big backfire, why?

[revilo]

Hi guys,

I was out for a ride last night on my 1980 GS1000G and got on the throttle in second gear in a rather spirited manner. Around 5 or 6000 rpm, the bike had a big backfire, with flames blasting out the exhaust and a huge bang. I immediately pulled the clutch and went for the shoulder, only to discover the bike was fine. No unusual noise or furter ill-running. It's never done this before. Anyone got a hypothesis on what happened? Simply a momentary richness, or indicative of a bigger problem looming in the future, like something in the valvetrain? Is this something I should be worried about? Thanks in advance!

[chuck hahn]

I am thinking a plug fouled and the cylinder keeps pumping raw fuel to the pipes. Plug shakes its carbon bridge and comes alive sparking the mini explosion.

[BigD_83]

I am thinking a plug fouled and the cylinder keeps pumping raw fuel to the pipes. Plug shakes its carbon bridge and comes alive sparking the mini explosion.

That, accompanied by a hanging exhaust valve...

[Brendan W]

Lean will do it too.

[BigD_83]

Lean will do it too.

Flames coming out the tailpipe suggest raw fuel making it through the system to be ignited downstream, no?

[revilo]

Good points, all. I wondered about a hanging valve, as the valvetrain is pretty much the last part of the bike I haven't gone through. Cracking open the top end is the next thing on the to-do list. Is a sticking valve symptomatic of a larger problem, or just a thorough cleaning? What do you guys recommend as the next course of action?

A bit of history on the machine, too, it's all stock in the engine department, and I got it with 67,000 km on the clock. Had to do fork seals, and stator/RR and some other electrical tidying. Looks like one of the PO's cared for the bike, as it came with hex head screws all over in place of the JIS screws, as well as other evidence of previous valve adjustment in the form of a big bag of shims, but I'm thinking it had been slightly neglected in recent years and the most recent valve adjustment is unknown. However, it runs well other than the one backfire. Really hoping it didn't nick a piston or some other mechanical damage. No smoking out the tailpipe, and it runs great. If I pull the plugs, will I be able to see into the middle two jugs, or will a borescope be necessary?

[chuck hahn]

Bore scope will help greatly if you can borrow one. Maybe the auto parts places that rent tools may have one.

[tom203]

Check out petcock and its vacuum line - fuel might be washing out #2 spark plug at low rpms, dumping fuel in exhaust .

[Brendan W]

Flames coming out the tailpipe suggest raw fuel making it through the system to be ignited downstream, no?

Misfiring would be the first suspect for me. However lean mixtures can burn real slow if at all and end up partially or not at all combusted in the exhaust. It will also have a good shot at burning exhaust valves.

[BigD_83]

Misfiring would be the first suspect for me. However lean mixtures can burn real slow if at all and end up partially or not at all combusted in the exhaust. It will also have a good shot at burning exhaust valves.

Ok...I understand where you are coming from on that