Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

Piston Blow-By - Strange?

Discussion in 'Technical and Troubleshooting Torque' started by T.Depta, Jan 4, 2009.

  1. Ooh Ooh, I got another annoying question(s)!

    The senario:

    I'm idling my engine (VTR-250) just after filling with fresh oil and have the bike righted so that the site glass gives me the right oil height. I usually kill it then open up the oil cap and put some more oil into the hole to give me the full measure and restart it with it righted to make sure i have maximum level and not over - so far so good. Just yesterday I didn't bother with killing the engine and just opened up the oil filler cap and whoa....!

    Almost got covered in splattered oil!

    So, questions:

    - Is it a measure of excessive blow-by (read below also before answering) if bits of oil get shot out the hole?
    - I smelt the air coming out the hole and it did not smell at all of exhaust. I smelt of warm oil (actually quite a pleasant smell). If i have a V(90deg)-twin then shouldn't the air just move around the engine and not rather blow out the oil cap hole?
    - has anyone tried this? Your results?


    I put my thumb over the oil-fill hole with the engine idling and it really didn't want to push my finger away. It actaully oscillated my thumb when I totally covered it so I guess i don't have blow by

    So, more questions?

    I'm confused now. Each cylinder alternates at TDC so the air volume is static inside the crankcase...

    so why was my finger getting sucked into (althought very slightly) the hole?
  2. because the crank case fumes are getting sucked into the intake via vaccum. when the engine is off theres no vaccum and may have built a lil pressure.
  3. Oil fill hole has nothing to do with blowby. Blowby relates to the oil passing the piston and cylinder into the combustion chamber.

    The oscillations are due to the pistons moving in the bore - TDC = lower pressure, BDC = higher pressure. Perfectly normal. The reason for an osciallation is that the Cylinders are not harmonious - you're taking the air out of one cylinder and shoving it back into the other cylinder. the resulting high/low pressures from this turbulence is what you feel.

    Due to oil flinging from fast rotating bearings, gears flicking oil onto teeth, chains carrying and flinging oil around the valve area, oil weeping from hard bearings, and the clutch picking up oil and flinging it around, you get oil vapour forming - droplets of oil small enough to carry a while midair.

    In an engine, they hit any solid surface, pick up heat, form droplets, and eventually make it back to the sump. if you make an opening, especially with such a location as the oil filler (usually located over the clutch basket), you are taking oil and flinging it at several metres a second, as well as the associated vapour travelling via these high/low pressure oscillations, would lead to oil spraying on your finger.

    Also, as there has been no time for drainback, to check the level with the engine running is foolish. Best bet is to start the motor, idle and let the oil light go off, give it a few seconds, kill the motor and leave it for a minute or two.

    Oh, and used engine oil is a confirmed carcenogenic.
  4. Exhaust gas blow-by is what I'm referring to. I would imagine that a heavily worn engine would even have blow-by up thru the valve guides as soon as it opens.

    It is starting to make sense, however.

    AFAIK, the oil flinging out is due to a big gear near the oil fill hole. I guess I really didn't look into the hole with the smell and oil flicking out with the engine running. I just had a looky with the engine off and I guess that gear is the fling-producer.

    So, i guess a little investigating answered my query.

    The push/suction of the two pistons when working on opposite cycles moves alot of air inside the crankcase, but with machiney in the way of smooth airflow, the open oil-fill hole provides the path of least resistence for the air to circulate, hence, the mild push-out/suction effect on my thumb.

    Looking thru the service manual mentions crankcase rebreathers (or something) which I traced two tubes going up from the cylinder heads and one tube from the crankcase all joining up to the airbox. I guess this is what is the PCV system.

    So with a little prompting from you guys, I'm understand alot more of the intracasy (sp) of four strokes since i really only dealt with the basics in Year12 Automotives.... so long ago.
  5. I trust you are also letting the oil all drain back into the sump after your first fill so you are not over-filling the engine. Best practice is to simply fill according to the volume listed in the service manual.
  6. Yeah, oil rated capacity at full level is 2.1L. I usually manage 2L. Its not that i measure it, i just stick enough oil in as a guess, right the bike and start it. Give it about 5 secs, then off, and check the glass at vertical. Typically it will be near the min. level, so my guess was pretty good ...and becasue the funnel holds about 1.5L anyways :cool: . I guessitmate that the maximum should take 400-500mls. I get it about 1mm below the maximum level using that stop/start technique, then all is good.

    I use mineral oil. I do not see the reason for full-syn since i tried both and i could not spot any difference in performance.

    Valvoline XLD FTW! *

    Ha! Performance! ...not on that commuter class bike!

    *and I'm not changing it to semi or full any time soon.