Miss and Low Vacuum

[Jim-n-Texas]

My '76 starts easily and runs - but with a miss at all speeds. I checked the vacuum and I only have about 10" of steady vacuum. I intend to pull the cam cover to look for problems - any hints, suggestions, condolences, etc.?

[Zed Head]

How long have you had it and what is it's history? Vacuum leaks are the first thing that comes to mind. It's not uncommon for the various hoses that run throughout the engine bay and even in to the cabin to be the original 36 year old hoses. The miss could be due to a leak leaning out the mixture.

[Jim-n-Texas]

I don't know much history on it - it sat for a good while - ran good and then current state. Checked for vacuum leaks, found a couple of small ones - can't find major one. When vacuum hose is disconnected from intake manifold engine speeds up. ?

[Zed Head]

ran good and then current state.

Based on data supplied I would say it needs a tuneup. Plugs, distributor cap, rotor, and confirm timing. If problem persists, take further action.

Seriously, there are many things that could be causing your problem. Ran good, now has 10" of vacuum and a miss could be running rich, running lean, timing is off, valve lash needs adjustment, vacuum leak, fuel pressure low, fuel pressure high, bad plug wires, bad spark plugs, timing is off, etc.

With EFI, in the long run, you're almost always best off to go to the Engine Fuel section of the FSM and start testing and confirming that things are right, if you know that you have good basics, like tuneup parts. Fuel pressure is probably the most critical spec. and is the foundation for supplying the proper amount of fuel to the engine.

Fuel pressure needs to be right, and no vacuum leaks. All air must pass through the AFM.

Edited September 25, 2012 by Zed Head