Loud BANG in my engine

I posted this as part of a thread on the carb side, but thought it may be better suited for this area.

I have an 83 KZ550 H that had not run for 10yrs. I cleaned out the carbs, changed plugs, put clean oil in, and luckily I was able to get the bike running. I've had this bike idleing for the past week, almost everyday for a while just to continue to work the engine. I had it running yesterday, and let it sit and idle for 5 minutes or so. About an hour later I went to start the bike and there was a large BANG in the engine. My guess would have been that it was coming from the first cylinder area. I tried to start it again, and BOOM, same noise. Tried a third time and it started with no issue and idled. What could that be? Is that a valve? I am completely out of my element here, so any advice would be very appreciated.

Sounds like a backfire. Check timing advancer.

Thanks bountyhunter. I don't believe it was a backfire, it sounded/felt much more physical. Of course I say that without much experience. But a backfire would have sounded through the tailpipe as well wouldn't it? This didn't, it seems very localized...and the engine had barely turned over when it happened.

I am guessing... To get any type of explosion in the engine you need fuel (fumes) and a certain amount of compression.
It does sound to me as so it is timing and possibly a valve (exhaust) not opening!.
RegardZ.

Thanks Gam. So I was running the bike again yesterday and noticed that carb 1 was leaking some gas. Looked around a bit more, and gas was coming out of the airbox from that carb. I pulled the carbs off, and there was a lot of gas going into the cylinder. All the other cylinder intakes were dry.

I checked my float level with a clear line and it is right at the float bowl level, and should be down 6mm or so. I guess with the slight lean forward of the carb, gas was flowing into the cylinder.

So if the noise I heard was a backfire, would this be enough to cause it?

Thanks

Yes. The flooded cylinder may have fouled the plug which means you had a pair of cylinders not firing but filling with fuel. Eventually when the plug fires, you hear a big BOOM. My 750 twin used to do it.

Garn wrote: I am guessing... To get any type of explosion in the engine you need fuel (fumes) and a certain amount of compression.
It does sound to me as so it is timing and possibly a valve (exhaust) not opening!.
RegardZ.

NO! No compression is needed to create a "Big Bang"! This is very important to remember, otherwise you could injure or even kill yourself. All that's needed is fuel and oxygen, no compression needed. A small flammable gas leak will turn a house into kindling instantly, or a garage or shop, natural gas, propane gas, gasoline fumes, or any other flammable gas fumes can be deadly. Engines that that get gas into the crankcase have been known to blow oil pans and valve covers off. Gas fumes sucked into a building exhaust system have been known to blow the roof off. And I believe a fuel/air bomb is the most powerful conventional bomb in the US Inventory.

I have either seen first hand, or experienced all the above, with the exception of the fuel/air bomb.

Sorry for the rant, I just don't want anyone thinking that gas fumes are safe without compression. You never forget the stink of burning people.