I received a 100cc quad because previous owner couldn't get it to run. No ideas of brand or specifics.
I replaced all warn and damaged components, as well as the carb. This includes the battery, starter, ignition coil and relay. Fired right up and ran very well. Even under my weight (260lbs) it would easily do wheelies. Could hit 30mph on the road. My small son ran it with the limiter on for weeks without issue. Was more reliable than a new quad we bought.
about a month ago it started popping and backfiring at the limited WOT. I was able to smooth it out by adding a small amount of choke.
Recently, reliablity has dropped. Now it will run ok, still popping and sluggish.
I bought new jets for it as well as checked valve lash. Intake was about a full turn in.
wlWith no limiter and WOT, the RPMs are very unstable and it will buck enough to nearly throw me.
It idles well and even. Low to mid throttle is snappy and even. Mid to high gets a little sluggish.
I have increased the main jet to a 90, than to a 96 currently. The 96 is less stable than the 90.
My current reading indicates not enough fuel, but increasing size has made it less stable. As well as I only have one jet larger. Am I missing something? I have pulled and checked the carb each time I have adjusted the carb. Needle is 4th highest setting.

 

Clean carb out and put jet back to standard, check fuel flow from tank. Fit a new plug and try bike. If it still isn't running right, check exhaust for loose baffles etc. If that is OK, you mentioned having to turn inlet valve adjuster a full turn. Either previous owner got the lash badly wrong or something has worn in the valve mechanism, I would try a new coil and CDI first, but it could be a valve problem, which would require a top end strip and check of all components.

 

I blew out the carb and checked for obstructions when I first changed the jet. Flow from the tank is good. I accidentally pulled the hose while messing with the carb and made a nice mess. This is the second new plug that I have put on it. A few weeks ago we were having a hard time starting it cold, so I swapped the original plug, which looked good still. Seemed to fix the starting issue. New NGK plug was installed before adjusting the carb again. Original coil had burn marks and a lot of wear, so that was changed when I first was fixing. Possible that the new one has issues.
The original owner was not mechanically inclined and anything more than a battery change would be out of the question. lol. So I assume intake valve lash was from wear. I adjusted the valve before changing the jet. It was not loose on inspection, so it was either set that way or wear. The jet was more of a last resort. I checked compression and it is about 90psi.
The CDI that came with the kit i bought may not be correct for this quad. I installed it and even the idle was uneven. It was jerky all through the throttle range.
I have worked to reduce all rattling and have played with the muffler a bit. There is nothing that sounds loose or off there. It is not excessively loud either. Similar in volume and tone to the new quad we have.

I will pull the carb again and blow it out again just to verify. While inside, I will drop the jet a few sizes. I am not sure what the original was size-wise. It does not have a number on it and I cant locate documentation. I have seen in other forums/site that carbs on similar quads seem to be around 65. 90 and 96 both have the same problem and 96 is the worst of the two. So I think I will try about 75.

 

A worn cam would restrict the amount of air/fuel mix getting into the cylinder, trouble is adjustment is for wear on the rocker, so you can't tell if the cam is worn without removing and measuring it. I would put the old coil back on and try it, try another CDI and, if you don't get an improvement, take the top end apart, may be worth checking rings and bore for wear too, 90psi is on the low side of acceptable. Can't see altering jets being the answer, if it once ran right on that original main, it should still do so.

 

I returned it to the stock jet and ran a top end cleaner through it multiple times. I have had good luck with the cleaner on my cars, so I figured it was worth a shot. Ran it up and down the street a few times, still had a shudder at the high end of the throttle. Kids wanted to run it a few hours later and it wont start. After many tries, I got it to start but it will not stay running. Rechecked compression. From multiple tests, I got results between 50 and 120. Mostly on the lower end. So I may have cleaned some carbon that was sealing it. I have now torn down the head and cylinder. I currently see no major signs of wear. Bore is polished. Minor scoring on one side of the piston. No light between the valves and seats. Going to further tear down the head and replace valves and piston/rings since they are already removed and are cheap. Will also hone the cylinder and replace seals.
This got WAY more involved than I had hoped it would become.

 

Check bore size if you can, and ring gap in bore on old rings, this gives an indication of how much wear there is on them. However, the only way compression can vary that much is if a valve was sticking, check the springs. Check the cam lobes for wear, you do need a vernier or micrometer for this. Wear on rockers is usually visible.