Sludge on exhaust studs

[Rage]

Wtf?  Nasty thick sludge on exhaust bolts. What would cause this?  Clogged muffler?  Blown gasket?

[Rage]

In addition to the exhaust sludge, I just pulled the plug and it shows lean..... which has never happened before. Any advice? 

[naughty lee]

Not all of this next post applies to you but the bit relating to setting the mixture does.
Identify which carb screws are which on your bike first, as mine is a zx50

Prior to speed test
Removed spark plug, cleaned and gapped to 0.9mil and refitted.
Removed the air filter, washed it gently in a small bowl of gasoline.
Wrung it out gently. Dipped it in two stoke oil then wrung out the excess gently.
Refitted air filter.

took the bike out and got it hot.
It must be hot before moving onto the next stage !

With the engine running, Park the bike on its centre stand on a block of wood to lift the rear wheel.
Locate the carburettor on the left hand side if the bike.

There are two screws located close together on it.
The highest one ( and the largest) is the idle. The lower is the air.

Turn the upper one fully clockwise (this allows more fuel in and your engine speed should rise)
Next turn the lower screw fully clockwise (to block the air) then turn it counter clockwise one full turn quickly to prevent the engine from stalling.

Continue turning the air screw counter clockwise and listen to the engine revs. Your aim is to make the engine rev as high as possible. Small adjustments of a quarter of a turn work well here, pausing for up to twenty seconds between adjustments to listen to engine speed.

eventually you allow too much air in and the revs will start to drop. You will then have to turn the air screw back the other way.
adjustments at this point can be very small, as little as 1/8 th of a turn. Find the spot where the bike revs highest..THEN turn the air screw 1/8 of a turn clockwise ( this is important as it makes the mixture slightly rich)
It's unlikely the revs will drop, but if they do, it will be something of nothing.
(i turn clockwise in very, very small increments until I hear just the slightest drop in speed)

Now turn the upper screw counter clockwise until the engine rev speed falls back down to regular tickover level.

Your mixture and tickover are now set. If the bike stalls in future then just turn the upper screw an eighth of a turn until it idles without stalling

Bike was taken to an undisclosed location in North Yorkshire
An indicated 64 mph

I'm gonna try refitting the iridium plug and leaning out the mixture a tad more
It will be some feat to hit 65mph on a genuine 49cc scoot, but the target is now in sight!