My 02 GS decided to rev at 3000 RPM at idle after it gets hot (1250 when cold).  How do I do a vacuum leak test?

ive read spraying wd-40 on the intake boots while idling..the rpm spikes if theres a vacuum leak...

p.s. im fairly new and used the SEARCH function! j/k...im sure one of the experts will enlighten u!  

Quote from: my_celica_is_jealousive read spraying wd-40 on the intake boots while idling..the rpm spikes if theres a vacuum leak...p.s. im fairly new and used the SEARCH function! j/k...im sure one of the experts will enlighten u!  

I am NOT an expert, but have some experience with what you just described. This is similar to using a volatile liquid to test for vac leaks in a high vacuum setup --physics labs, etc. When molecules from the sprayed on fluid get sucked into the vacuum leak, in this case the intake stream of the GS, they add to the combustion and cause the increase in RPM you mentioned. Sorta like a nitro boost. One site recommends spray on carb cleaner. DO NOT USE SPRAY STARTER FLUID. You are just trying to momentarily choke the leak with a fluid and NOT do a combustion boost. Once the stuff gets sucked into the leak, if it is there, the engine will go back to the previous state.

Usually a vacuum leak test is less invasive than a compression test. To add to the confusion, a Leak Down test is also in the arsenal of the diagnostic mechanic. The LDT uses a compressor to pressurized the combustion chamber at a specific point in the cycle to check valve leaks, ring leaks, case leaks, etc. Compression tester are cheap but do not tell the whole story.

You are talking about the vacuum line from petcock to the carbs, is that right?

Quote from: LEVOYou are talking about the vacuum line from petcock to the carbs, is that right?

Well, no. My_celica is referring to the connection between the carbs and the engine. There are two boots with o-rings that get compressed and dry. Also if the screws holding the boots against the intakes are loose, that could cause a leak. Spraying one of the previously mentioned fluids changes the leak and should also change the running of the engine IF a vacuum leak is there. This would be a rudimentary vacuum leak test.

The Vac line from the petcock to the carb is used to meter gas via a diagphram in the petcock. Checking that hose is a good idea anyway. It might also be interesting to have a T-connector there to put a vac gauge on while running normally, but that isn't where I am taking about in my post.

The compression test and leak down tests are done via the spark plug hole.

Let's hope our guru Srinath catches this thread and gives us some of his wisdom.  

Thanks starwalt.  Those are a lot easier to check out and spray around.   Rubbers look pretty good and pliable.  No cracks.  I try tightening the clamp screws and see what happens tomorrow.

I also saw one of pantablo's posts, he mentions that bike's with stock jets might idle as high as 3k rpm when they really warm up.  My bike is bone stock except for the progressive springs.

:dunno:



...hehehe

ok so now that i know one way of checking for a vacuum leak, what are the symptoms of a leak.  my friends kawi takes a long time for the rpms to drop to idle.  could this be a symptom?