I have 3516B Engine that drop on rpm after about 7 hours running, what could be the cause of the speed drop?

The basic concept is that in auto or defrost the air conditioner compressor is activated.
The compressor when activated via an electrical clutch adds an additional load that will reduce the engine RPM. The the RPM is increased slightly when the A/C is activated to prevent the engine from stalling. Power produced by the engine is increased by allowing more air and gas into the engine. When it is working perfectly the RPM will remain the same but the engine will produce the additional power to power the AC compressor.

Usually the bi-pass air valve is how the compensation for the additional power is accomplished.

Depending on your make year and model several methods to achieve the AC compensations have been used. In your specific case if the compressor is not working(no Air) and the compensation is open loop no engine control electronics I would expect a surge in RPM. With no additional load from the AC the additional air into the engine would increase the RPM.
On older cars there is a solenoid that has an adjustment for idle speed when the solenoid is activated. See the image above.
Hope this helps?

belt worn, loose or idler pulley weak --compressor clutch binding,moisture in system ,compressor internal damage

idles too fast.
what engine?
what year?

on a gas engine, the idle speed is 100% controlled by PCM at all times. hot.
so its out of control that means the PCM closes the ISC valve.
and RPM will not drop. (its stuck open) or vacuum air leaks
jeeps use speed density EFI system using the MAP sensor.
this means all air leaks on jeeps , cause wild fast idle speeds.
that is all you need to know , to find problem (find leaks)

clean the ISC yet (idle speed control) valve.?

yes, If you are going to normally boat with a full load, get a lower pitch prop to keep our engine in the recommended rpm range

What's happening is, the engine is under increase load when the A/C is running. Typically there is a solenoid (near the throttle linkage) that controls idle speed when the A/C compressor (clutch) is engaged. It activates each time the compressor cycles on to regulate the system. If idle returns to normal when A/C clutch is disengaged (or A/C is off), then the solenoid or some signal that controls the solenoid to maintain the idle is the likely to be the problem. e.g. The solenoid fuse maybe blown. This might be a good place to start investigating the problem. Best of luck resolving this problem.

When you get the under speed alarm is the generator under load? Try manually starting the generator with no load and let it run for a while, if no alarm, then put it under your home load and measure current draw (maybe have an electrician check it) if the amp draw and in rush caused by compressors, pumps motors etc are within the generator rating, check your fuel pressure, on average 8-11 WC Nat gas. Pressure may test well under no load but drop starving the engine of fuel unde rload. Underspeed is just that engine cannot make RPM/Frequency

You have a mechanical bind. Disconnect the main linkage and work it all by hand. Everything should be free and slopy. Aerosol carb cleaner will help, but only when the engine is not running.

almost sounds like the cruise control is engaging

Engine stalling and rough run issue at low speeds

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control

The problem that you describe could be fuel lock due to failure of vent system in gas cap. Easy to check; leave gas cap loose run engine as normal. If problem gone, replace cap with new vented cap. The newer gas caps (California caps) have much smaller vent holes that seem to clog when moving the compressor at too steep of angle causing gas to enter cap choking vent. Temp fix would be to cut small piece from rubber gasket large enough to let air to enter tank. Good luck with your repair.