Engine dies when I throttle a bit

I have no familiarity with Jawa motorcycles but there are some things common to most motorcycles you can inspect. I would first get a service manual and put the bike into maintenance mode. Older motorcycles typically require you to jump a connector that will cause the bike to flash a light to indicate faults. You can look these up in your manual. Newer bikes have OBD-2 ports that work with regular auto scanners.

1. Bank angle sensor - If your bike has one this is designed to kill the engine in the event of a crash to prevent oil starvation. If this is not positioned right, or the bolts are loose, it can move. In a turn the motorcycle will simply turn off.

2. Faulty start switch - these can get mucked up in humid climates or become old with time. As they age the connections in the switch become bad. Insure your start switch is wired correctly and the contacts are not dirty.

3. Unlikely but fuel starvation. I only post this because on my carbeurated motorcycles a significant lean could cause fuel starvation. Insure the routing to your fuel rail(s) is unimpeded. Also insure your vacuum hoses and overflow tubes are routed correctly. Depending on the way your fuel tank sits it could be pinching them which can lead to fuel starvation. Though this would be more consistent and not only in the turn. Additionally the bike wouldn't "suddenly stop". It would hiccup and lurch.

Unfortunately given the limited information this is all I can guess. If it occurs in the turn the three problems above would be the first I would check. Additionally it could be the result of an electrical fault. Inspect the wiring to insure nothing is pinched.

I am leaning towards either the bank angle sensor or a non-descript electrical fault you will need to pull fault codes for.

As a side note you should not be negotiating turns with the clutch pulled all the way in. You need power to your rear wheel.