Problem:
Engine idles great at 850 RPM, Hold brake and put in gear and the idle decreases by 400 and runs rough. I have tried the idle at 1050 but it still runs rough in gear. Go for a drive and it runs fine, but does get a bit rough when at a stop sign.

Things I have tried:
I cannot find and vacuum leaks. I have tried all sorts of combinations such as:
Plugged vacuum advance
Plugged PCV
Increased Timing, Decreased Timing
Swapped out carbs, new summit 750 for a a holley 750
Swapped HEI timing out for Pertronix Flame Thrower III
Engine at idle shows 7" of vacuum on manifold port and on the carb vacuum port.


Specs:
390 FE bored .30 over
Stroked 4.250 with 6.7 rods
Iron Heads with CJ Valves
Compression is 9.8:1
Comp Cam Hydraulic 280
2" Fender Exit Headers
Performer Intake
750 vacuum secondary carb
GM Style HEI Distributor - Now Pertronix FlameThrower III
C-6 Automatic Trans with Stock Torque Converter.

Thanks for the help.

 

You don't have enough vacuum - 7" is way too low. My 390 made 11 inches and it was a manual tranny.

That cam shouldn't be that bad.

One thing I notice, you have full vacuum at idle on the carb vacuum port, is that the "timed" port? Or it is "manifold vacuum" port?

If it's the "timed" port that shouldn't normally have vacuum, you have the throttle plates open too far. Which again points back to not enough vacuum.

What IS the initial timing at idle with the vacuum advance disconnected (and hose plugged) ?

 

Thank you for all the great response.

With the help of Summit i found the problem. In order to get the 850 idle I had the idle adjusted to where the primary blades where open beyond the orifice for the timed spark vacuum port. This allowed full vacuum at idle which was advancing the distributor by 10 degrees at idle. I adjusted the secondary blades to be open just barely and then was able to decrease the primary blades by 2 turns which closed the orifice to vacuum until throttle is applied. Now I have 2" of vacuum at the distributor during idle and when I place the truck in gear the rpm only drops by 200 and idles much better. Also the choke works better now.

Thanks again

 

There should really be NO vacuum at the timed vacuum port at idle. But you're on the right track

 

Trent,
That isn't just an 850 issue, that is with any Holley 4v carburetor.

Base setting is with the transfer slot in the primary *just* showing, or .020
Crack the secondaries so they are not fully closed.
Make small adjustments to secondaries first to obtain idle. On some you may need to adjust primaries some, but it shouldn't be much. You obviously do not want to be into the next circuit.

Idle screws should be 1.5 out to start. My experience on a 4 corner idle is that you end up somewhere around 3/4 of a turn out by the time tuning is done.

You have 15 degrees less duration, a good bit less lift, and a ton less overlap than the cam I have in my 445, and I can obtain 10 inches of vacuum out of gear at 950rpms. It took me a good lil while to get mine there, (I have 2, 600cfm carbs on mine) but it can be done.
Just remember to make one small change at a time, fire the engine and check the changes.
It takes awhile, but you can get it perfect if you are patient.

Also for initial settings, just disconnect and plug the vac advance until you get everything right. Later, make sure you aren't pulling vac advance at idle, adjust the vac canister until it is so. With lower than stock vac numbers, the vac advance normally is a little pain to get to where it engages just where you need it.

Good luck,
Drew