Some good info so far. I'm not so concerned about air leaks in the front of the engine as IROCBSA suggested, these are usually consistent leaks and can be tuned around. In fact, most engines have bit of air leak designed in to limit oil flow through the front bearing area. As CARSII suggested, glo plug could have a major effect on throttle transition. Several have suggested good plugs but whatever plug you choose. be sure it is a fresh one. As suggested, plug elements can get contaminated especially during breakin. Once contaminated, they will run just fine at full throttle and they will also idle OK but will suffer in transition when you most need them. Just because a plug looks good and glows well doesn't mean it is good to use. The easiest way to identify a potential glo plug problem is by noting RPM drop when you remove the battery. A good plug will show very little to almost no RPM drop at any throttle setting where a marginal or bad plug will show a notable drop.

Be careful using silicone based sealers on a glo engine. A bit of silicone migrating through the engine can easily contaminate a glo plug rendering it useless. Nothing wrong with using a bit of silicone where needed, just be sure not to over use it so pieces can get free to migrate.

One other area to look at. Is the tank well padded? Fuel foaming from the tank being too tightly held in place could easily cause your problem. Unlike gasoline fuels, most glo fuelsl foam quite easily and can cause all sorts of problems.