Vacuum Problems

Compression is the first test of all tests…

Spark is the 2nd test

Fuel is the 3rd test, and each of the above has a routine in order to follow, so you are not guessing.

Vac Leaks:

A vac leak is unmetered air allowed in error to get by the intake. This effects carbs and injection equally bad….

You do not want a vac leak on any engine ever, but they are very common.

Sometimes you might not know it if the leak is small, and other times a engine won’t start at all. So symptoms run a broad range of what a vac leak can do.

Bikes with CV (constant velocity) carburetors are prone to get cracks in the rubber diaphragm inside the carb, which is what raises the slide as a vacuum is drawn above it. This internal vacuum leak can make the bike idle erratically, and not want to rev up, to rev slowly, or sometimes to bog down and then rev suddenly. They are always “keyed” so that they only fit in one way, and must be installed carefully to avoid damage. The WD-40 trick doesn’t work as well here, as often the bike is already running a bit rich as the slides won’t rise to provide air. (See below)

Some symptoms are a sudden lean condition, which can result in loss of power instantly and a matching increase in fuel consumption.

This might be found to be true if heat cracked a vac line to the petcocks while you were riding, or the line just lifted off

Other types of symptoms cause what is known as “Hunting” Which is idle RMP that will not stay correct. The idle goes up to a given range maybe even to 2,200 rpm and then will drop to 600 RPM and go right back up as if a demon has the grips.. Or the bike might go to 2,200 RPM and stall forcing you to restart.

Lesser leaks might effect idle, and what was correct yesterday suddenly is high today.

Turning down the over all Throttle linkage screw (THE ONE MAIN ONE MADE FOR FINGERS)
will work to lower idle sometimes but is a mis-adjusting when you should not do that..

If the leak becomes worse the idle will do what ever the leak demands..

The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix indeterminable, ALWAYS lean…

On bikes each carb can have leaks, and manifold mount for any carb can have vac leaks.

Any throttle plate shaft can leak on either end. Any vac lines can leak on either end.

And any test port can have a bad cap, and so also leak.

Most bikes don’t have vac operated accessories, with the one exception of vac operated petcocks. A vac operated petcocks WILL say, Pri = prime, On/Run, AND Res = reserve.

There is NO OFF setting….. Also the petcock will have 2 lines each… One line is for fuel and the other is a vacuum line telling the petcock the engine is MAKING vacuum, and to turn on the petcock diaphragm to pull open the on off valve with in the petcock.

The way a internal combustion engine works creates vacuum. I have never seen any bike with a vac pump.

In my experience vac lines in general do not deal with heat and weather well. They crack, split, and become brittle, and should be replaced once a year, as well as gravity feed fuel lines.

To locate a vac leak you need a can of WD-40 which is probably the best thing you can use WD-40 for.

Also you can use WD-40 to test whether or not idle mix is right. This chemical beats ether hands down for use as a engine starter as well, and will not cause engine damage in moderate amounts.

WD-40 makes what you can’t see, and probably what you can’t hear findable.

You need to listen to know….

SO to tell if idle mix is right, Spritz a shot right at the intake with a running engine, and listen…

Does the idle go up? Or does the idle go down?

If things are correct the engine has all the fuel it wants and the idle will drop, as the engine wants no more…

If the idle goes up you are lean…….

If you have 4 carbs and all go down but one, then that one is lean…. Why it is lean remains a question.

Maybe the setting is wrong, and the pilot is in to far (mix screw on a car).

[ Often a book setting will say 2, or 2 1/2 turns out. That is a place where a fresh built engine should run to start, and IS NOT always the best mix for any given cylinder]

Or maybe you have a vac leak…and so adding fuel in the form of WD-40 causes the idle to jump to who knows what, and that depends on the un-metered air.

It is possible for a bike to run on 2 cyls out of 4, and have the two dead cyls fire up above idle speeds as the engine approaches mid range RPM.

So finding leaks becomes a bit of hit and miss, as you spritz about the carbs after a initial shot into the carbs.

Each time you spritz you must listen, so with a air cooled bike you might want a fan on the engine.

Places to spritz are the manifolds looking for loose clamps, throttle shaft ends, and any vac line ends and components vacuum operated. On injected bikes any Throttle body lines, and injector bases, also any vac operated components as you find them.

Often times vac leaks are misdiagnosed as clogged carbs, and bad plugs, wires, pick ups coils and more…

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