Questionkcfdguy@aol.com I have a 1980 suzuki gs450l that starts just fine but when I touch the throttle or throttle cable it revs up and dosent drop back to normal idle by itself.It will idle normally if I push down on the carb where the cable connects.If left alone it idles great and shows no sign of smoke or anything.Could this be a vacume problem?I believe I hear a faint sound of rushing air.Any help would be appreciated. Thanks, Joseph..
AnswerHi Joseph,
I suspect a vacuum leak or carburetor synchronization out of service limits (or both).
I'll send tech files to your email.
Respectfully,
Mark Shively
Vacuum Leak Testing
By Mac_Muz
An engine needs the elements to run: Adequate compression, spark at exact time, and correct air-fuel mixture.
Compression is the first test. Spark is the 2nd test. Fuel is the 3rd test. Each test has a routine order to follow, so you are not guessing.
Vacuum Leaks:
A vac leak is un-metered air allowed in error to get by the intake. This affects carbs and fuel injection equally. You do not want a vacuum leak on any engine ever, but they are very common. Sometimes you might not know it if the leak is small, and other times an engine won't start at all. So symptoms run a broad range of what a vac leak can do.
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 RPM 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. 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 will work to lower idle sometimes but is a misadjusting when you should not do that. If the leak becomes worse, the idle will do whatever the leak demands. Another symptom is engine starts, idles and runs well when cold, but stalls when warmed.
The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix undeterminable, 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 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 an 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. Same goes for 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 an 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 fuel screw is out too far. [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 unmetered air. It is possible for a bike to run on 2 cylinderss out of 4, and have the two dead cylinders 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 an initial shot into the carbs. Each time you spritz you must listen, so with an 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 vacuum leaks are misdiagnosed as clogged carbs, and bad plugs, wires, pick ups coils and more.
