Pilot goes out when turning knob to on position on water heater

Brand of unit?

Exact model number of unit?

If you have a standing pilot furnace then you can try to clean the thermocouple or replace. The best thing is to have it serviced every winter season.

brand of unit?

Exact model number of unit?

Model # SW6P
Direct vent water heater
It is from suburban manufacturing company
12000 BTU

Here is your manual

http://bryantrv.com/docs2/docs/operating/swinstall.pdf

The gas valve on this style heater (RV) has a built in ECO (energy cut off) and that can be a problem. Replace the thermocouple and make sure the pilot orifice is clear so a good strong flame is present. If this does not work then the built in ECO has opened and the gas valve will have to be replaced since it is not reset able on the standing pilot unit. I had one of these in a travel trailer 15 years ago and it was pretty trouble free. NOTE: Make sure you WD-40 the aluminum tubing at the nut areas since these heaters are exposed to the outdoors and the aluminum does corrode a bit over time. By doing this you will prevent twisting the aluminum tube necessitating its replacement.

Parts list and pic


Google Image Result for http://www.aaarv.net/sw6-p.14.gif (http://images.google.com/imgres?imgurl=http://www.aaarv.net/sw6-p.14.gif&imgrefurl=http://www.aaarv.net/sw6-p.htm&usg=__mwna0O3S32jJ-4aOgMa9Dn0Gv2g=&h=1126&w=900&sz=55&hl=en&start=2&um=1&tbnid=lvHfMHrl0yXhAM:&tbnh=150&tbnw=120&prev=/images%3Fq%3DSW6P%26hl%3Den%26sa%3DN%26um%3D1)

OK I changed the thermocouple and the pilot stays on but now it seems the gas flow is not strong enough to prevente the flame from back tracking

ok i changed the thermocouple and the pilot stays on but now it seems the gas flow is not strong enough to prevente the flame from back tracking


Back tracking? Explain in detail.

The flame reaches all the way back to the end of the burner where the gas is supposed to mix with air (o2) to help it burn

And then it burns only at that point and not at the burner end

Gas is burning inside the burner near the orifice and is it noisey like a blow torch?

Gas pressure to high? Restricted burner?

I cleaned everything and the gas flows well for a bit but then it starts to fail again should I replace the thermostat

No.

I have not worked on a rv htr. Like this, all the text book soloutions have been given... a technician should put a trained eye on the problem and fix it in a few minutes.

What should I do then to fix it

One would have to buy a gas pressure meter of some sort, train on using it and measure incoming and outgoing pressure, inspect the burner and ht exchanger and flues, and be sure all parts are there and where they suppose to be..