I'm looking for any advice that anyone is willing to offer. A little back round:


1979 F-250
400 Small Block (less than 2000 miles)
C6 Trans
Holley 600 cfm (vac secondary)




I had and electrical problem for a while with the ignition and finally solved the problem the other day. It was just a lose wire in the cab. I went out for a test drive to check everything out. Idled great as I left and for the first few miles. BUT, I noticed that it was having trouble idling at stop signs after a while. Seemed like it was starving for fuel (didn't smell rich). It ran fine once on the road. I've never had problems from the carb before. I DID have it off while trying to find my electrical problems, but it idled fine after I fix the lose wire. I've had it off many times before and never had a problem. I've noticed when it is cold (choke closed) it acts normal and appears to be fine. Once the choke opens, soon after it starts to have a up and down idle. Maybe 100 RPM's in change constantly.


Things I've tried:
1. Checked for vacuum leaks. Plugged ALL Vac ports to see if change. (NO CHANGE)
2. Took carb off, cleaned primary jets, needle and seat, and bowl. Check baseplate for problems. (NO CHANGE)
3. Checked Fuel pressure (7.5 psi) May be a little high, but this is what it's always been and ran well.
4. Changed secondary vacuum valve. Heard these can go bad and cause problems. (NO CHANGE)




I'm going crazy. This should be simple. It just feels like a vacuum leak, but how could this be? The carb was not dropped and how do I check to see if there is a leak in my baseplate? I've already sprayed carb cleaner as it run at the baseplate to see if the idle changed and it didn't. I've considered an engine problem since it is fairly new, but it runs perfectly other than at idle.


Any help would be appreciated.


Thanks Blake

 

If you're confident there are no vacuum leaks, have you tried just adjusting the idle mixture screws out a bit further?

 

Have you checked the ignition module? Mine was having similar issues before I replaced the module.

 

could the timing be out? you said less than 2000 miles on the motor, was it rebuilt? your still in the brake in time period at 2000 miles, maybe back your timing off and it should idle better. do you hear a 'ping-ing' noise from the motor when your driving and really shove it down all the way?

 

Start the truck then spray starting fluid around the vacuum lines and base of the Carb, it will bump the RPM's up by 300 or so for a second..

 

I have not checked the ignition module yet, but I keep a backup so can try that tomorrow. It does ping a little, I will check the timing also. Already checked for vacuum leaks at base plate and checked mixture screws. Thanks for your help.