[RGT]

Thought this might be of interest

[Rick Sperko]

Cool. Thank you for posting this.

[Blltrdr]

Thought this might be of interest

I'm not sure of the relevance. I have heard that cutting some of the flat off may help limit the noise produced by the duckbill. I cut approximately half the flat off mine to quiet it up, and it seems to have worked.

I do have an extra duckbill, maybe I will cut it back as advised and give it a go. I think I would limit the amount to no less than 3/16" because I would think at 1/32" the rubber may distort and not seal correctly. One could take 1/8" snips at a time and see how the breather reacts as the bike is running from idle on up.

Mine seems to work fine half cut so it is hard to figure the real advantage of cutting more. Maybe Ace can chime in and put in his two cents worth.

[ace.cafe]

As I see it, shortening the lips of the duckbill reduces their mass, and would make them faster acting.

That's the only thing that I can think of that would be of benefit from the mod.

I might try it on mine, and see how I like it.

[RGT]

I had cut mine down to an 1/8" I was thinking you might get a little better outflow with a shorter restricted distance and thus a little better scaveging.

[Blltrdr]

I had cut mine down to an 1/8" I was thinking you might get a little better outflow with a shorter restricted distance and thus a little better scaveging.

How long since you did the mod? If done recently please keep us informed how it works for you.

[mrunderhill1975a]

Would the shorter lenght reduce the "back pressure" from the piston decending in cylinder, thereby giving a slightly freer flow?  Any amount of free flow or less restriction would help the engine power, but  how much improvement could this possibly make.?

What is the reason for the duckbill in the first place? I assume it is to prevent dirt from entering the crank area during the compression and exhaust stroke.

[ace.cafe]

Would the shorter lenght reduce the "back pressure" from the piston decending in cylinder, thereby giving a slightly freer flow?  Any amount of free flow or less restriction would help the engine power, but  how much improvement could this possibly make.?

What is the reason for the duckbill in the first place? I assume it is to prevent dirt from entering the crank area during the compression and exhaust stroke.

The purpose of the duckbill is to provide a one-way valve in the breather line, so that the engine crankcase pumps down to a low pressure inside, which reduces pumping losses and helps ring seal, and improves fume evacuation.
If there's no duckbill or other one-way valve on the breather, the breather system does not work as intended.

[mrunderhill1975a]

Low pressure in the crankcase would only be have a power advantage during the intake and power stroke. Low pressure in the crankcase would have an equal and opposite power disadvantage during the compression and exhaust stroke.

Low pressure in crankcase would reduce the amount of oil being forced past the rings into the combustion chamber..

[ace.cafe]

Low pressure in the crankcase would only be have a power advantage during the intake and power stroke. Low pressure in the crankcase would have an equal and opposite power disadvantage during the compression and exhaust stroke.

Low pressure in crankcase would reduce the amount of oil being forced past the rings into the combustion chamber..

Is this a test?

The pumping losses result from air being pumped in and out of the crankcase, thru the breather hose on all strokes. If there is a check valve, we get no air coming back in thru the breather to speak of, and the crankcase is pumped-down to low pressure, and the low pressure in the crankcase serves as the suction engine for the fumes to be drawn down from the cylinder head and timing chest into the crankcase, where they are evacuated out the breather hose subsequently.
Once it's at low pressure, there's very little in the way of fumes needing to be evacuated out the breather, so it's very low pumping loss pushing them out, and none coming back in, so pumping  losses are reduced.
And yes the ring sealing is improved because less oil is pushed past the rings from pressure below. And reduces some oil leaks too.

All performance engines use techniques to reduce pressure in the crankcase, and in some racing engines with dry sumps, they even put vacuum pumps in to pull nearly a total vacuum in the crankcase.

In normal engines, the PCV system provides some of this effect.

[RGT]

I did it a little over a week ago, to be honest I don't notice any difference, nor did I expect to, I suppose you could tee in a vacuum gage before and after making the change and maybe see if there is any difference. I would imagine it would be a pretty small improvement and as long as you retain a closing seal on the up stroke you are not making anything worse. The big improvement I have made is from recomendations here in routing the vent/purge line up from where it exits the engine I now have a perfectly clean drive side of bike. The duckbill rest just behind the timing cover where the ignition drive comes off. I do get a very light sheen of oil where it points but no droplets or mess. I have also been running my oil level 3/8 of an inch or so below the full mark so that may be helping too. I have been getting the bike out for short rides every day which may also help, when I was away fo 6 days I did get a bit of smoke out the exhaust for a little bit a while warming up. Next time I plan on having it be idle for a few days I will leave it with the engine at tdc and see if I lose the wet sumping smoke....  

[Ice]

Thanks ACE.

[ERC]

Ace is right the main reason for the crankcase pressure on bikes is they are a form of dry sump with a seperate tank or cavity for the oil to reside in. If I can remember back to the 50s the motors in cars that I worked on just had a pipe going out on the road, If this became blocked the crankcase pressue would rise and gaskets would leak, burn oil etc.  ERC

[bullethead63]

Sweet...thanks for posting this,RGT...I love all of this old low tech stuff...typos and all...

[mitchell]

i find if i havent ridden in a couple of weeks, about a quarter cup of oil will pour out of the tube when it cranks. so, i pull it out onto the grass so as not to oil up the driveway.
a mechanic in india told me to just bend/crimp the tube back into the fins when i start it and after a minute unfold it and go.
just wondering why it happens in the first place.
any thoughts?

[ace.cafe]

i find if i havent ridden in a couple of weeks, about a quarter cup of oil will pour out of the tube when it cranks. so, i pull it out onto the grass so as not to oil up the driveway.
a mechanic in india told me to just bend/crimp the tube back into the fins when i start it and after a minute unfold it and go.
just wondering why it happens in the first place.
any thoughts?

It happens because oil settles down into the sump, after you park the bike.
It can be from various reasons, but the most common one is that the bike gets parked with the crankshaft down near BDC, and the oil runs out the crankshaft and into the sump. It's called "wet-sumping".

[elan]

I wonder why RE didnt modify the pipe starting with the next batch, they are still making them the same today!

[mitchell]

thanx ace. it is just so curious. when i am on the move daily, it never happens. and not always after a week or 2 off duty.
if i park the bike for the night in tdc, will that stop it from happening again?

[ace.cafe]

thanx ace. it is just so curious. when i am on the move daily, it never happens. and not always after a week or 2 off duty.
if i park the bike for the night in tdc, will that stop it from happening again?

Yes, that should cure it.
There can still be a little oil that can drain back down thru the oil pump lines, but that is minor.