What makes a motorcycle backfire?

Question

When I first start my motorcycle I let it idle to warm up. After sitting a while it always backfires very loudly when I first get on it. I have a Yamaha VStar 1100 if that makes any difference.

Answer 1

--Don't let your bike just sit there idling. The choke circuit is needed for initial start-up.
--If you leave the choke on, when it warms up a little, the choke circuit is putting to much fuel into the cylinders. The added fuel doesn't get fully ignited and just sits in the exhaust pipes. When you get on the throttle, the unburned fuel in the pipes gets ignited and explodes, causing the backfire.
--Use the choke to get the bike running. You should be able to start riding after less than a 1/2 minute. And during that time the oil is well circulated to lube all internal parts.
--As you're ridding, slowly turn the choke off in increments. As soon as the bike will idle on it's own, the choke should be off to prevent an over rich mixture and foul the plugs.
--Plus you're well into your ride and have saved a little on gas mileage.

Answer 2

You didn't say whether it backfires on acceleration or decelaration. If it backfires on acceleration, the mixture is too lean. That means that you got to keep a little bit of choke on untill you've ridden it for a couple of miles or so. If it backfires on decelaration, it could be you have an air leak into your exhaust system, when that happens, the exhaust sucks in enough air to make the unburned exhaust gases combustible and that's when the backfire happens.

Answer 3

There are two types of backfire. One is an explosion in the engine's exhaust manifold or exhaust system, this is most correctly referred to as afterfire. The other is an explosion in the engine's intake manifold.

Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Usually, backfiring occurs in carbureted engines that are running lean where the air fuel mixture has insufficient fuel. ("Running lean" is typically a sign of mal-adjusted carburetors or fuel injection where there is not enough fuel for the amount of air). Afterfire, occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in a decatted exhaust system. When a driver shifts up and lets off the gas, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.

Common causes of backfires are:

Timing – If car is distributor-less
Timing – If a two stroke engine is backfiring every other stroke.
Fuel pressure, fuel filter and pump – leaks and corrosion could cause lack of fuel during the fuel injection event.
Bad wiring in ignition – ..if car won't run for more than a few moments.
In older engines, backfiring may be considered normal. Backfire is rare in modern cars with fuel-injection and computer-controlled fuel mixtures.

Common causes of back fires in the intake manifold are bad spark timing, or incorrect (usually lean) fuel ratio.

When starting an engine, timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner. A common air filter will allow the gases to escape, but will block the flame front. On many small marine engines, no air filter is used, but a screen is placed over the intake of the carburetor as a flame arrestor to prevent these flames from escaping the intake, and potentially igniting fuel, or fuel vapors in the enclosed sump or bilge of the boat and causing a fire or explosion. Improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire.

In drag racing, backfires in the intake usually result in the complete destruction of the intake manifold, the carburetors, blower, and sometimes engine.

Answer 4

Sounds like the timing is off or dirty plug

Answer 5

raw fuel meeting oxygen in exhaust and igniting from heat or flame. find out why you are not burning most of the fuel and or have exhasut leaks (timing off? bad valve?)

Answer 6

Timing is off

Answer 7

timing is off or the plug needs replaced i would try the plug first its cheap

Answer 8

Condensation in the gas tank.

Answer 9

Check your valve clearances.