haarvik

I have been trying to figure out a problem with my 81 350. The distributor has been changed out as well as the intake and carb. Running an edlebrock with a Holley 4160 600cfm carb. Got timing set for 12 BTDC. Set idle and also set float levels. Starts right up without issue. Idles pretty good, but when you put it into gear it starts loping. I pulled the plugs and found the front 4 were black and the back 4 were tan. What would cause the difference in color? Doing a vacuum check this morning to see if it is fluctuating at all. Beyond that I am stumped. Oh, and it also has an ACCEL HEI Super coil on it.

JimLentz

Have you changed the wires? The front wires often get neglected because they are a PITA to change.

Big2Bird

Just to be clear, 1,2,3,&4 are black, and 5,6,7,&8 are tan?

haarvik

Changed plug wires, cap and rotor. Yes, that is the order they are in according to color. I checked vacuum and vacuum advance is working, and carb vacuum is sitting at 18. I adjusted the set screws to about 1 1/4 turns. Appears to be the best setting.

haarvik

Like I said, starts right up and idles pretty smooth. Put it in gear, and the loping starts.

Big2Bird

All I have is guesses.
I can only surmise the rears are getting more air.
SO, the secondaries are open more, the pcv is dumping air, the power brake booster is leaking, some other air is leaking into the manifold vacuum port. Maybe even the manifold gaskets are sucking air.
Your actually rich, but it's getting more air in back.

haarvik

But wouldn't a leak show up on the vacuum gauge? I sprayed carb cleaner around the intake and the engine didn't change at all, so I know that isn't a problem.

gungatim

agree. what intake are you running?

and no, the vacuum gauge may not show a leak, depending on where it is and how severe. you're reading total vacuum, it may be low, but what do you compare to, unless you have a baseline reading.

you could use a smoke machine to find the leak pretty easily. most shops would have one or make your own.

Big2Bird

Mixture differences bank to bank are easy to solve. Front to back, not so much.
Somehow the back is breathing, or the front is choking.
Maybe someone more awake has encountered this before.
I disclaim I am shooting from the hip here.

haarvik

It's an Edelbrock Performer.

haarvik

Ok, did a redneck cigar smoke check and nothing appears to be leaking.

toobroketoretire

Two years ago I let my spark plug gaps reach about .052" to .053" and all four of my front spark plugs fouled. Might be something to do with the slower runner velocity of the Edelbrock manifolds because I'm running an Air Gap (big block).

pltmgr

Gas leaking in the primaries? Secondary throttle plates properly adjusted?

haarvik

Ok, an update:

Tore down the top end and replaced the intake gaskets per Edelbrock instructions. Did tightening sequence and foot pounds on each bolt. Still getting the loping. I guess my next thing to try is carb gasket. I really can't come up with anything else.

procketus

When you say "lopey" to some of us "older" guys that tells me there's an aftermarket cam installed. Has it been replaced?

Maybe a cam lobe is getting wiped out?

Too low of an idle?

diehrd

Is the choke fully opening ? Any exhaust leaks ?

haarvik

Choke is opening as it should. Yes, there is an exhaust leak on the manifold.

Haggisbash

Have you tried blocking off all of the vacuum ports on the inlet manifold to eliminate the possibility of a vac leak? I had a problem with a misfire on the back cylinders, mostly 7 and 8 that turned out to be a loose vac hose on the wiper door valve on my 70.

haarvik

The only external vacuum hoses connected at this point are brakes, advance and PCV valve. guess I can try them to make sure none of them are bad. Easier than everything else I have done to this point!

diehrd

Did you ever check plugs prior ? And why are we not driving it ? Also your 12 degrees may not be 12 if the oem balance has slipped at all ..

Bump idle timing to 16 all in by 2500 mechanical , make sure vac advance is adding at idle what ever it is supposed to add with max vac .