On my MC21, I’ve always had to feather the clutch to pull away from a stop. The power is poor and the engine bogs until the rpms rise then it's fine. Midrange and top end, it pulls like a train.
It has a drilled airbox, Tyga pipes and #40 pilot jets, air screws out 1 1/2 turns. When it had stock #38 pilot jets it was even worse.
Is it a symptom of a lean condition in the idle fuel circuit or is this normal bottom end performance for an MC21?

I think you have 2 options. Return it back to a standard baseline and confirm it runs properly or take it to a 2 stroke specialist with a dyno to get it setup. There are so many variables like the age of the engine since last rebuild, what type of fuel, could be clogged carbs, battery condition, worn ignition pickups
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Why do things simply when you can complicate them

As far as I know, the engine has never been apart and has 22000 + km's. It has been compression tested at 120 psi on both cylinders. I've recently had the carbs cleaned by my local shop and set the float height to stock 13mm. I had to replace the float valves as both were sticking with aftermarket parts (I assume from China) but OEM are on the way. I also had to replace a bad rectifier and connecter on the wiring harness. Running on 91 octane (the highest pump gas available here), new spark plugs and new Mottobat sealed battery.

Another thing, the bike is now getting harder to start...30 kicks last night. When cold it's a b*tch but starts up first kick when warm. Could this possibly be a symptom of the dreaded NSR crank seals going bad?

I expect the only way to find out is to inspect. 22k is a fair amount for a nikasil barrel
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Why do things simply when you can complicate them

Ok, thanks. I have a home made leakdown tester, will take a look. If all's good I suppose it's time to do the top end.

I'm no expert in this, but I think a leakdown test will confirm if the centre crank seal is failing. The NSR-World Performance pages have some photos of failed seals
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Why do things simply when you can complicate them

Where are you located Mr Z? 91 octane....California?
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"If your not first, your last!" Ricky Bobby

scb67 wrote:Where are you located Mr Z? 91 octane....California?

Yes, Los Angeles.

Ahhh,,,,,I’m in the Bay Area.....a bit to far to meet up for a ride! Lol
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"If your not first, your last!" Ricky Bobby

Well, depending on the state of my engine, I may not be going for a ride with anyone anytime soon haha!

Quick update regarding the leakdown test: I'm definitely losing pressure, from 7 psi down to nothing in less than 5 minutes. I can hear air hissing out from underneath the flywheel, which means left crank seal is bad, correct?

Also, I'm measuring from the RIGHT side of the carb intake manifold (left hand side blocked off) and losing pressure. Does that mean that the center crank seal is bad as well?

For a visual indicator get a hand sprayer with water and a tiny drop of fairy liquid and spray away liberally, you will see it bubble up at the point it is leaking. Bare in mind it could be leaking in more than one area so Mark the leak with a sharpie and keep going until you are happy you have no more leaks.

I normally have a pressure gauge on both cylinders, if your centre seal has gone the you will see the pressure rising on the opposite cylinder to the one you are pressurising. Although I recently had an engine with a brand new centre bearing do this, the bearing wasnt quite seating in the cases so I had to dress the bearing seat to cure it but I would say that would be pretty rare.

Favourite places for leaks are the main seals, carb insulator and the r/h cylinder base gasket where the crank case halves mate.

The latest engine I rebuilt was a real head scratcher, pressure was dropping but no sign of a leak. Ended up dumping it in a bucket of water (only do this if you are going to strip it again, it gets everywhere) and it was bubbling out of the left cylinder water outlet. The crank case had some corrosion pitting on the left hand cylinder mating surface and was leaking from the case into the cylinder water jacket! That f****r really tried my patience!

I've done the soapy water spray trick around the cylinders/heads and carb insulator (which I found was also leaking, but has since been replaced with a new part).

I can't see any bubbles so far but I'm waiting on a flywheel puller and will spray on the left crank seal and do a visual inspection.

Thanks for the tip about using 2 gauges for both cylinders; I'll pick another up tonight. Hopefully I'll have a concrete idea of the scope of repair by the weekend.

Another update:

I took the flywheel off and performed a leakdown test with compressed air going into the right hand carb intake. Per Paul G's suggestion (thanks), I had another pressure gauge hooked up to the left hand intake.

Upon pressuring the right cylinder to 7 psi, I could see the left cylinder pressure rising at the same rate. I could also hear air hissing out of the left crank seal, which was made very obvious when I sprayed soapy water and got large bubbles at the seal. So, the left hand crank seal is bad and I can only assume the center seal is bad if both cylinders rise and fall at the same rate, meaning the pressure of the right cylinder is directly influencing the pressure of the left, correct?

Either way, t'is the season for engine rebuilding!

Sounds like it could be the centre seal gone.

It might be leaking from one side to the other some other way, as I said previously I had a centre bearing not seating correctly which was letting air past, the centre bearing has a small o-ring around the outside which could be damaged/missing or mating surfaces of the crank cases might be damaged and not sealing but I would put money on it being the centre seal.