The Sportster and Buell Motorcycle Forum - The XLFORUM®

The Sportster and Buell Motorcycle Forum - The XLFORUM® (
-   Sportster Motorcycle - Bottom End (
-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (

Hippysmack 31st July 2019 05:35


Originally Posted by dieselvette (Post 5766600)
Under hard acceleration, it is conceivable that oil is sitting against the rear of the crankcase and not being scavenged at all. Upon deceleration this should be picked up quickly and/or slosh to the front. .

Scavenge port is in the rear.
I agree but wouldn't that be in reverse?


Originally Posted by dieselvette (Post 5766600)
But at some point there's enough oil to keep the scavenge inlet covered, and at high RPM this would empty quickly - but at lower RPM, not as fast.

The scavenge is bigger than the feed side.
You will always have a spurt / no spurt condition from the return line in the oil tank.
Check it out next time you can.
There should be moments I'd suppose when it's pulling straight oil though.
High RPM is where you want the oil to return faster but inherent conditions hinder that.
In theory, feed / return ratio should be the same at idle as in high RPM.
It's other conditions that change that.
It's not the speed of the oil pump that "can't keep up".
It's other conditions hindering the pump.

Hippysmack 31st July 2019 05:49

Thinking out loud and may be full of it.
Mine lopes like Hell on startup.
Oil is thicker, no oil in the cup to drain from that little hole and air is pulled thru it.
Bunny says you get no measureable (extra) blowby on startup.
But that pesky hole...
Progressively, as oil heats up, it fills the cup, no air pulling in the hole.
Idle levels.
Just like when the oil cap goes back on.

I can't believe there is only a few of us seeing this with the cap off.

harley506d 31st July 2019 10:42

Well, just for peace of mind i just had to re-check my breathers were working.
Not as easy as you would think though.
The best way i found was to block with my thumbs over both ( and after a few seconds the motor lets you know it's not happy) and release each one individually. Both fine.
Also checked to see if there was any difference to RPM when the oil cap was loosened, no change.

dieselvette 31st July 2019 14:45


Originally Posted by Hippysmack (Post 5766605)
Scavenge port is in the rear.
I agree but wouldn't that be in reverse?

Fair enough then, oil slosh is not a factor worth considering.

bustert 31st July 2019 14:56

think "U_TUBE" effect
really not a vacuum but diff'l

Hippysmack 31st July 2019 15:13


Originally Posted by dieselvette (Post 5766661)
Fair enough then, oil slosh is not a factor worth considering.

Actually it is worth considering.
Think steep hills, on the way up and down.
Seems on the way down depend more on positive CC pressure.

Hippysmack 31st July 2019 15:16


Originally Posted by bustert (Post 5766665)
think "U_TUBE" effect
really not a vacuum but diff'l

There is a difference of course.
You mean on upstroke?

Hippysmack 31st July 2019 15:18

More food for thought.

Some potential air leak areas are in the pic below.
If these areas allow air to be pulled in the engine on upstroke, the added air will compound any other existing breathing problems.

60Gunner 31st July 2019 15:22

Maybe fine but could be better is my position.
As to venting out the cam chest needing a catch can, I'm not using one. Filter either.
I was using aftermarket breather bolts.
I think the question of the krankvent being an improvement would be best answered by someone with supposedly fine stock breathers trying it.
Didn't Tomcat already do this? Seems to me his were fine and went to the krankvent anyway and noted an improvement.

60Gunner 31st July 2019 16:27

I could move mine back to the heads. I don't think the location makes much difference. I can't test with or without the krankvent as my stock umbrellas are shot and I'm not replacing them when the krankvent clearly says it works best without them in the picture anyway and should be removed if using the krankvent there.
If I notice less dripping, I may leave it there. But if it drips any from there, I'm going back to the cam chest just for cosmetic benefit of no hoses dangling from the heads. I never could stand that mess or having to pull it and loosen the breather bolt just to adjust my carb.

All times are GMT +1. The time now is 04:44.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2022, vBulletin Solutions Inc.
XL Forum® - Linson Media LLC