what causes backfire?

Usually it is engine timing. It could be bad fuel also. I have seen Catalytic converters cause the same symptom. A bad coil could cause it by jumping posts. Excessive fuel out of a injector can cause that too. You need to take it to a shop to figure this one out. I hope this helps.

Engine Backfire

You answered yourself. With the choke almost closed. This means, it is not getting enough gas for the amount of air going in, so the choke has to be closed to suck in more gas. It could be a air leak, into the manifold, at the gasket. Or at the bottom of the carburetor, if it has a carb. Or a vacuum hose disconnected, loose, or broken. Something is letting in air. Cars that run "lean" tend to backfire. My Eagle had a leak in the manifold and that would really backfire, and the car would not stay running. I couldn't find it, as I didn't suspect the manifold, I just put it on a few days earlier. Or rather, I had a shop put it onwith two gaskets. Big mistakes, the gaskets got sucked into the engine and sprung a leak. But I didn't find it until weeks later, after trying everything else. I put a vacuum meter on a hose connection and read the vacuum. It was 5 lbs, where it should have been 18lbs. This told me immediately that I had a leak. I took the manifold off and resealed it, and now the car runs great. So, you are probably getting to much air. Check the vacuum with a vacuum guage. It should be up around 15 to 18 lbs.

The best way to cause a backfire is:

1) Get up some speed, say 50 kmh,

2) Engage the clutch (i.e. put your foot ON the clutch)

3) Turn the ignition OFF,

4) Pump the accelerator a few times,

5) Turn the ignition back ON

6) Drop the clutch

7) Enjoy!

Really good to time it to be going past a crowded tram stop as she fires!

Any mixture condition, lean or rich, that interferes with proper combustion can cause "backfiring" - actually, it causes "afterburning", where the unburned mixture gets ignited when it is exhausted into the hot manifold.

True "backfiring" comes up through the intake, and is related to really bad timing, cross-firing in the distributor or from bad plug wires, or from sticking intake valves.

If the fuel mixture is too rich you'll get unburnt fuel building up in the muffler. The easy way to tell that is when you accelerate you'll see a dark black smoke coming out of the exhaust.

Dark black smoke is unburnt fuel and light grey bluish smoke is oil

Once you get unburnt fuel building up the muffler a bit of heat or flame down the exhaust will cause it to ignite causing backfiring.

I think backfire is caused by preignition in the combustion chamber. The momemtum of the vehicle is moving the pistons and since they are dragging, the entry of the fuel/air into the combustion chamber pre-ignites when the rotation of the crankshaft is deaccelerating.

This is much more preferable to acceleration with preignition which causes knocking, which really sucks.

Backfire isn't typical in fuel injected engines. It's typical in carborated engines.

Back firing is caused by unburned fuel being ignited in the exhaust system, usually the muffler or catalytic converter. This can be caused by several things. Ignition timing, rich fuel mixture, or valve timing etc.You need to take it to a professional. You can not check it out without the proper equipment.

backfire

timing