kinda passed/failed leakdown but cant fnd leak.

[yamaha04]

hey guys.  been some time since iv been on.  finally got back on the banshee this year.  anyway   story little back story.    rode last year a few times.  bike ran good as usual.  this year same thing   2-3 short 30 min rips so far.  went to check fluids the other day and noticed I was down on trans fluid.  topped it up  took almost 3/4 liter.  seemed odd but I didn't think much of it at the time.   then I went for a ride and noticed my right side silencer smoking way more then the left. I stopped  smelled it . but couldn't really smell tranny fluid.    I rode it another 5 mins then came home   no change to the smoke.   I have replaced the packing   no change to smoke.  150psi both cylinders 21cc domes. pulled the motor this afternoon  did a leakdown   it will hold 5-6psi for 5-10 mins  so it would seem like its a pass but if I take it up to 8-9psi it will very slowly creep down over the course of 5 mins back down to 5-6psi.   iv sprayed the entire motor several times. iv tried moving the crank up and down  zero movement by hand  tried rotating it and looking for bubbles.    I did see 3-4 tiny little bubbles and I mean tiny.  they didn't really grow and blow up like an air leak does but they did appear non the less so at this point I think I'm just going to replace the seals. my question is.  can a crank seal pass a leakdown but still be bad.      

[JUSTWANNARACE]

Yes it can.. you are testing pressure (leak down down test pressurized from the inside out). You could have a vacuum leak (the seal not holding on the intake stroke). Most people don't think about the reverse pressure and only test from the inside out.  if you do pull it apart, check the crank snout for wear from the seal. If it is grooved it could be push against the side of the groove. So showing that it's sealed under pressure. But could be centering on the groove under vacuum  (ie sucking tranny fluid).. jmo

[JUSTWANNARACE]

A "true" leak down test on a 2 stroke should consist of a pressure test (which is what most people do) and a vacuum test!

[rjdgriff]

Yes it can.. you are testing pressure (leak down down test pressurized from the inside out). You could have a vacuum leak (the seal not holding on the intake stroke). Most people don't think about the reverse pressure and only test from the inside out.  if you do pull it apart, check the crank snout for wear from the seal. If it is grooved it could be push against the side of the groove. So showing that it's sealed under pressure. But could be centering on the groove under vacuum  (ie sucking tranny fluid).. jmo

This guy knows what's up. I would listen.


Giving away "TSS" for FREE!!!

[yamaha04]

Perfect.  I did think about the vacuum but didn't have the tools to preform it.   This answers my question perfectly.       Thanks very much.        I will post back how it is once I get it all back together   Thanks again 

[kawa110]

How is the vacuum test done?

[yamaha04]

I plan on building a vacuum setup up once I get it built.  Question now tho.  I understand you would watch the gauge for a loss in vacuum but how can you tell where it's coming from on a vacuum test ? 

[sleeper06]

Is the keyway on the pto gear sealed with sealer , it can creap through there slightly. Might wanna try pulling pto gear cleaning everything up to get rid of residual oil then reinstall and put keyway in with sealer and fill the shit outta the hole after install , put all gears washers n nuts together, let dry over night and test again. Could be as easy as that , worth a try before tearing down engine. 

[edgehanger]

I've had a leak drive me crazy before that I couldn't find. I was so frustrated that I submerged damn near the hole engine in water. Turned out that it was the seam on the boost bottle. I've seen leaks hide in between the cylinders at the base gasket. Check where the intake screws go in, on the back side of the cylinder, those are hard to spot sometimes.