You know what they say about the word assume? Depend on it to make an ass out of u and me.
Take nothing for granted check and recheck everything about a new engine, errors can happen even with the best engine manufacturers.
Before I start every engine new or used for the first time I make sure every fastener is tightened properly especially the prop/spinner assembly. Valve clearances and operation are checked. It’s only a couple of screws for crying out loud. Oil the rockers, springs and a little down the pushrod tubes while you’re at it.
I then mount it in one of my test stands in the upright position with the tank centerline on the centerline of the spray bar. I get the engine running perfectly in this orientation and once I am satisfied I then configure the engine and fuel tank as it would be in the airframe. When I run it now I note how much the reliability and ease of starting has changed if any. If your setup is good you should notice very little change and the engine should need little if any adjustment to duplicate the break in positioning.
If there is too great a change you should immediately rethink the positioning of the engine and fuel tank.
When you are satisfied with the engine being run in long enough to be reliable on the test stand mount it in the airframe. There should be no difference when run in the airframe remember you duplicated the positioning and spacing on the test stand. If there is a difference there is something just not right with the install. It could be something as simple as the cowl not having enough outlet area and its being pressurised when the engine is running. First is to remove the cowl and see if it corrects the problem. If it does you will have to provide greater openings to relieve the pressure. Maybe after a longer run the engine overheats then you will have to do stuff like baffle the intake to redirect air over the engine, enlarge the openings to let out the heat.
It’s not simple sometimes to chase down these problems but take the time to figure it out its well worth it in the end.
Dennis