1930 Pickup Backfiring/misfiring out carburetor

[eagle]

One thing to check is lift the spark plug straps about an 1/8 inch away from the dist posts and in the evening run the car. You can see if the spark gets erratic. The higher the RPM the more difficult but you'd be surprised how you can see an even somewhat erratic spark. It does sound like a fuel issue, but it can be very difficult to separate the two.

[sven_andretti]

Thanks everyone for all the suggestions. Tonight I’m going to compile a list and run through all the suggestions tomorrow. I agree it sounds like fuel and I’ll rework the carburetor passages tomorrow. I did have time to do a fuel flow test today into a pan about the size of a shoe box and did not see any decrease in flow over time. I also disconnected the headlights and horn from the generator with no improvement in the popping. Though running the GAV at 2 turns open substantially reduced the popping. More to follow tomorrow. Thanks again!!

[nkaminar]

This from http://model-a.org/symptoms.html. Although you have tried different carburetors so this may not solve anything. All the arrows point to a fuel issue. Running with the GAV open 2 turns would indicate that the main jet is too small. The site above lists the drill sizes to drill out the jets, using a pin vice, not a drill motor.

[johnbuckley]

Sounds like you've checked practically everything... Has the car ever run OK do you know? A few long shots....Were the carbs you substituted known to be good ? (if not maybe just check the fuel flow coming out of the float valve in case there's a block at strainer or float valve blockage ) Swap lower bodies on the carbs? Try hot wire like diagram below to eliminate any extraneous elec problem ? Check cam shaft wheel teeth are not 1 or 2 teeth out ?

[JOHN CT]

All of the things you have tried are all good! But I didn’t see anywhere where you tried different make spark plugs Like a motorcraft. My model a doesn’t like champion plugs. It won’t idle good and back fires I changed to a different plug now it runs great!!

[sven_andretti]

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Good thought on the main jet. I have the test rig to flow test jets. Tomorrow I’m going to run a stiff wire through the carburetor passages and can test the jet sizes. I’ve tried removing the headlight/horn harness and bypassing the ignition switch with no luck. Also, that thing about Champion plugs is interesting. I’ve always had best luck with them, but it worth a shot.

But I have also run into an interesting thing on timing. I had been testing with a known good distributor and decided to put the owners new manufacture original style distributor back in today. I set #1 cylinder to TDC using the pin and verified by pulling #1 plug. For years I’ve timed a Model A with a light as described in Les’ Model A Mechanic’s Handbook (using a light to show when the points open). But recently I have been using the technique in the Victor Page’ book. This is where the trailing edge of the rotor tab lines up with the back edge of the #1 cylinder contact in the distributor body/horn. My experience has been that either method delivers the same result - specifically; the points open when the spark arm on the column is moved down one to two “clicks” or notches on the sector. But I noticed today when I used the rotor method, the points were open at that point. Conversely, when I set the timing just using the light and then put the rotor on the cam the trailing edge of the rotor tab was still an 1/8 inch away from the back edge of the #1 cylinder contact in the distributor body/horn. Like I said, my experience has been either method yields the same result. But not on this truck. Any thoughts?

Thanks!

[Rob Doe]

Are you still using a known good carburetor?

The ferrule at either end of the fuel line should be no more than 1/8" from the end of the line. (I believe 700rpm mentioned this above.) Everything that I've read here (that you have done so far) would not have resolved this problem. The replacement carbs would have restricted flow just as your own.

The new line that you installed item #39, did you install it against the 1/2" bolt? You could make a temporary line with 1/4" copper tube from the hardware store, but it's not actually good enough for the constant vibration of driving. (So I've read. It is a fire hazard but could be used for a static test.)

I once cleaned my fuel system back from carb to tank and tested the flow at the carb inlet. It filled a 20 ounce bottle in 34 seconds. It was a full stream for sure.

[sven_andretti]

Unfortunately, this does not have vacuum wipers.

I’ve disconnected the ignition switch and it was still popping. Today I checked the ferrule at the carburetor end of the fuel line with a 1/2 inch bolt in the front fuel screen hole. The 1/2 inch bolt was visible when looking into the fuel line connection. It mated up fine, but there was still about 3/16 inch of fuel line extending beyond the ferrule. So I ground it down to less than 1/8 inch and dressed the end with a rat tail file. No change. Still popping at high rpm.

Another friend found a post with a Model A with the same symptoms. That solution was replacing the distributor intermediate shaft. I put on a new intermediate shaft to no avail.

Tomorrow I really going to tear down the carburetor and run a stiff wire through the passages and flow test the main and cap jets.

Thanks again for all the help!

[nkaminar]

Sven, Also check to see if the main jet exit is at the proper level in the venturi. It should be at the narrowest part. The thickness and number of washers under the main jet will set the height.


See http://model-a.org/main_jet.html

[Rob Doe]

Originally Posted by nkaminar

Sven, Also check to see if the main jet exit is at the proper level in the venturi. It should be at the narrowest part. The thickness and number of washers under the main jet will set the height.


See http://model-a.org/main_jet.html

Sven, I've used a short piece of welding rod for that level test. I cut it close and ground it 'til it fit across the shelf in the lower carb body at its maximum diameter. (The shelf where the venturi will sit when assembled) It allowed me to see the heights of the main and Cap jets better. I'm a rookie at this. Hopefully others will chime in with a better method.


Again, I thought we had tried multiple known good carbs. That would be carbs that were running recently on other engines.

[Rob Doe]

I used a paper clip to push through the passages as per Tom Endy as I recall.

I put slight bends on the probe end and a 90 to help me turn it with my hand. It took patience, but I managed to work that clip all the way to the main jet, the Cap jet from the bowl, the GAV jet, (ground into the casting on my carb, no brass jet), and my carb is a Zenith 2 with a 90 degree passage off the GAV that goes direct to the base of the CAP jet.It even went around that turn.

Never give up. Success will be that much sweeter and you and the rest of us will be that much the wiser.

[katy]

Originally Posted by sven_andretti

when I set the timing just using the light and then put the rotor on the cam the trailing edge of the rotor tab was still an 1/8 inch away from the back edge of the #1 cylinder contact in the distributor body/horn.

Have you compared that rotor and distributor body w/others?

[sven_andretti]

I wanted to take the time to thank everyone for your time and suggestions. They were a great help. As we all know, Model A's can be very humbling and this particular instant was a good lesson in "what you think you know." It was a fuel issue. The main jet was only flowing 62% of the minimum flow for a 37.25 in water column. I went back and tested the main jet on my "known" carburetor and while it had run fine on one of my cars, it was only flowing 68% of the minimum. What really threw me was after re-sizing the main jet I got in one 2 mile test drive with no popping out the carburetor. But the next morning I was doing one final test revving the engine in the shop and blew the head gasket between the cylinders; resulting in the same symptom. Popping out the carburetor. So once I realized I'd blown the head gasket and got that replaced, all is good.

Thanks again for all the excellent suggestion. I don't post off, but research a bunch here on Ford Barn.

[Rob Doe]

Thanks for coming back with an answer. I too study here daily.