atv galazy has some good deals. i bought 2 sets of front shocks from them, i have the dune edition i think there like 400ish, they can set them up for what every you want.
i had the same problem with my passion 4mm. and i was only runnng 4degress of timing and i was running race gas. the one fell right out. but know i have a different set of ported cylinders and still the 4mm and no problems. i think it was my domes from what the other builder told me.
i have both a 4mm hjr dune port and a 421 both have the same pipes carbs everything is the same but the cylinders. the 421 has more power and hits harder but is not near as smooth as the dune port. i think it dependes on what you want the power for. i love my dune port.
i have 2 sling shots, one on ny 4mm herr bike and one on my 421 cub. both work great. ilike the fact you dont have to have a different cover and there alot cheaper. i only paid 190 each for mine new.
im thinking of having my 421 dune ported, but i dont want to lose what bottom end is there now. do you think it will make a big power difference. thinking of having herr juggs port it. any ideas