If you had to name a fault on an engine that causes no end of diag. grief, it's an inlet air leak. From a worn carburettor to a knackered O-ring, any extra air sucked in with your [perfectly] metered fuel will have you chasing round the bike looking for obvious problems. Not always easy.
So, if you're trying to put together an old bike, be sure to change the O-rings on the inlet rubbers while you're at it. Why? Take a look at the picture below. This is the combined result of thirty years of engine heat cycles, and intense Australian sun. The O-ring on the left has turned into a flattened, plastic ring - no longer supple, and no longer sealing. When compared with the new O-ring on the right, it's easy to see why it won't work.
But before changing them, check the manifolds themselves. Although the rubber tubes on my GSX have hardened, I can't find any splits and a quick clean up should have them ready for use. The next step was to run a blade around the inside of the inlet tube where it meets the cylinder head. There was a rough, raised edge on each of them (either rubber or carbon) so I cut it away and smoothed it with 180 wet & dry.
Next was to make sure the surface was flat. Anyone brought up on Brit bikes with Amal Monobloc's or Concentric's will know this scenario all too well. Previous gorillas (ok, owners) tightening up a two-bolt flange like this tend to warp them, allowing a nice supply of unmetered air into the engine. Luckily, these particular inlets are held by Phillips screws, which generally prevents over tightening, but I still clean them up on a flat plate with a sheet of 180 emery cloth. Why take chances?
See how the new O-ring sits proud of the groove, ready to seal both surfaces together.
Make sure the mating surfaces are clean and shiny. Any dirt left here will cause you grief.
Sorted! I've even made sure the "lefts" are fitted to the left, and vice versa. Is there any stopping me?
And moving on...
With old gasket eventually scraped off, I thought I'd slap the clutch cover back on. It's far easier to clean gaskets off with dowels removed. Again, you need every bit of the old gasket off. Don't expect a new gasket to seal around tiny lumps of old gasket. Spend the time to get it right. A clean surface with a new gasket shouldn't need any additional sealant.
While you're in there, clean any crud out of the threaded holes with a tap or thread chaser. Even run a larger drill around each hole (by hand) to remove any raised/stretched metal from over tightening. Easier to sort out this stuff now.
With dowels back in place, the gasket has something to locate on.
If you don't have the correct tap or thread chaser, make your own using a bolt of the correct thread - see this post.
Personal preference is a light smear of grease on the dowels to prevent corrosion in the alloy cases. Then offer up the cover and push it on squarely, before tightening the bolts in an even sequence.
Great! Hopefully oil tight too.