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1980 E350 460 Lacks Power and Backfires Through Carb

2K views 2 replies 3 participants last post by  The Mad Porter 
#1 ·
A few months ago I bought a 1980 E350 4x4 ambulance with a 460 and Holley 4180. I drove it about 2 hours home and it ran beautifully. I drove it around the yard and distances of a couple miles or less a few times and noticed it was idling eye wateringly rich. Knowing it had been sitting for a while before I bought it I ordered a rebuild kit and tore the carb apart. Since putting it back together I can get it to idle perfectly and it revs with no load but under load it sputters and backfires through the carb. Here's what I've determined so far.

  • Plugs, wires, and distributor cap are all new. All cylinders have strong spark and good compression.
  • Base time is 8* BTDC
  • With vacuum advance unhooked I can adjust the carb to idle smoothly at 600 RPM with 8* base advance.
  • With vacuum advance plugged into manifold vacuum I can adjust to it idle smoothly at 600 RPM with 28-30* advance.
  • The symptoms are the same whether the vacuum advance is disconnected, plugged into ported vacuum, or manifold vacuum.
  • Mechanical advance increases by about 20* at 3000 RPM.
  • I found and fixed a couple vacuum leaks and now have 17 inches of manifold vacuum.
  • Fuel pressure is steady at 6-7 PSI both at idle and under load.

I'm guessing I screwed up on the carb rebuild. What else makes sense with these symptoms? Thanks in advance for any suggestions.
 
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#3 ·
Given that your symptons appeared after the carburetor overhaul I would look there.

After sorting out this issue properly curve your distributor.

14 degrees initial and 34 to 36 total (less vac advance) @ 3K rpm.



S
 
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