Hang on!?
Steve; cant you even start the engine on the cold output of your cell?
One would think it would idle on cold volumes?
Now; to double hydrogen volume! 
http://www.pondtech.net/uv.html
http://www.tootoo.com/buy-uv_water_sterilizer/

There are some issues on the coldstart. First, you need more fuel.
I think i do not produce enough HHO to make it run cold.
At the top of my current tests, my cell produces around 9 ltres per minute....
Thats basically because otherwise i blow upmy variac. I already have mounted a fan on it, preventing it to burn up. Its a max 4 amp variac and i punish it with a load till around 11 at top.
At 9 amps, the engine idles. At 11 it revs up till up 3500rpm.
Second, the engineparts are having more tolerance and hydrogen goes anywhere.
I think i do not produce enough HHO to make it run cold.
At the top of my current tests, my cell produces around 9 ltres per minute....