kz550ltd starter wont crank when hot.

First a little background. "83 KZ550LTD Mac 4-1 exhaust, uni pod air filters, jetted to 108 mains, larger than stock rear sprocket for more low end around the city. Last season i realized i had terrible compression, so had the cylinders bored and got oversize pistons, did a valve adjustment when I had everything apart. Now have great compression, bike runs great, although sometimes hot, I chopped my plugs, doesn't seem to be running lean at all. I figure maybe it runs hot due to there being less material in the cylinder walls to dissipate heat, just a guess. It only seems to run hot if I've been running on the highway for a while, due to the oversized sprocket its revving higher than it should at highway speeds, and the engine feels 80 mph when I'm doing 55. once i come to a stop i notice a bit of smoke out of the exhaust if i rev it when stopped, is this a sign of overheating? The engine seems to still run fine at this point. I have a new stock sized sprocket that I'll be installing this week, as I do more highway riding now than when I put the bigger sprocket on.
When I stop the bike once its running hot, the starter doesn't have enough oomph to crank the engine. it will turn a little, like its trying to crank it, but when a compression stroke comes up, it just stops. the battery is definitely fine, because when the engine cools back down it cranks great. Is this because the bike has higher compression now that its been rebuilt, and the starter is old? Or am I actually overheating badly? If the former, how much of a pita is it to remove and rebuild the starter, I see kits online for around $15-20 USD. If the latter, what can I do to bring my engine temp down aside from the rear sprocket that I'm doing this week, Does anyone know if a GPZ oil pan will fit an LTD engine, so I could use the oil cooler, that's so far the only thing i can think of.

First things I would do is check check to see if the alternator/regulator is OK and they are pushing the +13vdc to 14vdc at higher RPMS with the engine hot or cold.
Then when hot, I would check the ignition timing to be sure it is fully advanced to specs at +3000-3500 RPM and retarded when the engine is at idle. If the timing is running retarded at high RPM, the engine will over heat.

And yes, higher compression and an over bore from stock is going to mean more BTUs from the engine.

Turn engine by hand when is super hot.... try to remember what that feels like and do it again after the engine is cold.
If there is a major difference then you might have some kind of heat related bind or seizures in the piston skirts... or maybe the rings... etc.
Not knowing how you redid the cylinders and what clearances you gave the pistons with respect to your overbore, or if you gave the rings a large enough gap (bigger is much better then too small) ...it is hard to advise you.
A tear down might be needed.

Yes, you might need to install an oil cooler to help bring internal temps down a few degrees.

Starter bushings can go bad over time. With higher compression you are noticing a weak starter.
Steve

Thanks, I didnt even know that about the timing. Ill definitely look into that as it makes sense that the bike running at higher rpms (5000-5500) for extended time is when the bike gets hot.

do you mean brushes?

Both

I've never had this happen with a KZ starter but I could see a scenario where the starter motor is well used and getting weaker.
Like if the brushes are flaky, commutator is carbonized, shaft bearings are going bad and the whole thing gets really hot from a hot running engine... suddenly the starter bearings start to seize up so the motor is fighting itself in rotation... etc.

That was happening on my Z1. WTF it's only 30 years old. When I started getting it ready for the toad, I took it apart (I had extra starters) and used the parts to make one good one. The bushings were worn out. Egg shaped holes. That made the commutator rub against the field coils. The brushes would straddle the the armature contacts and fight itself.
Get another starter.
Steve