the header bolts.  Put one in hoping that it would make a difference, but it didn't.

This whole project has been a step forward and then two steps back, seems like.

Aaaargh

[Lt.Del]

if choke is open after engine runs for a bit, that is fine, it is supposed to be open.  No problem there.

I'd focus on vacuum leak around carb or intake or vacuum line. 
If not that, carb adjustment.
Play with timing until you can get her to idle to check it correctly.  Retard it or advance it until you may find a sweet spot to see if she idles.

 

[TexasRed]

Are your idle mixture screws all the way in? If so, set them about a turn and half out. You can get the initial timing close to correct if at 3000 rpm, the total advance with the vacuum advance can disconnected and line on carb plugged is around 32 degrees. That'll give you around 12 degrees of initial.

As SgtDel said, check for a vacuum leak first.

[VileZambonie]

Vacuum advance HEI?

[ITSANSS]

HEY!  Buddy of mine came over and helped me out.  The carburetor gasket was leaking, letting air in causing it to run lean.  Got that fixed and got her in time (sorta').

I didn't realize it, but the timing tab on the new engine is off to the side and the old engine was straight up and down.  So, without a timing tab, we got it to where it seemed to run the best.  Also, the timing mark was nearly straight up and down looking at it behind the water pump.

Like I said before, this thing has been a constant battle.  lol

Ya'll were certainly on the right track with a vacuum leak somewhere.  Ya'll rock!

About to take this thing for its maiden voyage.  Just got back from parts store with a new belt and new springs.  Had the engine running a while earlier and one of the belts twisted up by itself on the pulley.  It was a little too wide for the pulley.  Got one that should fix that up.  Also, had to get some new throttle return springs, the old ones were letting the engine run about 150 RPM too high because the throttle wouldn't come back unless I manually pulled on it.  The new springs are definitely tighter.

Thanks!