1970 340 won’t idle

[Cuda1970]

I am having issues keeping the motor running long enough to check timing to know what it is set at. It seems like a vacuum problem but nothing visible. I can get the motor to start and the throttle is very responsive, but once I let off the pedal it dies immediately. I can get the motor up to temp by starting and continually pressing the throttle to idle at a much higher RPM but even at temp it dies, but does last slightly longer. I changed the jets in the carb to the recommended size for 7000' elevation, but Not much change in operation. I have researched a bit but have not found anything other than timing and vacuum which I can't keep it running long enough to diagnose either. The Distributor is a MSD 8534 with a 6 series ignition control. I am not familiar with the springs/weights on the distributor for timing, so this could be the issue.

Any and all help is greatly appreciated.

[FE5CUDA]

Turn the idle screw up until it runs on its own, back out the screw and see if you get to your idle RPM.  If that works, the chances of not a vacuum leak are good.

[Chryco Psycho]

I agree , you are not getting enough fuel supplied at idle so it could be a restriction in the idle circuit , you don't say what carb you are using but you may have to remove the idle screws & put compressed air / carb cleaner or even fine wire through the opening to remove blockage or it could just need the screws opened up enough to flow extra fuel at idle while you get it tuned

[Cuda1970]

I finally got a decent afternoon to open up the garage a try and start it up. I adjusted the idle screw and can get it to idle above 2000 rpm but it surges. I start to back off on the idle screw and can get it below 2000 rpm but the surging gets worse and it will die. The current carb is an AED 650HO. It does feel like a restriction in the idle circuit, but would that cause surging? I have 6.5psi on the fuel line.

[torredcuda]

Changing jets does not affect idle mixture. I would suggest you first buy a Holley book on how to tune, that will tell you what circuits affect which rpm range, diagnosis etc. so you better understand it. Is there a choke on it, auto or manual, make sure it is working properly? If choke is working next thing I would do is check float level then adjust idle screws with a vacuum gage or if you don`t have one adjust for highest/smoothest idle.

[Shane Kelley]

Sounds a lot like a vacuum leak. IMO    Get it running on it's own and use a spry bottle of water to spray around the carb base and intake to head area. See if the idles changes while doing that. Try plugging off PCV valve and power brake hose if you have them as well. 

[RUNCHARGER]

If it's that bad I would be tempted to pull the carb, tear it down and blow out all passages with air, carefully inspect everything including the float control and put it back together carefully setting everything. Inspect all gaskets and hoses or plug off all the hoses to get it to start. Has the intake been removed before this happened?

[jimynick]

I like the K.I.S.S. principle. Why not borrow a carb, swap it out and see what happens? That'll kill several diagnostic birds with one rock and may make your life a wee bit easier?