Breather box - Rex Bunn breather alternative

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models

Updated: 2011 December 06

Thanks to Rick Duarte who posted this information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Rick's own words:

I installed the Bunn Breather on my eldo, testing it, and reporting the findings back to Rex Bunn, as he has an old Guzzi himself that he has been using as a test mule.

What I did first was removed the stock breather and hoses, then removed both rocker covers, I covered the end of the small oil return pipe on top of the case, and hooked up a brass plumbing fitting with a ³⁄₈ inch barb on the larger breather pipe. I attached a piece of hose to it, and did a blow test confirming I had good air passage from the crankcase up to the rocker box. Air passes mostly through the 2 cutouts in the block where the lifters set into. I can block the small oil return hole in the head with my finger, and still blow air freely in through the engine.

I then drilled, and tapped both my rocker covers to receive brass fittings, where I hooked up a piece of clear hose, then the bunn exhaust valve in each hose (fuel line) going down by the transmission to the bottom of the bike, the valve sits about mid way in the hose, near the carbs. On the brass barb I installed in the stock breather pipe (big pipe) I hooked up the Bunn intake valve, and two filters, the bunn supplied small filter, and a UNI breather filter on the end (very dusty here in southeast Arizona).

When you run the bike, you can see the flapper/disc in the valves fluttering, and air moving from the crankcase through both rockers, like an air compressor, I even put a latex glove on one of the exhaust lines, and it inflated it!! you can feel the air being drawn into the intake line, and if you block its end, it stops the glove after a few seconds...I think the pumping action of the pistons will actually draw a vacuum (slight) in the crankcase, thus Bunn designs it free flowing on the exhaust with 2 valves, and a slightly more restrictive intake with the filters and one valve to try and maintain a slightly sub atmospheric pressure (to prevent oil leaks) in the crankcase.

After riding the bike at speeds up to 80 MPH, there is no oil out of the rocker hoses, and the engine runs freer, with less compression braking, and it does seem to rev easier. So far, the bunn system works great as a bottom > Top breather setup. I will try to put up some pics.

Additional explanation from Rick on how the Bunn breather works:

How it functions in a nutshell is since you have a pumping that occurs under the pistons, due to the air they move in what is basically a sealed space (except for the two hoses on top of the crankcase for the breather) if you put a one way valve at one end allowing flow only out, and a one way valve at another place allowing air to only enter the sealed space every time the pistons move down and displace air, it will exit the crankcase, when the piston goes to TDC, it creates a sort of vacuum in the crankcase, so at this point the valve that allows only air IN opens and outside air enters the crankcase. Its just like how an air compressor works! Basically you are constantly bringing fresh air into the case at the intake one way valve, and flushing out through the two exhaust one way valves, essentially you are moving fresh outside air into your crankcase at a rate of about 2 liters per minute, while the engine is running. This frees up the engine, as there is no resistance when a piston goes down the bore, as the displaced air simple exits, pushing open the one way valves. It even makes a funky sound like an air compressor out the two exhaust hoses. you can see the flappers in the valves moving rapidly as the engine is running, and you can feel the air coming out the 2 exhausts. Its amazing how much pumping takes place BELOW the pistons. I hope this helps....