Blow-by: How much is normal? CD125

[wright123]

Hello. I've been doing some work on my CD125 recently when I traced a mystery hose from the cam cover. With the engine warm and running, the hose emits a noticeable but small amount of white gas (in puffs that matches engine RPM) and occasionally sprays small quantities of petrol.
Research told me the hose was connected to the cam head breather tube, and a small amount of gas should be expected to escape. But something tells me that fuel most certainly shouldn't be expected...
What do we think, is this normal?

Cheers

 

[ancientdad]

Check the dipstick for fuel smell in the oil. If you're leaving the petcock on all the time, fuel could be seeping past the float needle and into the bottom end through an open intake valve, contaminating and thinning the oil. Not good for the bushed surfaces in the engine as well.

 

[RobMan]

A small amount of pressure/oil vapor is normal from the crankcase vent tube but should not be excessive. I typically see about one drop of oil from the vent hose on the sheet of brown paper I keep under my 450 to spot oil leaks after a ride. Fuel droplets/vapor is not normal although the residue from the vent tube will have a little gas smell. If you do smell fuel in the oil and you have been turning off the petcock then it may be leaking.

 

[stl360+450]

The white puffs could be water vapor escaping as moisture in the sump is boiled off. I have been cautioned in the past about making short rides which can promote accumulation of moisture in the sump. Getting the motor up to full operating temperature helps avoid this situation.

 

[ancientdad]

Should also mention that the oil level could be overfull if the petcock is leaking when off, or if you've left it on overnight or longer. When fuel dilutes the oil, it also raises the level in the crankcase which reduces space for crankcase pressure so blow-by gasses are more likely to push a little more oil mist, or even fuel/oil mix, out of the breather tube.

 

[wright123]

Thanks for the advice chaps. Working on the bike just now I gave the dipstick a whiff and surely enough there was a noticeable scent of petrol. I have forgotten to turn off the petcock before but that was a while ago so I'm more worried the petcock is leaking, I'll test it once I'm done cleaning the fuel tank and carb.
@stl360+450 Insightful, I noticed the puffs after a short ride around the block so I'll check again after a longer ride.
@ancientdad Also very insightful, this may explain why my oil level seemed to rise slightly. Previously I just assumed I was just remembering the level wrong (why would oil level rise in an air cooled bike?)
Again, thanks for the advice

 

[ancientdad]

(why would oil level rise in an air cooled bike?)

Exactly. As I've always said, these engines consume a little bit of oil as a way of life, which is why you should check the oil before every ride and every time you fill the gas tank - especially if you're out on a long (and potentially spirited) riding day. On a 150 mile day of mountain riding with its fair share of redline rpms in the gears, my 450 will go from the full mark down to the add mark.

 

[Tony Clifton]

Point of order: I have been vexed with leaking petcocks, the flow of which is not checked by one float valve or the other, and then I get dripping from the carb bowl overflow. A drop at a time, all day and night. Is this not the purpose of the carb bowl overflow mechanisms? Put another way, shouldn't one see leaking carbs as a symptom of petcock failure, as compared to expecting this excess fuel to run directly into the crankcase? Are there differences between various models that mean "it depends"?

 

[ancientdad]

Point of order: I have been vexed with leaking petcocks, the flow of which is not checked by the float valves, and then I get dripping from the carb bowl overflow. A drop at a time, all day and night. Is this not the purpose of the carb bowl overflow mechanisms? Put another way, shouldn't one see leaking carbs as a symptom of petcock failure, as compared to expecting this excess fuel to run directly into the crankcase? Are there differences between various models that mean "it depends"?

Yes, and not all carbs do that but for whatever reason some overfill the bowl and it seeps up into the venturi through the main jet emulsion tube I guess. I know for a fact that the CV carbs on the DOHC 450 are prone to it, first time I ever saw a bottom end way overfull was on a rebuilt 450 that sat for a couple months in the back of the Honda shop where I worked and the tech left the petcock on.

 

[RobMan]

Thanks for the advice chaps. Working on the bike just now I gave the dipstick a whiff and surely enough there was a noticeable scent of petrol. I have forgotten to turn off the petcock before but that was a while ago so I'm more worried the petcock is leaking, I'll test it once I'm done cleaning the fuel tank and carb.
@stl360+450 Insightful, I noticed the puffs after a short ride around the block so I'll check again after a longer ride.
@ancientdad Also very insightful, this may explain why my oil level seemed to rise slightly. Previously I just assumed I was just remembering the level wrong (why would oil level rise in an air cooled bike?)
Again, thanks for the advice

If you think the oil has been contaminated with gas I would not ride it until I had determined how the fuel got in the oil, petcock leaking or left on, repair and change the oil. Running the bike with the oil thinned with gasoline will very possibly damage bearings and machined surfaces such as the cam and cylinder walls.