'72 - '74 250 and 360 Enduros Rust on the crankshaft - magneto side - right at & behind the seal.

All engines in "my collection" have this condition, bearing in fine, engine will run.

What troubles could this cause?
Would it affect the fuel / air mix into the cylinder?

Do you see rough, crusty rust or just light rust. If light rust where you describe chances are exactly where the seal rides will still be shiny. If crusty rust that doesn't sound good. If you replace the seal and it rides in a different location, it'll get torn up quickly. I had pitted rust in that exact location and elected to send the crank in for build up and machined down to provide a smooth surface.

It's quite rusty however smooth enough to not destroy the seal. I would not get that area wet for sure. Those bearings receive lubrication from fuel / oil flowing in that area, so I was curious if there was any possibility air from a poor seal could affect the engine. Perhaps run lean?

If the seal leaks, it will usually run lean.

Yeah I was wondering about the feasibility of having the cranks repaired / machined. Any recommendations on a good machine shop?

If crank surface is damaged where seal rides, you can get a speedi-sleeve to install over the damage. Sand rust off first.

www.skf.com/us/products/industrial-seals...es/skf-speedi-sleeve

I repaired my rusty crankshaft with Speedi Sleeve # 99114 cost about 18$ online


after

Oh awesome. I had heard of something like that but had no idea what it was called or where to get them. Hell, looks like Amazon has 'um.

Just FYI... You need to install after crank is installed in bearing and before installing seal.