Good morning. What causes an engine to run good on the sweet spot for a time, then begin to backfire intermittently and go back to running good again for a short time and begin to backfire again. When it backfires, the pops are random, 2 to several seconds apart. It does this when idling with the spark fully advanced. Doesn't seem to matter where the throttle is set or whether the engine is cold or hot.
When not backfiring, the engine runs smooth. I just sent my coil off to Ron Patterson to check them to see if they are off. He rebuilt and set them 7 years ago in 2010.
Last week when I removed my plugs they were set at .025 and all were covered in a dry soot over a thin layer of hard carbon, so the inside of the head and valves are probably also coated with soot over a thin layer of hard carbon. I cleaned the plugs and set them to .030, but did not remove the head. I don't believe it backfired like this when set at .025. The mixture control was turned clockwise until it ran rough, then backed off about 1/4 turn until it ran smooth, but then began to backfire. No amount of adjusting the mixture changed this random backfiring. It would do this before the engine got hot, so I don't think it is pre-ignition of the carbon inside the cylinder. Jim Patrick
Hmmm. You changed the plug gap and now there's a problem. I would change it back to see if it fixes the problem. Zero cost fixes are the best.
Did you drive the car or just run it sitting in the driveway?
My carb adjustment liked to fall out when I ran my L-4. It was a constant
chase to keep it in the sweet spot and not choking on itself as you describe.
Too lean, too rich, hills, flat, ... now that I run an NH, problem is gone.
A possible is sticking intake valve. Maybe a broken valve spring ? Broken valve retainer pin? These are just some guesses
Umm switching off the car, retarding the spark and switching it back on while in a tunnel?
Along the same lines as the guesses by Les would be a bad lifter. I recently had to replace one that wore out with less than a thousand miles on it. Before replacing it, I had reached to point of running on only three cylinders.
I had a similar problem & it was the brass seal behind the timer... it had torn & would short the timer & cause intermittent backfiring. Once I tore the darn thing off, it fixed the problem.
Just a thought
He says it is an intermittent problem. Never heard of an intermittent lifter before LOL!
Realistically it pretty much has to be one of following in no particular order;
1. A intermittent timer problem
2. A mixture problem
3. A valve action problem, most likely intake
Clean the lint from under the mag post?
Upon rereading the original post I have had another thought;
By my definition a "back fire" is a pop back through the carburetor. It is of course possible to get pops and bangs in the muffler and exhaust which to me is a much different symptom with different causes!!!
Les, the backfire is coming through the muffler.
Jim, a back fire in the muffler means a spark problem. You can make a T backfire just as Chadwick posted. A non deliberate backfire can result from intermittent problems with the timer/loom connections, like Bill posted.
Hope this helps.
Allan from down under.
I had a problem just like this with a buddy's model A. We adjusted the carb, and it fixed it immediately.
If it pops in the exhaust, you have a problem with unburnt fuel going out and burning as an adjacent cylinder fires or if the errant cylinder fires. If it pops in the carburetor, it is usually too lean a mixture.
My first thoughts would be an intermitant spark in one cylinder. This could be caused by any loose connection or bad coil, timer, or magneto. Next thought would be a carburetor problem. Dirty fuel or water in gas. Third thought would be a stuck valve. Usually when a valve sticks it stays stuck and is not intermittant.
Pops in the exhaust are usually the result of ignition problems from my experience
Suggest looking at or replacing the timer. Also look for carbon traces in the coil box. Take the coil box cover off, wait till dark, start it. Watch the coils closely and see if only one coil is firing at a time. Think you will find one firing when it should not be and it will do it when you hear a back fire.
In a completely unrelated story, I can develop quite a serious backfire
after chowing down a #2 Burrito Combo at Rancho Chico. And as Barney
says: "Sharing is caring !"
I'd suggest checking the fuel system. My 24 Runabout sat for 30 years before I acquired it in September. I have rebuilt coils by the Coil Doctor, new wiring harness, recored the radiator, new plugs, carbon removed from head, valves lapped and clearance set, timing checked, new head and manifold gaskets. It was running like a top until Saturday. I took it out for a spin around the neighborhood and it started sputtering, let out a loud bang from the exhaust and quit. I knew it couldn't be ignition so I drained the carb and flushed out the sediment bulb. Lots of garbage came out of the bulb. I'd say a speck of rust made it through the screen, wasn't letting the float needle close and was flooding the carb. She started right back up and has been fine since. I guess it's time to open up the sediment bulb and clean or replace the screen. Probably a tank flush is in order too.
Here's an update:
1. I cleaned the plugs and set them at .030.
2. I sent the 4 spark coils to Ron Patterson and they checked out okay.
3. Drained tank and put in New gas and sediment filter in the tank.
4. Double-checked the NH carb to be sure float was set correctly at 15/64 and that float was good.
5. Replaced the New Day timer with a new TW Timer.
Started the car today and it ran better, but still pops and backfires intermittently, through the muffler, when the key is turned to MAG, but when turned to BATT, it does not pop or backfire. What should I check on the Magneto to determine why it backfires on MAG but not on BATT?
To test magneto output use the Regan - Patterson memorial test procedure:
Wire a #1156 bulb across from the mag post to ground. I made mine by soldering a foot of wire and two alligator clips to the bulb, then wrapping duct tape around the bulb base. Connect one wire from the bulb to ground, the other to the magneto post.
Start the engine and measure the voltage at the mag post. It ought to be 5 - 6 volts AC at idle and about 18 - 30 volts at higher engine speed, typically enough to burn out the bulb in a few minutes time. The cheaper the voltmeter the better, I use an analog unit from Radio shack that cost $9.95. I know Radio Shack is gone, similar meters are on Amazon and eBay in the $20 range brand new with free shipping.
I would be suspicious of the ignition switch (poor connection in the mag position)
The "Regan-Patterson memorial test procedure". Now that made me smile.
Here's the test i did on mine, It runs great on mag but i wasn't able to Kaput the new 1156 bulb.
Three things for sure I learned the hard way that caused a back firing rough running engine.
1. Air allowed in the exhaust pipe, due to a bad connection at the muffler or exhaust manifold.
2. Dirty fuel that gets in the carb and varies the mixture from lean to rich. A lean mixture will cause a backfire by itself.
3. Black paint on the coil box wood that was made black with carbon in the paint. Carbon conducts electricity. I had that problem for several years. My engine would run fine on Bat or Mag at low RPMs. When it was reved up on Mag the voltage increased and allowed the sparks to be shared, so whenever #2 was firing, a few sparks got over to #1 and #3, etc. These extra sparks caused a lot of back firing and popping noises, not to mention a very rough running engine with no power.
I did 4 valve jobs, but I could find no need for any of them.
I had the engine rebuilt and had the same performance. Then I realized the only parts still original were the carb, coils and coil box.
When those three items were rebuilt and the exhaust pipe and muffler were replaced, I finally had a good running engine. Doing the coils first made the problem worse, due to a hotter spark.
Thanks James. Seems like if the problem were any of the 3 problems you site, the backfiring would occur on both BATT and MAG. The backfiring only occurs on MAG. On BATT it does not Backfire. Jim Patrick
I think given what you have accomplished there are only a few things left that could be causing the problem. This is a lesson learned, good troubleshooting saves time and money.
Likely causes that you have not mentioned or tested are the magneto output voltage, coil box wiring, and the switch. Rather than replacing each one with a new pat spend a few minutes with a meter and a #1156 bulb. This can be time well spent.
Jim, my T didn't backfire as easy on Bat either.
Then too, it did not run half as good on Bat.
The real reason there is that the Bat is 6 volts and is converted to a square wave by the points and rounded into a kind of sine wave by the capacitor to pass through the coil.
A good Mag gives a much hotter spark, as it can be as much as 30 to 36 volts AC to start with and that voltage level is multiplied by the same primary to secondary coil ratio.
Jim, you said it only back fires on MAG. If you have a timer with a carbon brush, the magneto voltage is high enough that carbon dust may conduct firing a coil at the wrong time. 6 volts isn't enough potential for this to happen. We had a car doing this locally and it turned out that excess carbon dust was the culprit.
I see from your profile that your car is a 26. I have a thought for a definitive test of the switch;
Please read carefully;
Remove the wire from the mag connection on the hogshead. Also remove the wire running to the bottom of the coil box. Now run a wire directly from the mag connection to the coil box. Now start the car. Any half decent mag should fire up the car. I start my 27 often directly on mag. To stop the engine, two options, disconnect the wire or turn off the gas.
Anyway if the car runs good this way then you know about where the problem is. If it doesn't,then you also know where it isn't !!!
A variation of this test could be done as follows. When you have disconnected the wire from the mag post you could connect it to a power source (battery charger or?). Always being careful to avoid connecting any power source to the mag post of course.
I hope this all makes sense. If it doesn't, then please ignore!!
I wouldn't try to test the coils by hooking up a battery charger to them. I was trying to troubleshoot some electrical problem one time and hooked a battery charger to the battery side of the coil box. It seemed to work but not long and my battery charger started smoking. I asked on here and somebody told me why it burned my charger up. You can hook one of those jumper boxes and it will work fine, or direct to a battery. I've driven down the road hooked up to a jumper box. Also thinking it wouldn't hurt for just one short time, I fried battery charger #2.
Corey, it is quite possible that one of those coils was shorting some of the spark plug voltage and current to the battery line to cause the smoke.
I had one that did that and fried a hole almost through a New Day timer in a very short time.
When I did it I used a old charger with no fancy electronics. You are very likely right with on of the new ones