Stumble below 2500 rpm - 78 308 GTB

Hello all,
I'm hoping I can get some more good advice as in the past. My 78 308 has developed a bit of a hesitation below 2000-2500 rpm with backfiring through both the exhaust and carbs. It sounds like one or more cylinders are missing. Above that the car runs smooth with lots of power (more power than usual actually). Prior to this suddenly coming on, the car ran great at all speeds.
Here is a list of everything I've done so far:
-Changed spark plugs several times, going to hotter, back to stock and back to hotter again. Plugs are brownish. Some are whitish.
-Changed all spark plug wires
-Checked the resistance of all extenders and all are straight through
-The condensors are new
-I've removed, filed and reset the points, The idle points are disconnected at the microswitch
-The mechanical advance is properly lubed and fuctioning
-I've checked and re-checked ignition timing
-Re-synched the carbs to be dead on balls perfect

Looking through the oil filler, the rear bank cam that I can see is spot on. My plan of attack is to check the cam timing on the foward bank, but I wanted to see if there are any other ideas I may be missing. The fact that I'm getting slightly more power in the mid range, leads me to believe that it's a cam timing issue.

 

Yeah I thought of that, but I guess its worth a shot to spray the bases. Maybe I'll hook up a tach and re-adjust the mixture too.
Vince

 

Update: I found plug #4 to be a little wet and it does not respond at all to any change in the idle mixture. This is the same carb that runs the brake vacuum on my car. I don't have any problems with braking, if that means anything. Any suggestions?

 

I'm using a flowmeter. I'll check out the possibility of blocked jets and re-evaluate how the air flow balance screws are set up and get back with my findings.

 

I blew out all of the idle jets with compressed air and noticed some improvement, but it's still running crappy. My #4 is no longer wet though, its a nice blackish-brown. At all speeds I seem to get a hesitation/miss, but mostly when the engine is under load (i.e. accelleration).
I'm out of ideas.
Anyone?

 

You mean where the idle/mixure screw goes? I did it for the rear bank, but not the foward bank. I'll do that tonight and see if there's a difference.

 

OK, I blew out all 8 circuits, some improvement, but still hesitation at all speeds when acellerating. When stepping down on the gas, there is a long quiet pause before the engine sputters and then revs. It seems to rev OK when the car is at a stop though. I pulled the plugs and the rears are all a nice toasty brown, while the foward bank has three white and one bank. I am going to try and re-synch and then re-adjust the mixture on all carbs. If that doesn't pan out, I think I've exhausted all options and I'm going to have to pull the foward cam cover to check the valve timing.
The funny thing is that I saw the same thing with the plugs on another 308 that skipped teeth on the belt. If that's the case with this one, I don't think its more than one tooth.

 

Make that three white and one black

 

It seems that the idle jets were the problem afterall. I just re-checked the ignition timing and found the 5-8 bank timed to the 1-4 bank... DOH! After cleaning the idle jets, there was a significant difference, but it just didn't run right because of that silly little mistake. So much for double checking and triple checking if your not going to read what exactly the marks say. I sure am glad I didn't start taking cam covers off.
Thanks to all for your helpful suggestions.
Vince

 

The hesitation above 2500 was from the idle jets. After cleaning them and removing the distributor to cure an o-ring leak, I inadvertantly set it to the wrong timing marks.
Thanks for the help.