Hi all, first post from a long time forum viewer!

I have a '93 MC21 SE that I have been restoring over the last year or two. All was going well till I found a puddle of oil leaking from the left side of the engine. It's been really hard to track down the source of the leak. After much reading and searching it became clear that I may have a dodgy crank seal. It was also pretty clear that I needed to check the condition of the center seal. I purchased a leak down test kit and yesterday spent some time removing the carbs & expansion chambers. I have blocked off both exhaust ports and the carb manifold on both sides. The boost bottle is still in place so no leakage there.

The problem is that I cannot get any pressure at all into either cylinder. Not even a flicker of the pressure gauge. Another post on this forum mentions that I'll need to block the crank "oil feed port" otherwise there will be no proper seal for the test.

Can anyone tell me how I do this? Do I need to remove the oil pump and block off the port there? Is there something else that I'm missing? The engine actually runs OK so I am assuming that the crank seals are still in half reasonable condition (if both have failed then that might explain no pressure!).

Any help would be appreciated.

So a quick update. Tried the leak test again this time with the exhaust gaskets in place. Lo and behold, now the cylinders now will hold pressure.

This is where the real mystery begins though. Left cylinder holds pressure just fine. 5 psi for 10+ minutes and there was not any pressure drop at all.

Right cylinder is a different story. After about a minute it will drop from 5 psi down to around 2 psi. Clearly leaking. The weird thing is that with the right cylinder pressurized, I can see tiny air bubbles escaping from the apex of the left reed valve. If i give the left reed valve a little push, it gives way with a 'pop' and all the air rushes out. So air is obviously getting across from right to left cylinder.

Can the centre seal leak just one way?

Also, i blocked the carb manifold on the left cylinder assuming that I could eventually get enough air to leak across and equalize left and right. This was in the hope of eliminating any other leaks. I could never get the pressure to equalize I'm assuming that there is more than one leak on the right side.

I hope someone is able follow my thought process here. Would love to hear some advice from anyone with some experience on this topic.

Cheers

Sounds like it could be leaking across the carb Insulator from one side to the other. I would remove it, clean all surfaces, fit with a new gasket and some gasket seal and torque the bolts correctly.

Also check the mating surface of the inlet manifold, making sure it's not warped, due to uneven tightening previously.
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Great suggestion. Hadn't though of that one. I'll do as suggested and see if that is the issue. Fingers crossed I'll be able to come back and close out the thread!

New carb manifold gasket arrived recently and whilst preparing the surfaces for the new gasket I took the opportunity to do the leak down test "the other way" by filling each cylinder with petrol.

Left side seemed to hold pretty well. After 2 hours, nothing in the right cylinder. After about 6 or so hours there was a small (20-30 mLs) which had leaked across. Left side also held pressure well with the 'air' leak down test previously so these results are consistent.

Right side not so good. After about 5 minutes I had around 30 mLs of petrol had leaked across to the left side. There are no leaks anywhere else that I can find so all other crankcase seals seem to be good. No petrol leaking out of the motor anywhere and I could not find an air-leak with the previous round of tests.

I'm a little perplexed how these things can leak more one way than the other!

So onto my question for all you wise NSR lovers out there:

Is a small amount of leakage across the center seal a major issue do I have some leeway? Any cross-leakage will be from one side of the crankcase to the other so shouldn't the "mix" be much the same. Am I kidding myself that I don't need to tear the motor down?

My thoughts are that I am up for a full crank rebuild but I'm hoping to be convinced otherwise.