Prime the oil pump
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Any time the engine oil is allowed to drain completely from the oil pump (like when you remove the oil pump from the timing chest), it is very important that the oil pump be primed again before operation. If the oil pump is not primed, it is very possible that it will cavitate and your engine will die a quick, painful, and expensive death. I've heard more than once that the toughest time for an engine is immediately after rebuild...before the engine oil gets circulated. Of course, we all use engine assembly lube on our bearings during assembly. But, the vital oil still has to be pumped (and the sooner the better).
Sometimes the pump can be primed simply by removing both spark plugs and cranking the engine over for 30 or 45 seconds. You should see the oil pressure light go out. But, many times this technique is not enough.
Instead, you may need to crack open one of the banjo bolts securing a high pressure oil delivery line running to either cylinder head. Doing so allows the oil pump to move the oil to a location with very low resistance to flow. To get a picture of what is happening here, imagine filling a drinking straw half way up with water. Now, place your finger over the end and try to blow the water out. It can't be done unless you give the air some place to go. A similar thing is happening inside the engine. But, with the banjo bolt cracked open, you should see oil get pumped out of the line when you crank the engine over (again, with both spark plugs removed).
If you still don't see oil coming from the banjo bolt, you can pressurize the sump. Shove a plug in the oil breather tube pipe (I find a 3⁄8 inch drive extension fits nicely), remove the dipstick, and use compressed air (using the blower attachment and your air compressor) to push the oil through the oil pump and out the banjo bolt. You can use a rag to make a tight seal around the dipstick hole. Remember, you are not trying to inflate the crankcase like a balloon and turn it into an 8 cylinder. Only pressurize it a bit (20 PSI should be plenty) so that the oil is forced through the lines and out the banjo bolt. So, give it a little pressure and a little time and it will work. Once you see a good flow of oil, tighten the banjo bolt down. Then, crank the engine over to see if the light goes out. If all is well, install the spark plugs and fire it up.