back firing while kicking

[Dick Falcon]

my bike just started doing this a few weeks ago,the first few kicks it back fires.once it states 2-3 kick,and warms up its good to go.i checked the valves and there right on.anyone????

[grayracer513]

Usually, this is a result of the plug being fouled just enough to short out under compression, but still fire on the overlap stroke (top of the exhaust stroke). The unfired fuel air charge from the first couple kicks is ignited as it is being pumped out into the exhaust system. A new plug, especially an iridium plug, will likely put an end to this.

[Dick Falcon]

thanks grey!

[Dick Falcon]

how does a plug"half foul"?I need to fix this.

jon

[grayracer513]

You didn't say what kind of bike you have, so I can't give you a specific answer, but basically a plug is effectively fouled when there is some conductive or semi-conductive coating on the insulator that allows the spark to short from the electrode to ground via the plug shell instead of jumping the gap.

Oil, fuel, soot, and partially metallic carbon deposits are the common fouling deposits, but none of these are perfect conductors, and all of them have some level of electrical resistance. The trouble is that the air gap across the plug has a lot of resistance, and when it gets to be easier for the current to turn the corner and pass through the fouling than it is to jump the gap, the plug fails to fire.

One of the things that increases the resistance of the air gap is compressing the air. Air is a resistor, and denser, compressed air is more resistive than air at normal pressures. So, you can have a situation where the plug fouling is not conductive enough to short the plug at atmospheric pressure, but is conductive enough to short the spark under the higher gap resistance of a full compression load.

See?

[Dick Falcon]

Sorry grey it a 09 yz450f.

[grayracer513]

Check to be sure your grounds are clean and tight (loosen and retighten them), and then check to be sure none of the stator bolts are loose.

[Dirt-rich]

I have a 09 yz450 also and a similar problem The bike has under 15 hours on it I bought it from a guy and it sat for a few months. Takes alot of kicks to fire over and it backfires Once started it pops and doesn't idle any ideas

[Dirt-rich]

Also I did drain the gas put new gas in