Backfiring intermittent

T

TheFargoA

Junior Member
Hello Model A owners,
I rebuilt my 1929 A pickup engine, distributor, and carburetor. It runs great; however, I have intermittent horrible running conditions. It will run well, then suddenly lose all power, backfire, and pop out of the carburetor inlet. It then floods and blows black smoke. Intermittently, however, it will come back to life and run great again for another mile or so. Distributor maintains perfect timing; points look great; fuel-air mixture is generally at 1/4 to 1/2 turn CCW; and plugs run tan to gray (except during the intermittent condition when they become fuel soaked); and all test show strong spark from coil to plugs. I changed intake/exhaust gasket, carb gaskets, and vacuum checks come out good. Despite me changing the float and float pin - and all other parts of carb - could a poor float valve cause this? Thank you for your help. A
 
It sounds like you may have multiple issues, possibly a carb/needle seat issue, dirt in carb from the tank, and possibly some electrical issues as well
You need to check your primary ignition circuit to make sure connections and wiring are good from the ignition switch, terminal box, ammeter, and lead or Popout to the distributor.
Check for any voltage drop from the battery to the points and that will help tell you if you have any drop.
Same voltage on both sides if the terminal box?

I would say take the carb apart and look at the needle and seat. If it won't hold vacuum they will leak. Just that simple .
Be attentive to the details and do logical path troubleshooting on the electrical
Make sure all carb and intake connections are tight . Look for gasket issues and any cracks at the mounting flanges if manifold and carb .

If you have set the point gap properly and then timed as per the service bulletins it should be spot on if done properly.
Larry s
 
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Then there's Cooling, Compression...

I regret that I said anything at all because nobody can really diagnose a car that's not in front of them and you didn't give enough data for the dots to get connected.

Does this happen on startup? Does it happen only after X minutes of run-time? Does it run fine in the Driveway? All of this is important to know.
 
larrys40 said:
It sounds like you may have multiple issues, possibly a carb/needle seat issue, dirt in carb from the tank, and possibly some electrical issues as well
You need to check your primary ignition circuit to make sure connections and wiring are good from the ignition switch, terminal box, ammeter, and lead or Popout to the distributor.
Check for any voltage drop from the battery to the points and that will help tell you if you have any drop.
Same voltage on both sides if the terminal box?

I would say take the carb apart and look at the needle and seat. If it won't hold vacuum they will leak. Just that simple .
Be attentive to the details and do logical path troubleshooting inn the electrical
Larry s
Click to expand...

Loose wires are the bugabear. Almost impossible to find, but more likely when the car is octogenarian.
 
I installed a modern distributor plate with modern points and condenser (the short condenser that sits atop the plate). I do not have a popout; rather, I have an 8-gauge jumper wire connecting the coil to the inside lower plate directly. I wonder if this is vibrating and sometimes rubbing against the distributor. I agree with your guidance. I do not want to "throw parts at it", but I need to admit that this is my first rebuild, and I am learning how much I do not know - especially when it comes to diagnostics and details. A small library of books has been of great help so far. Your real-life experiences are a great help too. I'll be sure to let you know how this goes. Thank you much! A
 
Step 1: re-check your work.

As Murphy said, 'a dropped tool will land where it does the most harm'

Was the car running when you began the project?
 
The vehicle was running, but the engine was trash. A reputable machine shop did the machine work and I built the engine myself. It ran amazing for about 500 miles. I suddenly began having this issue. I first replaced the head gasket (not needed) and checked valves for sticking (friend cranked engine while I watched valves (no issues). I then rebuilt the carburetor and distributor. I should have mentioned that it temporarily runs smooth when I spray carb-cleaner in the carb. This may indicate that timing and spark is good. Today I replaced the float valve, but I blew head gasket with backfiring. Friend towed me home, and I'll redo the head gasket; then I can resume detail work. A
 
If it runs on Carb Cleaner (which means RICH) it's Timing or Mixture. Since you have visited the Carb, that's where I'd be looking for a mistake.
 
quite a scenario...... As I suggested in my original post.... In my opinion, the modern point plate and condenser is certainly NO improvement to the runability of the car. In fact the points are harder to adjust, and tend to be more trouble prone than a distributor that has the stock upper/lower plate WITH a pigtail ( no wireless plate - more junk) and a A & L burnout proof condenser.

since you did your own work you know what to do.... I suggest going back to a stock distributor, checking your carb, and fuel flow... and doing whatever service you need to do on the engine and see where you are at when completed with those tasks.

Good luck!
Larry Shepard
 
On a Carb you only have four choices:

There's 'right' (which doesn't really happen ever)

There's 'close enough' (which is often the case)

Then there's Rich or Lean -- BOTH of which can cause your problem. Rich is easier to find.

Again, I'm going to shoot from the hip and say this is Operator-error. The Fuel Valve under the Passenger side of the Dash does more than one thing. It is a Choke but it also changes the mixture.

Before you do anything. you need to see if tweaking that changes your results.

The car needs to be rich when you start it. Lean it out from there.
 
Do you have float-a-motor mounts by chance? If you do...make sure you have a good ground on the engine to the frame. I had a similar issue that after a long time trouble shooting was simply poor grounding. It was a royal head ache. Good luck. KansasRick
 
My bet is the problem lies in your informal ignition path. Get rid of the jumper wire and put a proper ignition cable and switch in. Also lose that modern plate setup. Larry is right: they are trouble asking for more trouble. Go back to a stock distributor.
 
Thank you again for your help. Here is some info: (A) rich-lean knob works well and is very sensitive; (B) I richen at start, then lean out. I generally drive with 1/4 turn CCW and then advance spark lever 1/2 to 3/4 down generally unless in town or highway where I adjust per "feel" of engine. Both work well and are sensitive to use. (C) I tested coil, tension wire, and ignition with multi-meter and spark tester. All came out good. I did am suspect of the rigged "popout" jumper wire - I will replace with correct popout cable. (D) I changed to modern distributor plate because I had trouble keeping perfect timing (point-gap at TDC) with original (and replaced original style) distributor. The new modern plate maintains perfect conditions, although I will consider going back to original when I figure our fun problem out. (E) The vehicle will sometimes run perfect at 180 degrees. It then runs terrible in the morning at start-up. Once it warms up, it MAY run well (sometimes with help of carb spray), but again it's totally intermittent as I drive around town. (F) I read on another post that a valve may be sticking intermittently. That sounds strange, but could be a possibility. I may follow the post's recommendations with kerosene or MMO. Thank you again. It may be a day or so before I can check this. A
 
The ign points available today (the ones you're using) are iffy. Try getting a set of Echline's or go to the EV8 forum and look for a post by BUBBA and contact hime. He most likely will have a good set of points for you, also a condenser. FWIW
Paul in CT
 
Sounds more electrical from your description... If you can get your hands on a clear distributor cap... Take her for a ride and when she backfires, pull over and take a look...
Wondering if you coil is has issues when it heats up??? A clear top will show a crap condenser not doing it's job and producing arcing....

https://www.youtube.com/watch?v=DX_P-pLC04w
 
TheFargoA said:
I installed a modern distributor plate with modern points and condenser (the short condenser that sits atop the plate). I do not have a popout; rather, I have an 8-gauge jumper wire connecting the coil to the inside lower plate directly. I wonder if this is vibrating and sometimes rubbing against the distributor. I agree with your guidance. I do not want to "throw parts at it", but I need to admit that this is my first rebuild, and I am learning how much I do not know - especially when it comes to diagnostics and details. A small library of books has been of great help so far. Your real-life experiences are a great help too. I'll be sure to let you know how this goes. Thank you much! A
Click to expand...

How is the wire connected to the lower plate?
Is this just a temporary testing 8 gauge wire?
8 gauge is way too stiff and overkill. 14 gauge is fine for the primary coil circuit.

I also wonder if the stiff wire is touching the distributor?:confused:
 
Thank you all for your replies. I followed your guidance and diagnosed all primary and secondary circuits. They are perfect. I rebuilt the carburetor and found a loose idler jet (no good) and the float was 1/2" too low. Now it's 5/8 from the rim-perfect. Still having issues. I carefully sprayed carb cleaner around the intake and exhaust manifold and found the problem. Toward the rear of the engine there is an invisible crack and warping of the exhaust manifold that sucks in the fluid, affecting the engine. This explains why my pressure gauge shakes between 5 and 15 pounds while the engine runs (i.e., vacuum leak). I ordered a mew exhaust and intake manifold(USA made!) and will let you know if it fixes the issue. If nothing else, I learned a few things in diagnosing the problems. Education never hurts anyone. I'll keep in touch. A
 
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