Backfire - The Science.

Following on from a couple of threads I have written, My single spark 02 1150 still makes a singular loud backfire even after money spent on it sorting out why it seems to run rich through servicing and troubleshooting.

All standard except using 1200 injection nozzles and decat downpipe with OEM exhaust.

Backfires maybe every 20-30 mins especially when riding at normal road speeds. Not in a position to see what exits the pipe.

Where would a backfire originate?

Answers on a Christmas card please.
Best wishes everyone.

Mine backfired with cat and standard end can, y piece and standard can, y piece and remus, cat and remus .. never stopped it in 9 plus years of ownership.

I think it is called character

I’ve always thought it was caused by unburnt fuel getting into a hot exhaust and getting ignited by the heat?

Udders said:

I’ve always thought it was caused by unburnt fuel getting into a hot exhaust and getting ignited by the heat?

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Indeed. But the curious bit is why some changes or in some circumstances you get a backfire and in others not. Eg, I put a decat end pipe on my Guzzi V85 and I had loads of burbles and nice pops and bangs. Great. Subsequently I added big bore headers. Counter-intuitively most of the pops and bangs have gone. The burble mostly remains though.

Probably always did it, but the cat muffled the noise.

Unburnt fuel needs oxygen to get a true, full on backfire - could there be an air leak in the system?

If there’s a misfire and unburnt fuel gets through, it moves into the exhaust until it reaches a point where there’s enough oxygen to permit combustion. Sometimes that’s inside the exhaust (if there’s a leak allowing air to be drawn in) or it’s near the exhaust outlet where you might even get a visible flame…

Interestingly if you put a free flow exhaust on a new Enfield 650 you get rid of the pops and bangs by adding a unit which ratchets the mixture by lying to the system about how hot the exhaust gases are.

Paul Young said:

If there’s a misfire and unburnt fuel gets through, it moves into the exhaust until it reaches a point where there’s enough oxygen to permit combustion. Sometimes that’s inside the exhaust (if there’s a leak allowing air to be drawn in) or it’s near the exhaust outlet where you might even get a visible flame…

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So, does that mean not all fuel is burnt in the cylinder head from the spark;
Or is fuel leaking out from the exhaust valve before ignition?
Do the heads need removing and carbon deposits cleaned up and maybe exhaust valves re-ground?

What about using a different spark plug or is it that they are firing too late?

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Rugged Path said:

So, does that mean not all fuel is burnt in the cylinder head from the spark - Correct
Or is fuel leaking out from the exhaust valve before ignition? - Unlikely
Do the heads need removing and carbon deposits cleaned up and maybe exhaust valves re-ground? - You tell me…

What about using a different spark plug or is it that they are firing too late? - No idea

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The point is, that there can be various reasons for unburnt fuel to get through the combustion process. A common one is that not every spark works (a misfire) or the fuel injection system runs more fuel that the spark can cope with in certain circumstances, so it doesn’t all burn in the combustion chamber. I can induce a backfire by backing off the throttle at certain revs and matching when I bring it back on to when the engine has just stopped pulling. It seems to throw more fuel in at that point and it hits the hot exhaust and enough oxygen, just behind the Y-piece and can shoot a once flame out of the back of the Remus race can (no baffles). I do mostly try to avoid it though… Honest

I can't say I know the cam timing specs, but there is bound to be a small element of valve overlap with any modern engine.

If there is anything wrong, it's more likely to be an air leak in the exhaust or an injector dribbling in my experience, but are you overthinking the issue I wonder. Pre decat, this would have gone unnoticed. Once the engine has warmed up, try blocking the exhaust can outlet with say an old towel & listen for leaks - the engine should soon start to stumble, but air leaks should be obvious; check the seal with the cylinder head & then for any splits/cracks down through the system. Could there be a split weld on that y-piece for instance. If all is well with the exhaust system, then try some injector cleaner - if you have a dribbling injector, it may show a sooty spark plug on one cylinder.

Carbon build up usually creates pre-ignition (pinking). By all means check the valve clearances, but a leaking exhaust valve would, of course, be leaking full time, not just the occasional backfire.

If you remain concerned, why not talk to Steptoe or Mikeyboy.