You are way under geared, and the t/c is fighting the engine. It's gonna take a BIG torque engine to pull a 3.7:1 gear with a 19" tire.
Do you have any other wheel sprockets or t/c sprockets to test with????
I run the aluminum weights with 1 pink spring and 1 stock spring for 3000 rpm engagement. This is good for stock to mild builds for an experienced rider that has good throttle control.
2 stock springs with aluminum weights should give you 3300 rpm. Good for more mild- not so wild builds, manageable but take caution on slippery surfaces.
2 white springs with aluminum weights should be around 4000 rpm. More for very big cammed engines that don't make much torque under 4000 rpm, only really good for straight line racing, imho.
If you have too much rpm engagement (stall) and the engine peaks torque at less rpm, it will never be able to pull hard into the higher rpm. In other words, you have blown passed peak torque of the engine before the belt engages, and the engine don't have enough power to pull that high gear you are trying to run.
IMHO, you need to START with 3000 rpm engagement, a 50t wheel sprocket, and a 9t t/c sprocket.
This will give you a 5.55:1 gear, but with the overdrive of the 6" driven, you will have a final drive of 5:1.
Your build should have plenty in it to pull a 5:1 final drive with a 19" tire.
You really need a tach to fine tune the t/c driven spring and gear ratio. This way you can see where the rpm drop is when the t/c shifts into high.
If rpm drops too far down, it's gonna take a long time and distance to get more mph. If it drops very lil, there is no more mph to gain.
I set t/c and gear ratios for the riders weight, and a 600ft flat level hard surface run. I live in the mountains and we don't have long flat stretches of roads, mostly old logging road twisty, hilly trails.
If the bike noses over before the 600ft mark, I need to drop gear. If it noses over after the 600ft mark, I need to add more gear. This is for modified non gov'd engines that easily hit 50+ mph in 600ft, and don't need to rev passed 6000 rpm.
Stock gov'd engines, i set for 350-400ft, so the rider still has plenty of power and speed (35-40 mph) to pull the steepest hills and not have to push the bike up a hill because some dummy didn't take time to set it up correctly for the terrain we run on.
Hope this helps, good luck, keep testing.