Backfiring.... And what causes it.

[Mech]

This covers most cases of backfiring, and by that I mean loud pops or bangs, not the un-burnt fuel spitting out the back of the carb that's sometimes observed.

Ok, so there are two types of what people describe as backfiring, one sort is when it backfires out the exhaust, and the other is when it backfires into the carb and airbox.

So, backfires out the exhaust happen under two different circumstances.
First is as you are riding along, the bikes going fine, but when you are going at a steady speed or throttling off it makes a loud boom out the exhaust... and that sort is caused mainly by an exhaust leak up near the head or front pipe. It can also be caused by a lean mixture, or a rich mixture, but either way, the bike is generally going fairly well.
The other circumstance is the bike is not going fine, it's missing, and then there is a loud boom out the exhaust... that is nearly always caused by an electrical miss... bad plug or coil something like that.

Then there is the backfire into the carb. This generally happens when the bikes being ridden and the throttle is open. It's not the fairly common, occasional pop as the bike is being started or turned off.
This backfiring is because the incoming mixture is getting ignited. There are a few possible reasons for that.
It can ignite because the valve timing is out, and then they don't idle so good, and they are down or torque when you drive them.
Or, the incoming mixture is ignited by carbon build up, or a real hot spark-plug tip, both of which needs the engine to run for a while till it's good and hot.
Or, the incoming fuel gets ignited by un-burnt gases caused by real late ignition timing, which is super rare these days. That also makes them gutless.
Or because the mixture is still in there burning because it was so lean on the last firing. That is the most common cause by far.

There is another cause of burning still going on in there when the new charge arrives and that's raw fuel deposited on the cool head surface and smoldering away, but that doesn't get caused by a rich mixture alone. Iif the properly atomised fuel mixture was that rich it would be spewing black smoke, but it can be caused by badly atomised fuel getting in there in big globs. That can be because the slide needle is loose and jumping up and down, a vacuum hose to an auto-fueltap or a vacuum fuel pump is sucking in fuel, something like that.

Hope that points you in the right direction when you are trying to figure a problem, or describing a symptom in here. Remember, there are symptoms, and there are circumstances relating to the symptom.. Take note of both and it helps mightily when you are seeking help or diagnosing the problem, and that applies to all problems, not just backfires..

[97kingquad]

13 hours ago, Mech said:

This covers most cases of backfiring, and by that I mean loud pops or bangs, not the un-burnt fuel spitting out the back of the carb that's sometimes observed.

Ok, so there are two types of what people describe as backfiring, one sort is when it backfires out the exhaust, and the other is when it backfires into the carb and airbox.

So, backfires out the exhaust happen under two different circumstances.
First is as you are riding along, the bikes going fine, but when you are going at a steady speed or throttling off it makes a loud boom out the exhaust... and that sort is caused mainly by an exhaust leak up near the head or front pipe. It can also be caused by a lean mixture, or a rich mixture, but either way, the bike is generally going fairly well.
The other circumstance is the bike is not going fine, it's missing, and then there is a loud boom out the exhaust... that is nearly always caused by an electrical miss... bad plug or coil something like that.

Then there is the backfire into the carb. This generally happens when the bikes being ridden and the throttle is open. It's not the fairly common, occasional pop as the bike is being started or turned off.
This backfiring is because the incoming mixture is getting ignited. There are a few possible reasons for that.
It can ignite because the valve timing is out, and then they don't idle so good, and they are down or torque when you drive them.
Or, the incoming mixture is ignited by carbon build up, or a real hot spark-plug tip, both of which needs the engine to run for a while till it's good and hot.
Or, the incoming fuel gets ignited by un-burnt gases caused by real late ignition timing, which is super rare these days. That also makes them gutless.
Or because the mixture is still in there burning because it was so lean on the last firing. That is the most common cause by far.

There is another cause of burning still going on in there when the new charge arrives and that's raw fuel deposited on the cool head surface and smoldering away, but that doesn't get caused by a rich mixture alone. Iif the properly atomised fuel mixture was that rich it would be spewing black smoke, but it can be caused by badly atomised fuel getting in there in big globs. That can be because the slide needle is loose and jumping up and down, a vacuum hose to an auto-fueltap or a vacuum fuel pump is sucking in fuel, something like that.

Hope that points you in the right direction when you are trying to figure a problem, or describing a symptom in here. Remember, there are symptoms, and there are circumstances relating to the symptom.. Take note of both and it helps mightily when you are seeking help or diagnosing the problem, and that applies to all problems, not just backfires..

When the carb is out and the needle is set should it have much play? When my needle is on the fourth clip from bottom (highest) it has a bit of play so gas can still get around. Is it meant to?