Exhaust backfire at startup

[JesseB]

Hello,

Lately my GS500 is has some issues with a loud bang in the morning, when it hasn't run for a couple of days. From what I've found and understand it's due to some gas not being burned and ending up in de exhaust. It only happens when starting, during the first few rotations of the starter motor. Than once its running on its own, it takes a while to run properly. Even with the choke fully open, it takes about 10-20 seconds to get really going. If I start it a few hours later (say 8 hours, engine is cooled), it starts just fine without choke into a nice idle.

There are a ton of similar questions being asked online, but most of them are backfires during riding or decelerating. This seemed a bit different to me. It started a few months back but it is becoming more frequent.

What is going on?

[mr72]

could be the same cause of both problems.

I think the "takes 20 seconds to get going" is likely caused by leaking float needles, causing the float bowls to get drained if it sits long enough. So it takes it running a bit to refill the float bowls.

If this is the case, then that leaked fuel has to go somewhere. Most likely, some of it it goes into the cylinder. So when you start cranking, some of it would get pumped out the exhaust valve before the bike starts, into the exhaust port. Then that fuel ignites once it starts running, you hear the pop out the exhaust.

Technically speaking a "backfire" is out the intake. Sounds like this is not what you are experiencing.

[JesseB]

could be the same cause of both problems.

I think the "takes 20 seconds to get going" is likely caused by leaking float needles, causing the float bowls to get drained if it sits long enough. So it takes it running a bit to refill the float bowls.

If this is the case, then that leaked fuel has to go somewhere. Most likely, some of it it goes into the cylinder. So when you start cranking, some of it would get pumped out the exhaust valve before the bike starts, into the exhaust port. Then that fuel ignites once it starts running, you hear the pop out the exhaust.

Technically speaking a "backfire" is out the intake. Sounds like this is not what you are experiencing.

Thanks for the reply. It is definitely the exhaust, so not a backfire. Is there a way to verify of even fix this? If the carb, or both of them, are leaking fuel, it must be dripping onto somewhere right? Perhaps check after a ride, when the bowls are full?

[mr72]

if the carbs are essentially "flooding" and leaking fuel then the first place it goes into the intake of the engine. Once it goes there it leaks into the cylinders and past the rings into the crankcase. If you overflow enough it will come out of the overflow fitting and you wind up with fuel on the ground under the bike and everywhere else. Try leaving the petcock on PRI overnight and you'll see

If my theory is correct, then the problem is not that too much fuel is making it into the float bowls, but otherwise the fuel is leaking from the float bowls while it sits, and some amount of it pools where it can end up being pumped into the exhaust port. Again, this is just a theory

There are other places fuel can leak from the fuel bowl and wind up in the intake tract. Rebuild the carbs and replace every single o-ring and you will get it mostly solved. At least that'd let you rule out this as the cause.

I am more confident that the bad running is caused by empty float bowls. You can also know this for sure by putting the cold bike on the center stand, put a piece of clear tubing on the float drain fitting, then open the float drain tap while holding this clear tubing open end up, the fuel will come out and fill the clear hose up to the level where it sits inside the float bowl. I think it's likely at least the left side carb will have an empty float bowl. You could also diagnose this sort of by putting the petcock on PRI for a minute or two before you try to start it. If it starts way better and runs better then that would make it likely the float bowls are empty. But there's not a whole lot you can do to solve this. If I ride mine every day then I don't have this problem but even with freshly rebuilt carbs my bike will drain the left carb when it is left to sit for more than a week or so. Yeah, I don't know where the fuel goes. Probably into the crankcase. So I change the oil like every 500 miles.