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Old 06-21-2021, 08:52 AM   #1
sven_andretti
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Join Date: Dec 2012
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Default 1930 Pickup Backfiring/misfiring out carburetor

I've followed the forum for years, but this is my first post. I really hope someone sees what I've missed.

I’ve been working on a friend’s 1930 pickup for a while now and I’m hoping you can point me in the right direction. It started with the pickup running rough and backfiring through the carburetor (misfiring) at high RPM. I found the distributor cap had worn down graphite nipple. We replaced that with a new distributor cap, a new manufacture original style distributor (point gap set to 0.022 inches), and new Champion 3X plugs (had had Autolite plugs). I checked the plug gap and must admit I may have had a couple of plugs gapped more than 0.035 inches. The car ran well after that on a 50 mile tour and parade, but it could not maintain speed on hills at the end of the tour when back to intown speeds it ran rough. I suspected a stuck float valve causing it to run rich fouling the plugs. A compression check revealed low compression on #2 cylinder. Compression readings were (#1: 55, #2: 50, #3: 60, #4: 62). The plugs had carbon build up after the tour, but they were not oily, so I recommended doing a valve job. After the valve job I set all the valve clearances to 0.013 inches on the intakes and 0.015 inches on the exhaust. I set the clearances per the Model A Mechanics Handbook, Vol I, using the “rule of 9s” and ensuring all the cylinders were at the same height when measuring and adjusting. Post valve job compression (cold) was #1: 64, #2: 55, #3: 60, #4: 60.

Now the fun begins. With everything re-installed, the truck idles like a dream. However, at high RPM (3/4 to max throttle on the column) it consistently backfires/misfires out the carburetor (generally low popping, but occasionally a bang shooting a little flame out). Here is what has been done since to try and address the issue:

The engine was run after each of these things were done and it still backfires/misfires through the carburetor.
1. Tested the ignition circuit per the Model A Troubleshooting manual. Strong, consistent spark from ignition cable to stud nut.
2. Tested the coil Model A Troubleshooting Manual, all resistances within specifications.
3. Verified point gap at 0.022 inches. No change, still popping at high RPM.
4. Checked fuel flow at the carburetor, solid stream extending 2 to 3 inches beyond the end of the fuel line.
5. Opened the GAV to ¾ turns open, slight improvement but still popping out the carburetor.
6. Removed the gas cap. No change, still popping at high RPM.
7. Check point gap again. No change, still popping at high RPM.
8. Found that the rotor tab was impacting #3 contact in the distributor body (horn). Adjust all gaps to 0.025 inches. No change, still popping at high RPM.
9. Added Marvels Mystery Oil to the gas tank in case we had a sticking valve. No change, still popping at high RPM.
10. Replaced distributor with a known good distributor off another Model A and set point gap to 0.022 inches. No change, still popping at high RPM.
11. Installed a new condenser on the known good distributor. No change, still popping at high RPM.
12. Sprayed the where the intake manifold mates with the block with WD40, no change in RPM.
13. Put a machinist straight edge on the intake/exhaust manifold, manifolds are dead plumb.
14. Changed intake manifolds. No change, still popping at high RPM.
15. Ran cold and hot compression test. Cold: #1: 55, #2: 54, #3: 53, #4: 57. Hot: #1: 60, #2: 60, #3: 59, #4: 60.
16. Had a Model A friend and retired mechanic check valve clearances using the companion cylinder method (head installed) and adjusted two valves about 0.002 inches. No change, still popping at high RPM.
17. Installed new coil. No change, still popping at high RPM.
18. Tightened spark plug porcelains, none were loose. No change, still popping at high RPM.
19. Added 6 gallons of fresh gasoline to the 3 or 4 gallons in the tank. No change, still popping at high RPM.
20. Reinstalled original intake manifold and tested for air in-leakage with carburetor cleaner. No change in RPM.
21. Installed different carburetor after verifying float level, still popping.
22. Installed 3rd carburetor. No change, still popping at high RPM.
23. Reset float level on original carburetor, blew out all jets and passages with carburetor cleaning and reinstalled. No change, still popping at high RPM.
24. Drained and cleaned cast iron sediment bowl and screen. No change, still popping at high RPM.
25. Blew air back into gas tank and hear bubbling. No change, still popping at high RPM.
26. Replaced the sediment bowl with new glass cup style sediment bowl with a fuel filter in the glass bowl. No change, still popping at high RPM.
27. Reset all spark plug gaps to 0.032 inches. I realize the specification for stock compression, such as this engine is 0.035 inches, but I wanted to close it up some since a larger gap has lead to misfires in some applications. No change, still popping at high RPM.
28. Found odd wire attached to the coil clamp. It was ground wire for the after market windshield wiper. Power for the wiper came off the passenger side terminal box post, removed both. No change, still popping at high RPM.
29. Check for arching between spark plug coppers and spark rod. None observed.
30. Installed used Champion 3X plugs from a known car. No change, still popping at high RPM.
31. Magna-fluxed intake manifold and found small crack where a vacuum line tap had been welded up. Installed a used intake manifold that was plumb with the exhaust using a machinist straight edge. No change, still popping at high RPM.
32. Tested new intake manifold and carburetor mating surfaces and carburetor throttle shaft with carburetor cleaner. No change in RPM.
33. Tested fuel flow at the carburetor again, this time it was not as strong as previous. Extended only about an inch beyond the of the fuel line and was not quite a solid stream, though it was not a dribble. Suspected no inlet screen in the fuel shutoff valve. Removed the fuel shutoff valve and found a very nice (nicer than I can buy today), clean inlet filter. When reinstalling the shutoff valve stripped the threads in the shutoff valve outlet and the fuel line flare nut. The shutoff valve outlet and the sediment bowl inlet are not on the same level. Replaced the shutoff valve with a rebuilt used shutoff valve and had a plumber friend bend a new shutoff valve to sediment bowl fuel line. No change, still popping at high RPM.
34. Installed a distributor body (horn), rotor and distributor cap from a known car. No change, still popping at high RPM.
35. Verified the terminal box was wired correctly, but found the passenger side post was loose. Tightened the terminal box post lock nut and verified all connections were tight. No change, still popping at high RPM.
36. The pickup has an original generator with cutout. Rather than changing the cutout, replaced the generator with an alternator from a known car. No change, still popping at high RPM.
37. Removed gas cap and opened GAV 1.25 to 1.5 turns. Noticed some decrease in the frequency of the popping and the loudness, but still popping at high RPM.
38. Installed Ford authorized aftermarket ammeter. No change, still popping at high RPM.
39. Changed fuel line from sediment bowl to carburetor. No change, still popping at high RPM.

Through out the pick up as idled like a dream. The only issue is at high RPM.
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