I've just had the same trouble. Insulators for the F (410) are no longer available from Honda. There may be some NOS ones out there somewhere, but I couldn't find them. K7/8 insulators (405) are still available - but only #1 and #3. These insulators are a couple of mm shorter than the F ones. To fix my bike, I bought two of each and flipped a #1 to become a #4 and a #3 to become a #2. Getting the carbs on was a struggle - these insulators are individually part-numbered because they each have a different profile, and are not designed to be flipped. It is not possible to get all four in line. But rubber being what rubber is, they do go on and after some riding I suspect they receive enough heat and vibration to settle into a less stressed state (they're really not that bad though, and two of them are correctly fitted anyway). I am running pods, and am making up stays to support the back of the carbs which will help.
