My bike is a cruiser with a Huasheng 142F 4-stroke, and a Q-Matic drive. Since getting the bike ready for the summer after its New England winter nap, it's been running great -- really feeling "dialed in." The engine is quite new. I'm only starting the second season of some rather light, short riding. I'm using a new, clean, sintered bronze in-line fuel filter, and my gas tank is brand new, thoroughly cleaned and lined with a state-of-the-art novolac epoxy tank liner from Caswell. But on one of my rides to work recently, the engine suddenly started to bog, and wouldn't go much more than 10-12 MPH. I was able to get it to run well enough to get home by choking it about halfway. Then upon further examination, I found that the idle was fairly "stumbly," not the nice smooth idle I had before. I eliminated the possibility of air leaks between the carb, the intake manifold and the engine block by spraying these areas with WD-40 and looking for bubbling. Then I removed and disassembled the carb completely. I made sure that the float bowl was moving freely, and that it was actually capable of floating, and I thoroughly cleaned all orifices with Gumout carb cleaner and canned compressed air. I cleaned and reinstalled the pilot jet, and then reinstalled the brass slow jet screw by turning it out my usual 1.75 turns. Next I took my valve cover off, and found that both the intake and exhaust valves were too tight. I reset the lash (COLD) to .004" intake and .006" exhaust. The plug (previously pulled to get the piston to TDC) was completely black. I replaced the plug with the same one I had before: an NGK CR7HSA. Through trial and error, over a few months, I determined that this plug is the right one for my motor. I set the gap on the new plug to .026" and screwed it into the Huasheng's little Chinese head. My crankcase ventilation tube is securely attached to my engine block and to the carburator's airbox. The air filter is clean and lightly oile per specs. I also checked the connections on my kill switch wiring because I heard that that could cause idle and bogging problems. I emptied all of the 87-octane fuel out of my bike, and I bought a few gallons of 89-octane for the bike's plastic 5-gallon storage can. I thought I'd try a slightly different octane -- and a different gas station for scientific purposes! I added the appropriate amount of Sta-Bil to the gasoline as I usually do I gassed up the bike, and finally, I refilled the crankcase with Mobil 1 10-30 synthetic (I had previously drained the old oil from the engine when it was hot). But my problem persists: The idle is stumbly (but not really all that bad) and I can accelerate for an 1/8 mile or so (with a bit of hesitation/bog at times). Then when I brake the bike to a stop, it ALWAYS stalls. I can restart it by simultaneously twisting the throttle, pulling the starter cord. It will easily start, and it will easily idle. And with the rear tire lifted (I have a center stand) I can rev the engine pretty high and let up on the throttle, and the bike will idle instead of stalling out as it does when it's under load and rolling to a stop. I'm baffled! I feel like I've covered all possibilities of air flow, fuel flow, valve and plug settings -- even the addition of neew gas from a high turnover gas station. Any help will be greatly appreciated! Thanks.