yes, i think you are glowing too long. is the engine hard to start? also the carbon build up could be causing a problem, perhaps you are damaging them on installation. you are using the original loop type? the fast glow retro fits on the 115 i had found to not last very long... perhaps from glowing too long.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.