i had that same exact problem with my tomos. It turned out that the reed valve wasnt opening wide enough when i hit the gas. the little prong that keeps it from opening too far was bent, and was keeping it shut. Now, dont go bending your reed valve in half, just make sure its working right. might also be due to the piston expanding in the cylinder, or something related to the eninge heating up after a while
Check your plug. Sounds like maybe you're running too lean (crazy acceleration while cold), and running the engine too hot (which is slowing you down). If the plug is almost white (too lean), maybe your main jet is clogged or you have a vacuum leak between the carb and the intake manifold. Check your points, too, just for good measure. You haven't messed with the timing (following some recent threads)? Too much advance will make your engine run hot, too. Just a couple of thoughts.
How do you check if the reed valve is working right? Do you blow through it backwards, or forward or is there some other way? Never had to deal with one before.
Going to a richer main sounds backwards.
Try a leaner main ... Because as engines warm up their fuel to air ratio gets RICHER because warm air is less dense (which means less of it) mixing with the same amount of fuel.
However... You should do a "plug chop" first.... That means getting the engine totally warmed up and running it as fast as it can go for a good half mile or more... then stop on the spot and pull the plug... If it is not going white... then you need a LEANER main jet... and only go 1 step at a time.
And once you put the leaner one in.... do another plug chop.
When you are doing plug chops... It is important that you stop ON THE SPOT from high speed and check the plug color... because idling down and riding at low speed will color the plug.... What you want is the color while the engine while is running wide open.
And for checking the reed valve... put it up to you mouth and try to suck air through it backwards... it should let little (if any) air through.
I'm with Fred, sounds too rich. Have you tried running without the air cleaner.\?
When I do a plug chop, I hit the kill switch or turn off the ignition before closing the throttle. The plug will get wet but will not change color. If you close the throttle from high speed to stop, the mixture will change a lot and may give you a false reading.
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