The pilot tube had been removed and cleaned. You can see pilot flame along bottom of tubing, but peeters out when it hits top tubing(u bend) and doesn't seem to hit sensor to open main gas, until vent is plugged.
Have you tried to increase the pilot gas pressure and did you check the orifice on the pilot tube?
I got a bunch of these I work on, they SUCK!!! What I've found is the newer crossover tubes didn't get drilled out right from the factory... If its an old one that is in decent shape should be good there, don't change it unless you like drilling out the last straight section of crossover tube... It takes about an hour to do.
But the first thing is check the pilot orifice as has been said.. Always use a orifice drill kit with the right size bit.
Couple of other things to check are the baffles in the heat exchanger. Are they in there or rusted out? Air leakage around the draft motor will cause some issues too.
You can also run the pilot pressure in and out to see if it makes a difference.. I still haven't made a tee yet to test the pilot pressure.
I also can't say I've never made the pilot orifice a size bigger.