I had the same problem when I installed my Walbro, turns out that I forgot the lock collar that goes on the neck of the pump AND needed to put something between the bottom of the pump and the locking piece that closes the pump in the assembly. So, the pump was sliding out and fuel pressure couldn't build, the car would start but would barely idle.

 

let it prime and run a fuel injector cleaning or get it tuned mine dose the same thing

 

I thought the pump doesnt prime while in the "on" position? I've heard so much of both and really have zero idea

 

On the Evo 8 and 9 the pump doesn't prime until you are turning the key to start the car. On the 10 it primes in the on position.

 

Thanks for clarifying

 

I have the same issue with my 2003 EVO. Starts after holding gas down, bounces rpms (sometimes stalls) then idles out smooth and have no issues again until I full up.