The glow plug is getting cold at idle, so if you add some glow fuel to the gas it will help a lot. There's another long thread on that idea called glow/gas. One fella had good luck with using 10% nitro fuel with 10% oil at 1 parts glow and 2 parts gas giving about 3% nitro in the final mix. I've used 15% nitro at that mix with E85 ethanol fuel with good results in a .40 size engine on glow. On my ASP 1.08 I only had good results on ignition. One thing you can do is just use a glow driver system at idle to heat the plug. What kind of carburetor do you have on that TT 90? It's difficult to get a glow carb to work with gasoline on the low end and transition.