I own two Hoover Convertibles; a model 70 and model 1030. I never tried an F&G bag in the 70 but do have an F&G bag in my 1030 and just leave the snaps open on the outer bag, seems to work fine. I agree the filter bag is a bit big but I ensure the folds are open and by leaving the outer snaps open, helps airflow.
The 1030 outer bag appears to be a bit smaller than the 70 outer bag but that could be my eyes, lol.
I did read on here, not sure who posted this but they said when you're ready to change a C bag, turn the cleaner off in the operating mode, that seems to help the dirt stay in the bag and not settle down if you were to turn off in the upright position, I need to try this. I will have to find the post, but know I read it on here.
I am a die hard canister guy so I have very few uprights in my collection, the two convertibles mentioned above, a dial-a-matic, and a windtunnel.
I will say I recently purchased my first ever Eureka ESP Upright from the early 80's and I am thoroughly impressed, moreso than the convertibles. oops did I just say that? but that's my opinion ofcourse.
