Are those older GP4000 clinchers? I don't have any problem removing the newer 4000 sII from my ENVE SES 650c rims. I also don't have a problem installing them back on by hand. To avoid breaking tire lever, use plastic ones with metal core.

Don't know if they are older GP4000s as these are not my wheels/tires. The lever I broke was plastic so switched to metal core levers and it still takes considerable effort!!! I would not want to tackle a flat out on the roads based on how tight the tires are on the rims. Especially not during a race.

Will take your suggestion and check the vintage of the GP4000 tires.

Are you pushing the tire bead to the center of the rim before trying to remove the tire?

Yes - trying all the tricks. I've dealt with plenty of 650 and 700 tubular tires and clinchers over many, many years and these rank near the top of the PITA list. Not quite as bad as some Conti 700 tubulars ages ago that left me with bruises on the tips of my fingers but close.

This issue was common across all the carbon/aluminum composite Zipp aero wheels. Had 303, 404 and 808 depth rims. They all were terrible for mounting tires. Some tires were much more difficult than others. I didn't run Contis at the time, but found Vittorias and Zipp's (rebadged Vittoria) Tangentes very tough to mount. More than once I ended up with huge blisters on my thumbs mounting up tires by hand.

If I still had any of those rims I'd pull off and cloth or nylon rim strips and replace with a couple layers of tubeless tape such as Stans. I'd also recommend using a Kool Stop Beadjack for mounting tires.

Why did you have a problem with tubular tires?

Why did I have problems with tubulars? I used them for years and rarely had issues. However, once in a while some brands of new tubulars were tough to stretch onto rims. I kept old tubular rims solely to pre-stretch tires so they would be easier to deal with on the road. And well-worn tubulars that still held air were great for spares as they were easy to install especially when you are miles from home with a flat and the sun is rapidly setting.Simple as that. Would imagine that everyone who has mounted enough tires, whether tubular or clincher, find some more problematic than others.