So Vacuum Petcocks failing is EXTREMELY common, on my own personal Mc-23-150 I have replaced the stock petcock twice, then upgraded to a "Mikuni style" and that failed, as well as the next one.
On this bike (and about 75% of other scoots) the fuel tank is located HIGHER than the engine, which, for some reason, can cause fuel on the tank-side of the petcock to rot away the "rubber" diaphragm inside the vacuum petcock, causing a "constant on" problem. This really is quite an issue, if your bike is left sitting with fuel pouring through the petcock, it will flood out your carb, and then sit in the combustion chamber, possibly ruining the engine as fuel seeps past the rings and then mixes with the crankcase oil.
I discovered this while doing a monthly cleaning to my carb because my bike was bogging, and when I took off the fuel line from the carb (POST petcock) all the fuel in the tank poured out into my drain pan. I then check then oil level and found the oil to be way too high and thin and REEKING of gasoline. I immediately drained the oil and found it to have almost 2 quarts of fluid! Luckily I did not drive it, or I probably would have ruined the engine.
So confirming that the petcock did not hold vacuum with a hand-pump, I was not very enthusiastic about replacing another 30 dollar petcock. Luckily, as I was messing around with the bike, my landscapers came to mow my building's lawn, and my bud saw what I was doing, and asked why I did not simply install a manual cut-off switch, like he had on his gy6 power lawn-mowers. I felt kinda stupid for never considering such a switch, and he showed me the switch on one of the mowers, and gave me a part number. Turns out, the switch is a common part, readily available at power equipment shops;
Petcock on amazon:
https://amzn.to/41HIUvj
Cost about 7 bucks (with new Tygon fuel hoses
).
So now, I have the switch instead of the vacuum petcock, and what a difference! The first thing I noticed was much improved throttle response, and a large boost in top-end power, which I surmise is do to no vacuum being expended to operate a petcock.
If your bike is NOT fuel injected, and has a tank that sits HIGHER than the engine, I strongly recommend this mod, it's cheap and easy, and fixes a critical engineering failure in these stock-style petcocks.
I know some people will be quick to poo-poo this idea, but I propose:
2 weeks and no problems (300+ miles)
Much improved throttle response because of no wasted vacuum
And many many motorcycles, scooters, marine engines and power equipment use a manual petcock, and have done so for 100+ years.
Thanks for reading!