back firing 253

[Big Rob]

Cool. That's the best thing it could be.

It's tough diagnosing a problem without having the engine in front of you, and since you'd replaced the plugs it was the last thing you'd expect, so that's why we were all thinking it was a stuck valve.

Anyway, that's a good reminder to everyone to have a good spare plug in the toolbox as a test plug (along with a long lead that can fit any spot on the motor).

Good job mate, and I'm happy we were wrong. (I was secretly hoping it wasn't a lobe worn off your cam!)

[wbute]

Except the bloke who said it would be a plug or lead....

[thatotherguy]

Thanks guys,had to be something simple because it was running fine on gas but it also ran ok when i drove it the first time but the next day it struggled to get up the driveway,just shows you cant always rely on new parts to be ok. always helps to get other peoples opinions,

[wbute]

With a stuck valve you have no fuel going in or out of the cylinder. It can't backfire. Backfiring through the exhaust is unburnt fuel igniting in the exhaust system. Through the carby is timing. It will miss with a stuck or burnt out valve.

[Taily]

It will miss with a stuck or burnt out valve.

Nope, not always (but I do understand your reasoning), but it will pop through the exhaust with a burnt exhaust valve or seat (badly recessed). I've seen a burnt and split exhaust valve in a low comp 308 run almost as smooth as silk yet pop like crazy out of the exhaust. As I pointed out my own 308 had a riding valve that wasn't quite closing properly a few weeks ago (after sitting for quite some time). The guides had been done 10-12,000km ago (which equates to about 7 years ago the way I was using the ute). It freed up with a gentle persuasion and getting the engine up to temp, but I was getting close to pulling the heads off again....

[Innuendo]

You can certainly have a valve that is sticky and not stuck. Can be running fine one minute and popping the next and then running fine again. (a cracked valve spring can do similar).
Valve lubricant can help along with another product called engine tune-up. It will help remove carbon build up. Mind you a cup of water down the carby at high revs can also remove carbon in a hurry!
Wouldn't do it to something you love, but if it's giving you the sh!ts and your over it; then it becomes an option.

[Big Rob]

With a stuck valve you have no fuel going in or out of the cylinder. It can't backfire. Backfiring through the exhaust is unburnt fuel igniting in the exhaust system. Through the carby is timing. It will miss with a stuck or burnt out valve.

Not if it's the exhaust valve that's stuck open. In that case a lot of fuel will go straight out the open exhaust valve into a hot pipe and burn in the exhaust system before the spark plug can fire it.

[wbute]

True I suppose. If its stuck open. If its stuck closed though there is no way anything can pass through the cylinder.
It would only be the smallest amount that would go through with a stuck open or burnt out valve as there would be no compression or vacuum to draw it through. Only the slight ventury effect from the exhaust system.
It's a miss you hear. No firing. It may backfire under back off like a cam with big overlap causes.
Fuel needs to be forced through a cylinder. It won't just fly through by itself.