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Leak-Down Testing

Leak-down testing on a two stroke is another essential tool that can get you out from troubles. In contrary to the compression tester, this test measures pressure loss but both can be used in conjunction by an experienced user to reveal possible issues on an engine. 

Testing can take place after a newly assembled engine to verify that everything is leak free, on a bike you just bought and you need to check crank seals condition etc. A leak down tester can be fabricated easily out of parts found on a hardware store but there also kits dedicated for this work at reasonable prices. 

Procedure is pretty simple, you need to block all air passages out of the engine, like the carb and exhaust manifolds and the spark plug hole. For the later one, a simple spark plug will do the job fine but you will have to construct some bungs for the carburetor and exhaust manifolds and also acquire a pressure gauge and an air hand pump. 

Here is a pic of a plumber pipe plug on the exhaust outlet of a Honda Nsr250 engine. These plugs squeeze the exhaust walls and seal very well but can pop out some times. 
Next step is blocking the carburetor manifolds. This is the best place to position the pressure gauge and air pump. The gauge should be in the region of 0-15psi, as bigger readings will not be very sensitive to a possible pressure drop. Any style of pump will do the job well. Depending on application and cylinder configuration, you may or may not have to block all of the inlet manifolds. 
When everything is in place, it's time to make the test. Pump the engine up to 5PSI and wait for 10 minutes. If there is a pressure drop of up to 0.5psi at max, then the test is successful and the engine is assembled correct. 

If there is a pressure drop of more than 0.5psi, then use soapy water to find the leak. In the aforementioned test, left hand side of the engine tested fine but the right hand side had a pressure drop of about 0.5psi in only just a few minutes. Using some soapy water on a detergent spray bottle, a small air leak was revealed between the mating surfaces of the crankcase and RH cylinder.  
Sadly, even though a leak down testing is an easy job, most of the time any possible air leaks will require the dismantling of the parts that leak. Using higher torque values on the bolts etc is not the way to go and actually this might increase the air leak as well. I suppose it's your call, do it right and correct the leak or use a bigger main jet on the carb and live with it. But I believe if you've gone that far you have already took a big breath and started picking the right tools. 

Have fun, keep the two strokes alive. 


  1. Good job, thanks for inspiration.
    I think in addition to the pressure test one should also see whether the engine can hold a vacuum. Crank seals might seal well when under pressure from the crank side but may leak when under pressure from the outside.
    Also, I would suggest that the test is performed independently on the two cylinders to find a leaking centre seal.
    And: Wouldn't it also be good to check whether the reed valves seal well?

    Keep up the good work, I enjoy your blog!

  2. Thanks for your comment. Keep the two strokes alive.

  3. What? if any amount of air leak from an exhaust flange is acceptable? And what would the results (consequences) be on 1 cylinder? Bike will not hold pressure after 5 minutes. Is exhaust leak effect bike the same way as other leaks?

  4. Hi. I'm not sure I'm following you but even though air leakage from the exhaust flange is not as critical as e.g an air leak from the crankcase mating surfaces or cylinder base gasket... it should be avoided as well.

    That said, you cannot test exhaust flange air leaks with the aforementioned device as the exhaust is not bolted on it during the test and actually the air pressure at this point is quite lower than the crankcases itself.

    Exhaust air leaks will have a consequence of poor performance and in extreme cases can damage the engine due to lean mixture.

    I hope I've answered your question and you understand my messy English. Have fun!

  5. Thank you for the reply, I will try and explain. At the end of the drop down pipe there is a flange seal, and then the flange (place where the exhaust hooks up with springs), the bike is leaking at the flange seal. It is a honda rs125. How much is too air leak is too much? I am just wanting to do a track day with it. Not proper racing.........

  6. I'm aware of the issue that you describe but I cannot answer with numbers. You will have to seal the pipe the best you can (e.g use High Temp silicon gasket like Permatex) and if you feel that mixture is lean, check your spark plug and adjust the carb accordingly. In case you're not aware, I'm also fabricating expansion chambers for the NF4 models with good results so far.


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