That will not help. That only makes it worse. The problem is two fold. The main issue is the float valve leaking and the fuel having no other place to go but into the engines cylinders. The manual thing just allows you to leave it on and flowing even more so than a properly working automatic petcock. So lets assume the stock automatic petcock fails and flows all the time, that would be the same as you turning on a manual valve before starting the bike up. It only takes a few seconds to fill a cylinder. The same thing occurs when shutting off the bike. The few seconds it takes for you to shut if off right away (if you remember) is enough to cause hydro lock.
The best option is over flow tubes
With overflow tube you can have a float valve leak and leave the petcock open or have the automatic petcock fail (or turn it to PRIME) and you will know right away when there is an issue and you will not have hydro lock. Fix the problem, do not add to the problem. If you still want to install a manual fuel valve that is fine, but you really need overflow tubes installed or you will still run the risk of hydro lock
How does that not work?
Vacuum operated petcock fails-doesn't turn off the fuel. How does having a shutoff valve after the failed petcock make the situation worse? If you don't turn it off (your fault) it's just like it not being there. And a properly working automatic petcock DOESN'T FLOW when shut off. So the bike doesn't know the shutoff valve is there even if it's on. There is no fuel flow, so how does it make the situation worse?
What FIXES the problem is carbs that don't constantly fail for this reason. Replaceable seats would go a long way towards that goal. And ya don't know they have failed unless the petcock fails. Being able to turn off the fuel and doing so prevents the problem.
Now, tell me how turning off the gas makes the situation worse. Isn't that the job of the factory unit? problem is, it stops doing its job. And it's not a matter of IF it fails but WHEN.
