Installing a check valve...I know one end you are able to push air through and the other you are not able to get pressure back. Which end flows towards the muffler?

Are you trying to pressurize the tank? If so the arrow goes away from the tank.

The end you are not able to get pressure back flows towards the muffler.

The exhaust gases should be free to flow from the muffler into the tank.
There should be no flow from the tank to the muffler.

Hi brrzee
Why will you use a check valve? I never have needed one. Twin engines and only one fuel tank?

Using one to hold the prime...

there is no need for the check valve....no reason why you can't have one either...

I just can't see any advantage to one.....you will loose any pressure in the tank when you refuel.....you should only have to prime on the first start of the day after that there should be enough fuel in the engine to start and run......

In my line of work the arrow will be with the flow usually

I see 2 more possible leaks and no advantages.

Another problem with having a check valve in the vent to the muffler is when you stop full throttle flying. When you reduce throttle to a lower point, the extra presure NOT required has no place to go, but push the fuel to the engine. This can cause all sorts of idling & mid-range problems. I have seen some do ok with these check valves, but most take it back off due to more problems with it than without it.

MildBill said it right. I tried a check valve in a Cap with a Mag 1.08 2C in the nose and there was nothing but problems once the throttle was brought back from WOT. The engine WILL NOT have consistent fuel metering below WOT. I thought the pressure would be constant with a check valve but it wasn't (I was using a Pitts style muffler with little back pressure).

The fix I found was to REMOVE the check valve and add some stainless steel window screen in the outlet pipes of the Pitts muff. This increased the back pressure slightly and toned down the exhaust note a little AND solved the fuel pressure/feed problem. Give it a try. It might work for ya.

Didn't think of the fuel flow problem coming off WOT...thank you

If you prefer avoiding the problems that have been mentioned above, and still holding the prime, you could try a three-line tank set up and place the check valve between the tank and the carburetor.
The third tube would be for filling the tank (must be plugged after re-filling) and the muffler line would have no obstruction (pressure in the tank will be low at low rpm's).

Eliminated the check valve period...having trouble with the engine as it is...gonna try running it without one and see how it goes...