Welcome to NSR-WORLD.COM Sprintboatbob. Sorry to hear your first post is not a positive one though.

My first question is what fuel are you using?

It may sound silly, but the first thing to do is check you have 2T oil in the reservoir! No word of a lie, but I had a mate seize his MC21 saying "the oil light never came on", only to find there wasn't even a bulb in the instrument cluster!

Next pull the expansion chambers off, and check which barrel is damaged. Look up the exhaust port and slowly turn the motor over. You'll almost certainly see scoring on the exhaust side of one piston... maybe even both.

After you ascertain which piston has failed, before going any further, I would check the airbox is seated correctly, and that there's a filter in place. If it's loose, or there's no filter, it could've run lean. Does the airbox have any holes drilled in it?

If the airbox and filter look OK, I would then pull the carbs and check the jetting. (Standard jet settings can be found in the Technical section of the Main Site.) Before you actually remove the carbs though, check all the air correction hoses & 2T oil feed lines are connected properly and that none are damaged, and possibly leaking. Once the carbs are off, closely inspect the inlet manifold. It's likely the original 33 year old manifold, and they can warp or crack around the rubber carb mounts. When checking the jets, make sure you note the sizes in each carb, as they can differ. Inspect each jet for any blockages, and ensure the needles are correct.

Check the wiring at the PGM-II, to ascertain if the ignition has been delimited. The 1989 R5/6K ignition is delimited in the same way as the 90-93 MC21. Details of delimiting are in the Tuning section of the Main Site. Basically, you're trying to ascertain whether there have been any modifications made before really tearing into it. You don't want to [hopefully] just clean up the barrels, sling a pair of pistons in it, just to have it go pop again because it's been fiddled with and not set up correctly.

Pull the barrel(s) off and inspect them and the pistons. Check the heads for detonation, and also keep a note of which spark plug was fitted to which head.

That should keep you busy for an hour or two.
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Andy.
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Last edited by Andy on Sat Aug 06, 2022 2:46 pm; edited 1 time in total