When mine do that, it is normally because there is a plugged up jet in the injector lines...

I have installed a strainer on the header feed line for just that reason. 

But X2 on the trying to make the lines from the valve the same length, even though mine were not even close

My Bassett valve was actually mounted to the right header injector pipe and a hose (6AN) ran from the valve to the other side, seemed to work very well, even though it really shouldn't have, the right header just started (steamed) sooner by about a second.

GT