All internal combustion engines experience positive crankcase pressure. Primarily a function of blow-by gasses. Unburnt fuel/air, water vapour and combustion byproducts leaking past the rings. Into the crankcase space. As the engine wears the problem gets worse.
- contaminates engine oil
- contributes to sludge build-up
- causes corrosion
- promotes leaky seals
- introduces windage (drag on the rotating crank caused by an oil cloud in the crankcase)
The parallel twin configuration – both pistons moving in unison – overlays alternate high/low pressure pulses onto this.
Solutions are either make it easier for the pressure to escape or create a vacuum (optimally 14-15 Hg/7 psi crankcase vacuum. More, sucks oil from the valve guides, rings and bearings). Vacuum can be created by pumping, one-way PCV valves, routing through the intake or exhaust and by dry sumps (the option used for Yamaha’s second foray into 4-strokes, the TT/XT/SR500). Systems can be open or closed.
Being 2-stroke people Yamaha knew a lot about crankcase pressure. Their first foray into 4-strokes, the XS650, gave them a few headaches however.
Originally they opted for easing pressure escape. A baffled, single orifice, open system vented to the atmosphere. The amount of oil/air was rather significant. This leaked like a true brit. Later, after experimenting with venting and baffle options they chose to run with a closed system routed through the air intake. Vacuum.
baffled, single orifice breather vented to the atmosphere … produced excessive oil … crankcase oil capacity spec 3.0 ltr
early single outlet breather
the next three generations of breather … twin orifice straight and angled and single orifice reduced box size, restricted outlet … baffle configurations were also changed throughout the series
The first 9685 machines apparently had a single outlet breather with single hose venting to the atmosphere. This was followed by a straight 2 spigot outlet, same baffles, also vented to the atmosphere. Spigots were changed (angled down) and baffles reworked. No effective change to excess oil leaking. They even tried plugging one spigot completely. Over-pressured the crankcase causing further leaks and blown seals.
Eventually vacuum was added by routing the 2 breather hoses directly to each air filter box. Closed system. Restriction plugs were added to each spigot. Again the baffles were reworked. The air intake becomes the vacuum source. Basically the engine is setup to consume it’s own blow-by gasses. Not brilliant. It coats the intake in oil residue, promote carbon build-up on piston crowns, combustion chamber, valves and lowers the effective fuel octane rating. The vapourized oil ignites at lowerenergy levels than 87 octane gas. The more that enters the cylinder, the higher the potential for detonation. A real issue for forced induction systems. It is, however, efficient, practical and good for the environment.
In the end the baffle box was resized. New baffle. Single reduced outlet. Single hose, split to feed each air box. Easier for carb removal.
steel wool baffle in original breather boxes … if this gums up pressure will have a problem escaping
baffles … left hand required 2 gaskets – this was modified to sit inside the box (later this was again modified, shortened, can just see it in the middle box, and the 2 plates shown were added – no steel wool) … the later box had a simpler single plate baffle as shown
baffle plate modified to sit nside the box … this was later shortened (as shown above) and 2 extra plates added
need to take care when using silicone type sealents – too much can block the return hole … oops!
venting to atmosphere, single hose, exiting above the chain
breather box with restricting plugs
Interestingly, early models specified 3000cc oil. Shell Thuet swore this was too much (and was ignored) only putting 2500cc in his motors. Eventually (’74/75) Yamaha’s engineers saw the light. Solved much of the oil leaking problem.
early cases – cast with oil spec, 3000cc – too much
eventually yamaha accepted Shell’s advice – 2500cc
specs modified again to allow for oil filters (when not removed during oil change)
tech bulletin M5-051 – 1975oil specs and dip stick modification
early open system vented to atmosphere
closed system schematic
early closed system airbox … blow-by gasses introduced after the filter, brfore the carbs – both sides
A common mod today is swapping stock air boxes for pods. Loses the vacuum effect (and may require rejetting). A one-way power brake valve can help here. It allows positive crankcase pressure to escape and prevents air returning when the pistons resume their stroke to TDC. A small vacuum is created helping seal rings and prevents oil leaks.
pod filters with breather pipe routed into the end … closed system
another attempt at replicating the original closed system
I have seen people use a short hose positioning the check valve over a pod filter. Any oil drips on the pod recontaminating the intake (and on only 1 side). Others pipe from the check valve back below the engine so any oil drips on the road. Hopefully off to one side and not on the tyre. OK if you enjoy cleaning your bike. If adding a filter ensure the hose ID and filterinlet OD match (12mm).
Using a catch-can resolves this and is relatively simple.
open system with catch-can
simple catch can
Fred Fleury’s soda catch-can
catch-can with filter … chromed
mine, made from an ikea pepper shaker
Comparison of differing methods of dealing with positive crankcase pressure.
|
|
OF |
AF |
CC |
AP |
DS |
|
De-clutters the engine space |
? |
N |
N |
N |
? |
|
Simplifies crank case ventilation system |
Y |
N |
N |
N |
Y |
|
Helps remove more oil from the air |
N |
Y |
N |
Y |
Y |
|
Helps evacuate the crank case |
N |
Y |
Y |
Y |
Y |
|
Maintains or improves the stock flow capacity |
N |
Y |
Y |
Y |
Y |
|
Helps promote ring seal and prevent blow-by |
N |
Y |
Y |
Y |
Y |
|
Helps prevent oil smoke in exhaust |
Y |
N |
Y |
Y |
Y |
|
Helps prevent dip stick from popping out |
N |
Y |
Y |
Y |
Y |
|
Helps prevent oil leaks due to pressure |
N |
Y |
Y |
Y |
Y |
|
Helps keep intake track clean including carb body |
Y |
N |
Y |
Y |
Y |
|
Helps keep intake manifold/plenum clean |
Y |
N |
Y |
Y |
Y |
|
Maintains or reduces amount of pollutants |
N |
Y |
Y |
Y |
Y |
|
Maintains prevention of un-metered air from entering the intake |
Y |
N |
Y |
Y |
Y |
OF: … open flow … single or twin hose
AF: … routed through air filter
CC … routed through PCV valve and catch-can
AP: … air pump routed through catch-can + filter
DS: … dry sump
thexscafe 
Also one may modify the later one hose breather cap (1980+) for use on earlier engines by drilling out the hole to match the combined diameter of the two restrictive plugs and drill the inside plate correspondingly to match the surface area of the earlier plate.
The result is only one hose needed to route to catch can versus two hoses to join and then run to catch can.
Or if venting to atmosphere loosely shove some steel wool up the end of the hose to keep insects out.
I do agree that the smaller, single orifice box is worth putting on earlier engines, simply to make carb removal easier. If keeping original early air boxes some hose modification is required. Easy to route to a catch-can.
As for drilling the passage and return out I am not so sure. Yamaha went to the effort of continually reducing the outlet without changing the basic layout of the motor – and it works fine on the later engines as it is. Seems to me that increasing the hole sizes allows more oil to exit unnecessarily. I can see the benefit for performance and race engines but not really for daily rides.
Good point about the insects – have had that problem myself.