The breathing system is the carb and manifold, filter boot, the bottom end in a two-stroke and the top end. Any mating surface, gap or hole in any part of this system will cause additional air to be pulled into the engine, thereby leaning the mixture. Any of the parts of the system can develop a leak, either through neglect, accident, or improper manufacture. We’ll take each component and describe the problems, what to look for, and suggest a cure.

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One of the most common air leaks is around the intake manifold. It can leak at the clamp on the carb, at the flange on the cylinder, or head on four-strokes, or the rubber will split with age.

On four-strokes the intake manifold is bolted to the head, so the mating surface must be clean, with gaskets in good condition.

Another big source with air leaks is the base gasket on two-strokes. Improper torque on the cylinder fasteners, old or pinched gasket, or a warped cylinder, or cases, are usually the causes.

Check the carb and all its fittings for leaks.

A spark plug can be the culprit if the washer is wasted, or missing.

The top of the carb is a must-check area.

Improperly torqued down head or base bolts can cause a leak.

FILTER

Air filters, by the nature of their construction, limit the amount of air that is pulled into the engine at a given engine speed. Your jetting is set with this “obstruction” in mind. If the filter develops a large hole, or comes loose and loses its seal between the air box and its lid, then it allows more air into the engine and leans out the mixture. Proper filter maintenance and replacement is the cure. If your filter is bolted in, use nylon lock nuts, double nuts, or some other precaution to prevent the filter from coming loose. If the filter is torn, toss it, and find a replacement.

AIRBOX BOOT

This connection between the carb and the filter will produce the same lean condition if there is a hole or tear in it. Also, if the clamps on the ends are loose, or the seal on the carb or box is not tight, then you will have an air leak.

CARBURETOR

Normally the carb will develop air leaks in only a couple of places. One is the top of the carb, whether it be the bolt-on or screw-on type. On the screw-top type, like a Mikuni, ensure that the rubber gasket is in place. If the top is loose, it will pull extra air. (It will also probably come off, causing a stuck throttle situation.) If the air bleed screw, the idle screw or the choke assembly have backed out, or fallen out, the carb will suck more air through these “holes.” Make sure that all parts are present and properly installed, and that the clamps on the boot and manifold are tight.

MANIFOLD

Rubber manifolds will eventually harden and deform, or will crack from age, or from an unsupported carb/sock filter combination. Some manifolds are bolted to the cylinder or head via a gasket which fails to seal. Sometimes over-tightening a two-bolt rubber flange will cause it to warp and pull air. If the mating surface on the cylinder is found to be warped, or gouged, then resurfacing may be necessary. A fuel-impervious gasket compound can make up for a small gap. Replace rubber flange manifolds regularly. They are reasonably inexpensive, especially compared to the cost of a total end.

CYLINDER (TWO-STROKES)

Since the cylinder on a two-stroke acts as a seal for the pressurized bottom end, it has several places from which to suck air. In this particular area, there is a two-way leak — it not only sucks air, but it also blows out a small amount of gas/oil air mix, depending on the position of the piston during the stroke.

As a result, a leaking base gasket will cause wet drool around part or all of the base gasket flange area, including the cases and the base of the cylinder. This mating surface can become warped from excessive heat. It should be noted that a greased, fresh base gasket will not always seal a warped base condition. Either resurfacing, or a quality gasket goop, is the cure.
Gouging from previous gasket removal is many times the cause of a base gasket leak. The cylinder gets loose once, or is improperly tightened down, a good seal is lost and the gasket should be replaced.

If the cylinder head comes loose and the gasket gets burned, or if a new gasket is improperly installed, or the head is not torqued in the specified sequence to the proper specifications, then the head will leak. It will pull air into the cylinder during the intake stroke and will also cause a loss of compression. Seepage almost always accompanies a leaking head gasket. Look for the wetness between the head and cylinder.

Another place to look for an air leak on the head is the spark plug and its hole. If the plug is loose, or the sealing washer has been mashed down too many times, or is missing, then air can be sucked past the threads on the plug. Spark plug manufacturers and tuners both specify that if a plug is removed, a new gasket should be installed before the plug is reinstalled. It is also possible (though rare) that the plug itself is defective and is pulling air through its body from a leak in the insulator. Usually, the plug will fail at that point, though.

CRANKCASE (TWO-STROKE)

Since the two-stroke has a pressurized bottom, leaks can occur easily, the largest causes of air leaks in the cases can be found in the cylinder/cases mating area discussed earlier, but another biggie is the mag seal, or dry seal.

It is one of two seals on each end of the crankshaft that keep the crankcase air-tight. The other seal usually rides in gearbox oil, and causes different problems. The mag seal, though, is the seal between the crankcase and the magneto case, which is normally full of air. If this seal starts to leak, it’s harder to detect, because the leak is normally not visible unless the mag cover, and sometimes the flywheel, coils, or points plate, are removed.

If there is a leak at the mag seal, there will usually be seepage around the seal, or there will be a fine oil-mist coating on the flywheel, or any of the other parts. If you pull off the mag cover and goo is puddled in the bottom, then you definitely have a mag seal leak.

Mag seals leak for a number of reasons; if the crank twists and gets out of true it will ruin a seal in a hurry. The seals also get a lot of abuse from the high-revving shaft, and normally have minimal lubrication. They can also get hard just from age.

Sometimes a mag seal will get blown out, or the lip will get ruined after a faulty petcock or carb float needle sticks open and floods the bottom end. If the unsuspecting rider tries to kick the machine over, or bump-start it, the pressure can partially blow out the lip of the seal, causing a leak. The only
cure for the leaking seal is replacement. Most mag seals on most engines can be replaced externally, but some require splitting the cases.

ODD STUFF

Some rotary-valved two-strokes have two dry seals, as well as sidecase sealing between the filter element and the valve. Also, keep an eye on the rubber grommets around the cable and choke and the access plugs in the carb shrouds on these rotaries.

Many two-strokes have crankcase drains. Make sure these bolts are not loose or missing. There are also other factors that will cause lean symptoms, which may appear at first to be air leaks. Crud in the carb, that partially obstructs a jet, will cause a fuel/air imbalance, so ensure that the carb is functioning properly before you start to search out the air leak.

Air leaks that are nearly impossible to detect, but have cropped up in the past, are cracks in the cylinder liner, cracks in heads, and improperly machined or damaged cases. (Look for wet spots around the front and underneath the crankcase at the case seams)

But if you keep seizing the same engine, or run lean to detonation, or run hot, and you’ve checked everything else, then go hunting after the wild air leak. Now that you know where to look, it should be easier to find it.