Breather pipe venting oil after Mikuni Single Carb fit

Hi Andrew,

Sounds to me running the breather to the rear of your bike will only shift your problem to the rear of the bike and most likely over the rider following you.

You say the engine was covered in a sheen of oil. I'm not sure how the engine was covered in a sheen of oil from the breather pipe when the breather pipe is aft of the engine, from my experience when the breather is dumping, it's basically from the battery box back that cops the oil.

I own a '72 Commando, I've had the original twin carb set up, single amal and single mikuni. Changing the carb will not affect engine breathing. When i first bought my bike, the breather went into the air filter. All this did was fill the filter with oil and create a mess. I pulled the breather out and ran it down below the gearbox. This meant getting a longer piece of hose and instead of going from the oil tank into the back of the air filter, I just continued it straight down from the oil tank to just lower than the lower gearbox mount. If you look under my bike, you can see it hanging down maybe an inch.

If the engine is covered in a sheen of oil, are you sure it's comming from the breather. You say your recently acquired '72 Commando.... Is this your first Commando? because there are a couple of other favourite oil leaks on Commando's that definately would cover the engine in a sheen of oil. One is the tacho drive, another is the 5/16" head studs at the front centre of the head. These studs can leak and stream oil down the front of the engine and back through the fins and down the back of the engine. The tacho drive oil leak will usually puddle just behind the tacho drive and smear back across the right side of the engine. leaking base gasket??? Leaking exhaust tappet cover gaskets. if you have a look at an exhaust tappet cover gasket they are much wider than the sealing surface on the tappet cover. The gasket can make a little dam inside the cover holding a little puddle of oil actually in the cover itself. This can leak down through the bottom stud hole and across the engine. You need to trim the gasket back so this dam does not form, allowing the oil to drain back into the head chamber.

I guess you could run the breather to the rear of the bike and go for a good test ride to eliminate, if, indeed the oil leak problem is from the breather or from somewhere else, because if the breather is spraying the rider behind from your rear number plate, it's not going to smear up your engine!!

Then you have to ask yourself why is the breather blowing so much oil????

The engine is obviosly not blowing smoke or you would have said so. oil rings could be ok, but compression rings might be leaking compression into the sump and causing excess blow by......

Do a compression test, do a leak down test.

I find a leak down test is one of the best, easiest, simple and most straight forward diagnostic methods you can use. Remember, rings will always leak in a leak down test, it's the amount of leakage you need to be aware of.

On the inlet cover breather. I have had this fitted to two Commandos for many years and am a great believer in it. However, as Steven says, you'd be suprised how much oil comes from this breather. When I first fitted it to my bike I ran it into a catch bottle and went for a long run. I ended up with the bike covered in oil and I lost a considerable amount of oil doing it.

Your crankcase breather on the '72 750 comes from that funny attachment at the lower rear of the crankcases, up to and into the oil tank, then there is the oil tank breather that used to run down to the air filter, but now you have it running straight to atmosphere. Correct...??? Check these breather hoses are the right way around connected to the oil tank. If they are the wrong way around, you will blow oil out!!!!

Maybe you need to experiment with a level of oil in your oil tank that suits your bike. You may have the wrong dip stick for the oil tank, meaning you may be overfilling the oil tank. I know this has happened before. My bike is not that fussy with oil tank levels, but I know of others that are. You should be able to give the bike a good thrash without worrying about oil blowing out from an overfull tank. let me know how you go, I can measure the dip stick marks on my oil tank for you to compare with yours.

I run my inlet breather hose to a tee piece fitted into the crankcase breather hose, directing both breathers into the oil tank, simple as that. I could prattle on about Bob's theory on cylinder head breathing, but you don't need that at the moment, but it was a definate improvement. Look at any performance engine and crankcase and cylinder head breathing is always important.

All the best

Bob

Thanks Bob

I think that I did indeed cause some of the problem - overfilling the oil tank. However when I got the bike there was still an oily residue coating the plate the the airbox used to fit onto (and out of which the breather vents.

You mention the inlet breather tube being joined to the drank case breather & both therefore being directed back to the oil tank. Kind of a closed loop system which sounds a very neat solution. Have I understand you correctly?

Andy

Hi Andy,

The breather as I have it is more like an extension of the existing system. I'd messed around with a few ideas on one of my Commando's. It was a circa '69 750. I'd collected a few bits over the years and had an 850 top end sitting all alone & lonely on the shelf. I fitted it to the 750 bottom end. It was a simple enough job, machine out the moth of the 750 cases to take the 850 cylinder spiggots and that was it, instant big bore. All the talk about 750 better than 850 etc etc I had the capacity difference in exactly the same bike.

One of the dramas was the 850 head was of the porous bastard variety, so oil leaked out of the alloy. This prompted me to look at ways to fix the problem. One was to get the head dipped in some sort of goop that was supposed to seal the alloy. That didn't work, probably because the alloy was so full of oil nothing would stick to it anyway.

Thinking about the regular crankcase pressure got me thinking that after the motor began running the pressure pulses were allowed to escape out of the cases via the crankcase breather. Remember mine was a '69 ish motor and should have had the timed breather on the end of the camshaft, but a previous owner had done away with that idea and installed the Combat breather thing at the lower rear of the cases, hosed to the regular spot on the oil tank. This appeared to work ok, but despite good fitting pistons, a nice bore and new rings (at various times, both as an 850 and a 750) I could still get a wet air filter. I pulled the breather out of the air filter and had it go straight to atmosphere below the gearbox. Note, oil did not pour out of the breather, it did not wet up the rear of the bike, but after a good brisk to hard ride it would wet up the air filter element to the point of being annoying. After I took the breather out of the air filter a frothy puddle would often appear on the shed floor under the hose after the bike had been parked.

On top of this the constant weeping of the porous 850 head. Considering the crankcase pressure (pulses) were able to escape from the cases via the breather, then any pressurisation of the cylinder head had nowhere to go. The way upstairs for pressure pulses was past the cam followers and / or up the inlet valve chamber oil drain gallery down the back of the motor. These routes already being chockers with oil would offer no real avenue for the pressure to escape or equalise with the cases while the motor was running, so in my thinking, the head would always contain a posative pressure.

Hey, I'm not talking tyre pressure, not even 10psi, not 5psi, I'm saying a posative pressure. Around this time a fellow by the name of Alan Lewer (I think!!) actually screwed a pressure gauge to a valve cover and measured it at about not very much psi. I'm sure it was written up in the NOC archives somewhere.

My reasoning is with a nice hot engine and hot oil with the viscosity of thin water with a constant not very much pressure behind it, the oil will find it's way out of where ever it possibly can. An hours ride, two hours.....all day, gives plenty of time for the nice runny hot oil to be pushed out of where ever it can find a way. Take the pressure out of the equation and just maybe the incentive for the oil to seep out of the tacho drive, the 5/16" studs at the front of the cylinder, out the rocker cover stud holes, weak base gasket, porous bastard 850 head.....whatever, might also go away.

There was lots of talk of check valves, PCV valves, someone on a US list even thought up an idea of having a vacuum pump running from the balance lines from between the carbs and on it went.

I drilled and tapped a 1/4" BSPT hole into a spare inlet rocker cover I had, screwed a fitting into it and ran a hose to a tee piece spliced into the existing crankcase breather and ran it to the oil tank as per Pa Norton would have wanted it!!!

Oil weeps from the bastard porous head were drastically reduced, no more frothy puddle of oil under the breather hose on the garage floor. I was excited..., so excited I did the same to my '72 Combat. It was a simple exercise. If it didn't work or I became nervous about the originality thought police I could easily replace the rocker cover with a new one, bury the bastardised item in the backyard and no one would know. I sold the '69 back as a 750 with the breather still in place. My Combat still has the inlet rocker cover breather and the world still turns without a frothy puddle under the breather pipe.

Why was it so...........I don't think or worry about it too much. Sometimes someone crawls in under the Interstate tank and points and laughs, I don't care, it worked for me.

At the same time, I might add, I know lots of other Commandos without the extra breather and they do just fine, while others sprayed a mist like yours and mine. One thing I will never do is paint all Nortons with the same brush. Whats good for one, ain't necessarily good for the next one. You gotta fiddle with your own and get it how it works for you.

Bob