Spotted this little modification courtesy of the very knowledgeable Mr Forchetto in the RAT forum. "Jesus!...not another of Forchetto's long-winded and tedious write-ups...yes lads, get the popcorn out... Removing the blowby gases from the crankcase is especially important on 360 degree engines as the simultaneous rise and fall of both pistons moves a huge amount of air within the crankcase. 180 degree engines on the other hand, tend to just move the air around the crankcase. Older bike engines had all sorts of weird and wonderful timed breather valves and oil traps, some so inefficient that makers directed the flow of oily gases to the drive chain and claimed it was "an automatic chain oiler". Most would just let the gases spew towards the rear of the bike through a long hose pipe. In the 70's emission laws came in banning open breathers so makers re-directed all the crap into the carburettors to be re-cycled and burned. These blow-by gasses effectively lower the octane rating of the incoming mixture by diluting the charge, displacing oxygen and, being hot, also lower the charge density. Both efects tend to harm Torque/power output. Oil mist carried in the gasses can also deteriorate the O2 sensors in the EFI bikes, and does no favours to things like the Air Intake Temperature and MAP sensors and other components in the throttle bodies. Redirecting blow-by gasses away from the air box also helps keep carburettors or throttle bodies and other internals cleaner. Paper filter elements soon become contaminated with oil and moisture as a cursory examination inside the airbox will confirm. They're often found to be oily and/or damp inside. Most of today's airboxes incorporate a more or less effective coalescing filter arrangement to separate the bulk of oil mist and moisture from the gases, this muck collects in a blocked-off tube under the airbox. This has to be drained periodically by removing a bung. Have a look under your airbox to see it. Doing away with this recycling of gases from the intake gives a consistent and measurable power gain, typically 1 to 1.5%. As an example on a 82-odd BHP Buell it has been found to give a nice and consistent 1 to 2 hp gain over a wide rpm range. The gain on a lower power Bonnie will be smaller but as a percentage it might be just over 0.5 bhp. On large-engined powerful cars as much as 5 hp has been seen. Recycling the blow-by, the way the factory does, cost a little power over a wide range. A little bit of the power gains noted by our posters on removal of airbox are to be credited to the enforced elimination of the breather arrangements by removing the airbox. I've done mine by replacing the pre-formed hose with a suitable 12 mm car hose formed into a "U" shape, some hose clips and a rubber-lined "P" clip to hold the filter on to one of the manifold bolts, have placed the small filter to stop muck getting in on full view, it's a nice chromed thing and adds a bit more bling as well as prompting questions from the general public: "What's that thing mister?". The hole that's left in the airbox can be blocked by a suitable rubber bung or screwing an old M8 bolt in there. The easiest way would be to use the existing hose, disconnected from the airboxand fitted with a suitable filter on the end. This can be tucked in close to the airbox, on top of the crankcase. It was easy - here's mine... using the original hose.
I like it. I did the same on my last Mustang Coyote motor. The factory design allowed oil mist back into the motor which eventually gummed up the TB’s. Question, is this mod basically the same on my Thruxton 1200 motor? Thanks for the post mate.
Great little mod, Steve. I wonder how often will you need to clean/replace that filter. I'm also assuming rain falling on the filter won't affect it? A couple more questions. Did you just put a plastic bung into the hole left in the airbox? And of course, do you feel any difference in power in the Bonnie at all?
One of the best mods I did to a BSA C15 was to drill a couple of half inch holes from the crankcase to the primary chaincase, then vent the top of the chaincase cover by welding a bit of old handlebar into it. Then ran a hose from the handlebar stub up along the subframe to the rear of the bike. I did it to fix oil leaks figuring that the oil would not be forced by crankcase pressure through the smallest of gaps. It worked a treat.
To be fair, my Bonnie hasn't sprayed any oil into the airbox. There's no sign at all in the box or in the breather pipe so I'm not too worried about too much oil in the filter. I imagine it might need changing every so often but not as a regular thing. I just used an old bolt and a rubber washer to seal the hole in the airbox. All seems fine but I may seal this later. No, I can't say that I have felt an increase in power but I like the idea of not recycling the "mucky" air from the crankcase back through the air intake.
The sceptic in me says these mods are for the inveterate tinkerers.....you know the ones.....the ones that keep topping up the oil and over-pressurise the crankcase in the first place. The bypass arrangement exists to recycle anything that blows by the piston rings and any oil film that would otherwise escape to atmosphere. Feeding it to the bottom of the airbox outside the filter makes a nonsense of any idea it will foul the carbs. It would whizz past injectors! I understand the visual appeal for anyone who wants to personalise their bike but let’s not kid ourselves it makes much difference!
I am not discounting what you say. My 865 carbed model produces the very lightest grease mark and no discernible fouling of the filter. If your 790 is otherwise then go for it. Manufacturers make loads of minor tweaks born out of experience without ever admitting why. If you use an oiled filter though, you might think of it as a Scottoiler!
All interesting reading so I just thought id take a look at that sealed bulb plastic catch tube thingey that comes from the airbox and hangs a couple of inches past the swing arm and there is a minute drop of odd coloured oil in the bottom of it not even worth draining off like sure reading somewhere that it gets drained off at service intervals if stuff is present in there on other bikes owned in the past there was a plastic bung in the tube that could be removed to drain tube on this bike would need removing from airbox to empty or fit a bung as im sure just cutting off and leaving it open would allow the gunk to get all over the rear tyre re-routing to an external filter may have its uses.
