A load on the engine will be required to make the governor work propperly. It would be much better if the original generator was still on the engine. The governor will be your problem.
Here are three options but with out a load none might work:
1. Lower the amount of HHO so it draws about 5 to 7 amps. 1% electrolyte mixture and try again. It is difficult with the small diesels if you can not change the amount of diesel being used. You will need very little HHO. To measure if it is doing any good with out a load will be impossible.
2. Run some of the exhaust back into the intake and use the same amount of HHO you are putting in now. Keep adding more and more exhaust until it starts to run bad and then reduce the exhaust going into the engine until it is smooth. You still might need to reduce the HHO too but maybe not. Again you need a load on the engine to do this right.
3. The last option is spray a little water into the intake with the same 30 amps of HHO with a spray bottle or use a paint spray gun and a separate compressor. Be careful not to get to much water because it needs just a little to make it work. I prefer exhaust and water together but will be hard to control on a small engine like this.
What is happening with the exhaust/water is you are slowing down the burn of HHO and increasing HP with steam yielding more torque and HP. When done right you get more power over a larger crank angle. Depending on what the governor does will determine the amount of fuel saving if any. The governor is most likely controlled by rpm, centrifugal weights, and you will need to put a load on the engine to see any gain.
To measure fuel savings on a governed diesel you need to put a load on it. Other wise you will not be able to tell if you are saving or not.
Larger diesels that have much more torque are easier to get reasonable savings. The hardest ones to do though are generating sets because of the governor and the fact that they need to run at a given rpm range. You have no control over the throttle which complicates it even more or makes it impossible. Because HHO/water/exhaust gas increases torque and HP in diesels the engine can run at lower rpms and make the same HP but this can not be done on a generating setup because the generator needs to run within a certain rpm range set by the manufacture, so unless you can lower the amount of fuel some way and keep the rpm in the right range you will see no gains. Diesels are very efficient engines and just a better burn will yield no measurable gain.