Hey,
I have a '90 MC21 R, have had for about 2 months. When i got it it didnt idle well at all. On a cold start revs shot to about 7k for a minute or so before it settled down. While riding around town, stopping at lights etc, idled at about 3k for about 30 seconds before dropping down to 1.2k. I opened up the carbs and gave them a bit of a clean, floats were balanced at 12mm (was expecting them to be out). cleaned out the tubes (one was very blocked). Put back together. Everything was working well for a day. did 20-25ks, was running better than ever so decided to open it up properly for the first time, as soon as revs hit 12k bike locked up. I pulled over (well coasted after skidding for a bit i guess lol).

The kickstart is now stuck, like it has a bit of give at the start, like it always did, but does not budge once its at the point where its suppose to be doing something. Even with a decent amount of force. So somethings pretty seized. crank bearing collapsed maybe?

Bikes done just over 50,000km so was in need of a rebuild anyway. Planning on ordering top and bottom rebuild kits from tyga. Noticed tho that it has mc18 top end, (hexagonal heads etc). Is there any difference in mc18 and 21 bottom ends? (R models). If so how can i identify it? Am thinking someone just replaced top with 18s when it needed doing at some point. would hate to buy crank only to realize its the wrong one tho. Read somewhere that you can tell by gear sensor, could anyone elaborate on that?

Thanks in advance

Second thread on page 2 of the forum asked the same question - hopefully reading that will answer your question.
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yes thanks, very helpful, didnt want to hijack someone elses thread tho.
So honda isnt being very helpful with engine number. Is there somewhere i can find out what my MC16E-1040461 is?

Also whats the deal with pistons? tygas mc18 top end kit comes with a 21 unmarked piston, and the 21 kits letters dont match the piston letters they come with? is it just the head gasket that is different between 18s and 21s?

The rebuild guide on the main site is awsome, very informative.
Is there anything else anyone would recommend doing during rebuild? while everythings opened up?

1. Take your power valves our and clean them.
2. Inspect big end bearing play
3. Remove the reed valve assembly and inspect reeds for damage.
4. When everything is buttoned up, do a leak down test and make sure you don't have any leaks.

Even more...

1. adjust power valves after assembly.
2. Inspect water pump oil and water seals.

21 crank webs are 2mm bigger than the 18. so the cranks aren't interchangable.pistos etc are same . send new pistons away with your barrels if you need them replated and they'll match them to the 18 barrels you have.
with the kits it's just the head gaskets that are different.

Thanks for the help guys.

Is this the gear sensor? if so, 18 or 21?