HEY!!! I had a NELSON Q40 with a ST4500 carb on it and it ran great!!!! Idled at 4000rpm and 27,400rpm on the top end with a carbon 7.4 x 8 prop. I don't see what the problem is here?[sm=shades_smile.gif] I have ran big bore Supertigres for years. Yes the O.S. 7D is a great replacement carb. I have had no problem with the stock ST carbs but have incorporated a Perry/Conley VP-30 pump and a Electro-Dynamics EDR-103 Glowlite on-board digital glowdriver. 0-5% nitro and 15% Klotz KL-100 lube. FOX R/C LONG w/idlebar ( thanks Duke!!!) or O.S. F glowplugs. Absolutely no problems but now that I have written this comment I will probably have nothing but problems! The NELSON story is absolutely true!