I am setting up a Magnum 65 in my P-51. I have the engine inverted to fit in the cowl. I am having a hard getting it to start and run. If I turn the plane upside down, so the engine is in it's normal position, it starts and runs fine. What are the basics for fuel flow when you invert and engine??????
Thanks

It could be that your tank is a Lot higher than the carb when the plane is upright, so the weight of the fuel is flooding the engine when off, and making it run rich when on. When you turn the plane over, the tank is lower, and your engine is working well to pull the fuel up that amount.

If theres anything you can do to lower the tank so the center of it is even with the center of the carb, then do that. If thats not feasable, (as in both of my p-51s) one trick that can help is to run a long fuel line from the tank to the carb, and make it loop up to the top of the cowl, so that the fuel has to go up after leaving the tank before it can go down to the carb.

I set up my inverted glow engines with the vertical mid-point of the tank at the same level as the spraybar in the carb. This gives the same head pressure to the carb in both upright and inverted flight.

The difficulty with this setup is that there is a positive head of pressure with a full tank, such that once you establish fuel flow from tank to carb, a siphon effect takes over and if you don't control it, you'll have fuel dripping out the carb (which would be inverted if you're starting with the plane upright, so you should not be flooding the engine).

So first off, you need a way to close off the fuel feed to the carb when you're filling the tank. Once the tank is full, leave the fuel cutoff closed until you're ready to start the engine.

Once the engine is started, the rest of the trick is in the engine tuning, which must be a solid tune on both the low needle (especially the low needle!) and the high as well. Basically you set the high needle at a preliminary setting that gives a clean two-cycle at full throttle, then you lean the low needle to the point where it dies when you try to throttle up from idle. When you get to that point, just richen up in tiny turns until it will throttle up smartly from idle without dying or gasping. If you're lucky, you can do this in five minutes... or some tough cases I've spent half an hour getting them right. After you're happy with the low needle, then reset the high needle. The pinch test is as good a way as any for doing this. At full throttle, pinch the fuel line closed for about half a second. If the engine revs higher, set needle leaner. If rpm's sag with the pinch, open the needle a click or two. I like to get it to where a pinch will cause the rpm's to just rise maybe a hundred or two, and fly it like that.

For the best high needle tune, start the engine with tank about 1/3 full, hold the nose straight up after the engine is running well and at operating temperature, and have your buddy throttle up from idle to wide open. If the engine pulls well, without sagging, you're good to go. If the rpm's drop in the slightest, open the high needle a click or two until it will run with full power pointed straight up.

Cold start: with both needles properly set, throttle one or two clicks above idle, glow igniter on, now release your fuel cutoff and spin your electric starter. Fuel should ignite as soon as it's delivered to the combustion chamber, so no flooding as long as both needles are properly set. Engine should start easily and run steadily at any throttle setting.

An idle bar glow plug may help in getting things properly tuned, after which you can change to a non-idle bar type, or just leave it in as long as the engine is happy.

It may be easier to get everything tuned up to begin with by flipping the plane over in a cradle, such that the engine is upright.

If you land without having burned up all the fuel in the tank, you need to cut off the fuel flow again in order to avoid the siphoning effect.