1930 Pickup Backfiring/misfiring out carburetor

[sven_andretti]

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.

[ArtimusGordon]

If possible I recommend that you perform a VACUUM test! From the vacuum diagrams perform ALL the recommended throttle settings! You might see a vacuum reading indicating the problem.

[jayvee34]

You mentioned cleaning out the carb with carb cleaner and blowing out all the jets and passages with air. It ran somewhat better with the GAV opened up to 1.25 to 1.5 turns.
Sounds to me like you need to do 2 things. 1. Check the opening of the four jets for proper
sizing. 2. Use a straight paper clip and run through all the passage ways. I had a similar
problem with my A and followed these instruction by Tom Endy, and my car runs now with
the GAV opened About a 1/8 of a turn or just cracked. No popping or back firing, runs like a scolded dog. Best wishes.

[sven_andretti]

Thanks ArtimusGordon & jayvee34. I appreciate the thoughts and will try both suggestions. One question though. This pickup does not have a vacuum line tap on the intake manifold. How would you recommend performing a vacuum test in that situation? Thanks again for the help!

[700rpm]

I don’t see in any of your 39 steps mention of timing. Have you checked and confirmed that it is correct? Is the spark rod pulled down to 9:00 at driving speeds? Do you perhaps have a Model B timing gear cover?

Also, after step #4, are you certain the gas line was not reinserted too far into the sediment bulb? Or that the ferrule was not too far back from the end of the gas line, which can also cause the line to go in too far? That can restrict gas flow, which will cause problems as you increase speed and demand for fuel.

Step #33 also concerns me, specifically the filter and the plumber’s line. I don’t trust inline filters, and for proper fit and materials it’s better to buy a new line from a good supplier.

In spite of my timing comment, the more I think about your trouble the more I think it is a fuel delivery problem. I’d still check both, though.

[sven_andretti]

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Thanks 700rpm. I forgot to add that to the list. But yes, the timing has been check with both the original distributor and the known one from another car. But it is always worth checking again. I do not know if it has a Model A or Model B timing gear cover. What should I look for? Thanks again!

[sven_andretti]

Also, the timing was set so its between the 9:00 and 10:00 position at driving speeds. The points open about 1 click down. Thanks again!

[Ernie Vitucci]

Good Morning...sometimes the plugs in the headlights and the lights get to vibrating at high Model A Speed...you might consider disconnecting the lights and seeing if it makes a difference. I.E. a headlight short feeding back through the systemj...also, separately disconnect the tail light switch and see if you have any improvement...same problem, a short feeding back through the system. I am impressed at the work you have accomplished and how you have kept track of it...A third possibility would be the little mount many of us have on the side of the starter with the fuse in it...those mounts can vibrate so much that the swedges become loose and cause a vibration short as well...Good Luck...Ernie in Arizona

[sven_andretti]

Thanks for the reply Ernie. I can't take all the credit for everything I've tried, I've been working with my Model A buddies and research this forum. A lot that I tried did come from Ford Barn.

I will give your suggestions a try. Your suggestions got me thinking too. This pickup has an aftermarket ignition switch that sticks out an inch or so from the instrument panel and some had covered where the wires attach with electrical tape. I've run it with the instrument panel hanging loose on the speedo cable and still had the same symptoms. But that doesn't mean it may be time to lift the leads and hot wire it and see what happens. Thanks again!

[700rpm]

Originally Posted by sven_andretti

Thanks 700rpm. I forgot to add that to the list. But yes, the timing has been check with both the original distributor and the known one from another car. But it is always worth checking again. I do not know if it has a Model A or Model B timing gear cover. What should I look for? Thanks again!

The timing pin on the B cover is in a different position... further to the passenger side, if I remember correctly. Someone here can likely describe it more accurately. It’s an easy visual check if you can compare it to another Model A which is likely to have an A cover.

[sven_andretti]

Thanks 700rpm. I'll give that a check. I had not realized there was difference. Good to know. Thanks again!

[nkaminar]

Sounds like you have tried just about everything. I would go with 700RPM's suggestions. I would disconnect the wire from the alternator to the light switch to eliminate any problems in the headlights, tail lights, etc. Also check the fuse. I use an electrical grease there. You can temporarily wire up the coil directly to the battery to eliminate any issues with the ammeter and ignition switch or fuse. For the timing, take #1 plug out and turn the engine over with the hand crank until the distributor rotor is pointing to that plug, or a little before that. Then use a stick or wire to feel when the piston is at top dead center. Adjust the points to open when the spark lever is retarded at that position. That eliminates any issues with the cover bring Model B or not. At the same time look at the valves. They should both be cracked open just a touch. That will check the cam timing which could be off.

[Rob Doe]

I've read that a leak where the muffler meets the manifold can put enough exhaust out to foul the carb intake at higher speed???

On my car, I jumpered across the terminal box with a short wire to eliminate the ammeter.

The ferrule on the heavy spring shielded ignition cable can also ground out internally. I used one of the jumpers from Bratton's that you clip to the + side of the coil, that's the ignition switch side, and then screw it's ferrule into the distributor. Remove the red wire that goes to the ignition switch first.

The above two items will eliminate everything above the terminal box.

Tip on new fuel line install:

Remove the carburetor inlet filter and replace with a 1/2 x 20?? NF bolt 3/4" long. I think a 1" long will work too, but my oil filter got in the way. Make sure I stated the threads correctly on the 1/2" bolt so you don't strip any more threads. It should go in with your fingers, easily. Push the new fuel line all the way to the bolt and tighten enough to lock the ferrule. Remove the bolt and reinstall the inlet screen. The clearance between the screen and end of fuel line will be good.

[sven_andretti]

Thanks nkaminar and Rob Doe. I'll give them a try. I like the tip on seating the fuel line.

Thanks again!!

[Ernie Vitucci]

Good afternoon...It might be time to just change that after market switch to a new high quality after market switch from Bratton's or one of the other suppliers...I've seen after market switches that you could just wiggle the key and get a pretty good 'BANG'. Ernie in Arizona

[jayvee34]

All this information you are getting to solve your problem is one that could solve your problem. Try them one at a time then you know what the problem was. if you still have the popping, missing, and have to turn your GAV open to 1.25 to 1.50 go to post # 3. I know
that it seems kind of a intense job, but is not. I just feel that your problem is gas related,
and that it can be resolved. I clicked on Tom Endy's write ups on the Zenith Carb., and it
sure cured my problem which is the same as you are having. Do yourself a favor and check on it. Best wishes

[nkaminar]

Sven pointed out an error in my last post. To check the valve timing the engine has to be turned over 360 degrees from the top of the power stroke so that the piston is at the top between the exhaust stroke and the intake stroke. At that point both valves should be just cracked open. Probably an age related error.

[daren007]

Your problem is fuel not ignition.

[ArtimusGordon]

If you have vacuum wipers you could use the connection for attaching your vacuum gauge.

[ArtimusGordon]

Originally Posted by sven_andretti

Thanks ArtimusGordon & jayvee34. I appreciate the thoughts and will try both suggestions. One question though. This pickup does not have a vacuum line tap on the intake manifold. How would you recommend performing a vacuum test in that situation? Thanks again for the help!

If you have vacuum wipers you can use that connection for your vacuum gauge.