G16 sputtering and backfiring

[Kal]

Where to start..... I have a '96 that supposingly is bored .50 over. I had an aftermarket cone filter on it and it ran fine until I changed out the cone filter for a slightly larger cone filter(original cone filter rotted out in 6 months) and then there was some lag in the pedal (like there was dead space at the top of the pedal) but it would even out a bit when you pulled out the choke a little. It ran that way for a while and then all of a sudden it died on me. It would start back up again after playing with the throttle but die shortly after with some backfires followed by dying out again.
I pulled apart the carb and cleaned it, it wasn't very dirty at all. At the recommendation of a friend I also adjusted the valves which were way off. As per the advice of some of the threads here I used a screwdriver in the spark plug hole to find TDC and both valves were loose adjusting both valves to .004. After I got everything done it backfired at first, then evened out after adjusting the mixer screw a little. I took it for a short test drive and it did ok, even sounding smoother thanks to the valve adjustment and having a smoother throttle and powerband. I took my wife out to see how it would do with more weight and it was fine going down hill, we went to turn around and it backfired a lot and died out. It took a while to get it going again but it started to take off, backfiring here and there and went. When she tried to jump back on it died out again, like it didn't like the extra weight. Does anyone have any idea on what's going on? I'm about to pull what little hair I have left out it's so frustrating....


I forgot to mention, I have a new champion plug in it, the old one had a lot of carbon and was slightly wet. Not sure with fuel or oil.

[cgtech]

Since you have deleted the stock airbox & filter, You will need to re-jet the carb to make it run right. If you still have the stock airbox, put it back on & see how it runs.

[Kal]

I don't think I have one left. I modified the stock one to bring in more air and it didn't work very well. I think I know where I can grab one though. Supposedly the jet that's in it is already set up for high altitude, if I find another jet for it is that going to effect my altitude?

[sho305]

You have to jet richer if you mess with the stock airbox. Many yamaha just a little leak in the airbox lid will mess up how it runs. It can foul the plug and then they quit and backfire. With mine I used a torch tip cleaner to file out the main jet little by little until it ran good with no hesitation. I still had the airbox on a G9 I only removed the silencer part and it messed it all up had to jet it. After that it ran great for years until it burned oil then I swapped a 420 clone into it.

High altitude will have a leaner jet because you have less air. It will make this problem worse. Free flow airfilter will make it lean with more air flow you need richer (larger) jet.

[Kal]

Originally Posted by sho305

You have to jet richer if you mess with the stock airbox. Many yamaha just a little leak in the airbox lid will mess up how it runs. It can foul the plug and then they quit and backfire. With mine I used a torch tip cleaner to file out the main jet little by little until it ran good with no hesitation. I still had the airbox on a G9 I only removed the silencer part and it messed it all up had to jet it. After that it ran great for years until it burned oil then I swapped a 420 clone into it.

High altitude will have a leaner jet because you have less air. It will make this problem worse. Free flow airfilter will make it lean with more air flow you need richer (larger) jet.

Thanks for the advice, that's a good idea. I got a few spare carbs laying around here I can steal the jets from. Any guess on how much bigger I have to go?