Had this saved from another site. I'm not taking credit, but figured it was good info to share.
Here are all of the engine related differences between the 1987, 1988, 1989, and 1990 LT500Rs, and a brief description of how they changed.
CARBURETOR
The carburetor assembly used on the 1987 model, 13200-43B00, is changed on the 1988-89 models, to 13200-43B10, which supercedes to the one used on the 1990 model, 13200-43B20.
The differences are the float bowl gasket, jet needle retaining clip, pilot jet, needle jet, jet needle, main jet ring, main jet, floats, float bowl screws, piston valve spring, piston valve, air vent hose retaining plates, piston valve spring seat, air vent hoses, anti-slosh plate, fuel line retaining clip, and fuel line retaining clip screw.
The pilot jet is changed from 30 on the 1987 model, to 22.5 on the 1988-90 models.
The needle jet is changed from O-6 on the 1987 model, to R-2 on the 1988-90 models.
The jet needle is changed from 6DH6-3 on the 1987 model, to 6DK3-3 on the 1988-90 models.
The main jet is changed from 420 on the 1987 model, to 350 on the 1988-90 models.
The floats are changed from 2 round ones on the 1987 model, to 2 bullet-shaped ones on the 1988-90 models.
The piston valve is changed from 2.0 on the 1987 model, to 4.0 on the 1988-90 models.
The air vent hose retaining plates are changed from having 5mm mounting holes and being secured by the float bowl screws on the 1987-89 models, to having 4mm mounting holes and being secured by the top cap screws on the 1990 model.
The air vent hoses are changed from 4x7x400mm on the 1987 model, to 4x7x400mm and 4x7x325mm on the 1988-89 models, to 4x7x400mm on the 1990 model. There may be other changes as well.
The anti-slosh plate is added on the 1988-1990 models.
The fuel line retaining clip is added on the 1988-1990 models.
The fuel line retaining clip screw is changed on the 1988-1990 models. It is now longer by 1mm.
CRANK BALANCER
The crank balancer used on the 1987 model, 12651-43B00, superceeds to the one used on the 1988-1990 models, 12651-43B10.
The difference is that the new balancer has a lip on it where the seal is. This prevents the balancer shaft from being pulled farther into the bearing and having the weight scrape on the inside of the crankcase.
The left hand crank balancer oil seal used on the 1987 model, 09282-25004, is changed on the 1988-1990 models, to 09282-32004 .
The difference is the new seal ID had to be bigger to fit on the new crank balancer, as it had a lip on it, instead of being flush with the bearing area. It also has directional grooves cut into the ID area for some reason.
The crank balancer oil seal retainer used on the 1987 model, 12543-43B00, is not used on the 1988-1990 models.
CRANKCASE
The crankcase set used on the 1987 model, 11300-43840, superceedes to 11300-43841, which superceeds to the ones used on the 1988-1990 models, 11300-43842.
I've heard that the newer cases have steel inserts in them.
The lower front engine mounting bolt used on the 1987 model, 09103-08224, is changed on the 1988-1990 models, to 09103-08096.
The difference is the new bolt is 5mm longer.
The 1987 model does not use the engine mounting shims, 09181-08008, and 09181-08009, used as required on the 1988-1990 models.
This might have to do with the different frames.
CRANKSHAFT
The pistons used on the 1987 model, 12110-43B02-0F0, 12110-43B02-025, and 12110-43B02-050, are superceeded to the ones used on the 1988-1990 models, 12110-43B10-0F0, 12110-43B10-025, and 12110-43B10-050.
The differences are the newer pistons have a special wear resistant molybdenum overlay on the piston pin area on the inside of the piston, and the circlip groove diameter is now 19.5mm, instead of 19.9mm. My opinions are that the inside on the pistons were wearing out too fast, and the circlips were too deep in the piston, allowing the piston pin to pass them, and to cause significant damage to the cylinder sleeve, and perhaps other components.
The piston pin used on the 1987 model, 12151-43B00, superceeds to the one used on the 1988-1990 models, 12151-43B01.
The difference is the new pin is 63mm long, instead of 62mm.
The crank assembly used on the 1987 model, 12200-43B00, superceeds to the one used on the 1988-1990 models, 12200-43B01.
The difference is the new assembly has a different connecting rod.
The connecting rod used on the 1987 model, 12161-43B00, superceeds to the one used on the 1988-1990 models, 12161-43B01.
The difference is the newer connecting rod is lighter.
CYLINDER
The cylinder head used on the 1987 model, 11111-43B01, is changed on the 1988-1990 models, to 11111-43B10.
The difference is the new head has 7 stud holes, instead of 6.
The cylinder head gasket used on the 1987 model, 11141-43B01, is changed on the 1988-1990 models, to 11141-43B10.
The difference is the new gasket has 7 stud holes, instead of 6.
The cylinder head studs used on the 1987 model, 09108-08143, are changed on the 1988-1990 models, to 09108-08218.
The difference is the new studs are 3mm longer and are 10mm cylinder side threads and 8mm cylinder head side threads.
The cylinder used on the 1987 model, 11210-43B00, is changed on the 1988-1990 models, to 11210-43B12-0F0.
The differences are the new cylinder has 7 cylinder head studs, instead of 6, and the intake area is now smaller. Also, besides the obvious changes, the newer cylinder has a different boost port area. The 1987 cylinder has an aluminum vane on the outside of the intake port bridge, and seperating the boost ports. The 1988-1990 cylinder does not have the vane or the two small boost ports, the cylinder/sleeve boost port transition is much smoother and the boost port angle is steeper.
The reed valve assembly used on the 1987 model, 13150-43B00, is changed on the 1988-1990 models, to 13150-01C10.
The differences are the new assembly is smaller and uses 6 mounting screws, instead of 5. Why they made it smaller, I don't know.
The reed valve stoppers used on the 1987 model, 13152-43B00, are changed on the 1988-1990 models, to 13152-01C10.
The differences are the new stoppers are a smaller 6 pedal design, instead of 8, and use 3 securing screws each, instead of 4.
The reeds used on the 1987 model, 13153-14200, are changed on the 1988-1990 models, to 13153-01C10.
The differences are the new reeds are a smaller 6 pedal design, instead of 8, and use 3 securing screws each, instead of 4.
The reed valve stopper/reed securing screws used on the 1987 model, 02112-03087 are changed on the 1988-1990 models, to 02112-03086.
The 1987 model does not use the reed valve stopper/reed securing screw lock washers used on the 1988-1990 models.
The 1987 model does not use the reed valve gasket used on the 1988-1990 models.
The intake manifold used on the 1987 model, 13110-43B00, is changed on the 1988-1990 models, to 13110-43B10.
The differences are the new assembly is smaller and uses 6 mounting screws, instead of 5.
The gasket set used on the 1987 model, 11400-43840, is changed on the 1988-1990 models, to 11400-43850.
The differences are the new kit has a different cylinder head gasket and reed valve gasket.
EXHAUST VALVE
The exhaust valve retainer used on the 1987 model, 11265-01B00, is changed on the 1988-1990 models, 11265-43B00.
The difference is the new retainer has "500" stamped on it instead of "250".
FUEL TANK
The fuel switch assembly used on the 1987 and 1988 models, 44300-43B00, superceeds to the one used on the 1989 and 1990 models, 44300-43B01.
The fuel line hose used on the 1987 model, 09343-07111, is changed on the 1988-1990 models, to 09352-70113-300.
The fuel switch mounting screws used on the 1987-1989 models, 02112-06167, are changed on the 1990 model, to 02112-36168.
The fuel switch mounting screw lock washers used on the 1987-1989 models, 08321-21067, are not used on the 1990 model.
MAGNETO
The rotor used on the 1987 model, 32102-43B00, is changed on the 1988-1990 models, to 32102-43B01. The difference is the new rotor has a bigger hub and bigger rivets.
MUFFLER
The exhaust pipe mounting nuts used on the 1987 model, 08316-16064, are changed on the 1988-1990 models, to 08319-31065.
The muffler used on the 1987 model, 14330-43B02, superceedes to the one used on the 1988 model, 14303-43B10, which superceeds to the one used on the 1989 and 1990 models,14303-43B20.
The differences are the one used on the 1988 model added a clean out tube and 2 mounting bolts for it, and the one used on the 1989 and 1990 models had only 1 drain plug, which was different, along with a gasket. It was also changed to black.
The muffler connector used on the 1987 model, 14771-43B00, is changed on the 1988-1990 models, to 14771-43B10.
TRANSMISSION
The 4th drive gear used on the 1987 model, 24241-43B00, is changed on the 1988-1990 models, to 24241-43B10.
The difference is the gear used on the 1987 model was bigger for a faster 4th gear set.
The 4th driven gear used on the 1987 model, 24341-43B00, is changed on the 1988-1990 models, to 24341-43B10.
The difference is the gear used on the 1987 model had 21 teeth, and the one used on the 1988-1990 models had 22 teeth.
The 5th driven gear used on the 1987 model, 24351-43B00, is changed on the 1988-1990 models, to 24351-43B10.
The difference is the gear used on the 1987 model had 19 teeth, and the one used on the 1988-1990 models had 20 teeth.
