[TrialsRider]

Not ALL CV carbs need a velocity stack. Most, maybe.

Show me one that doesn't need a velocity stack. If he looks at the parts he took off that motorcycle, it will include the stacks and that would be a darn good indication they are needed.
... maybe those pods have stacks inside them but I seriously doubt it and that would account for his need for additional tweaking .

Did you see his crankcase vent, how long will that work before it is saturated with oil and starts spewing black mung all over the back of the engine?

 

[jsmith800]

my issue is going from idle to mid range the engine stumbles some. Top end it screams rpms. velocity stacks look sweat but prefer pods for air intake quality, ingesting dust and gravel would be bad for the engine, velocity stacks have none to minimal filtering. i increased my pilot jet slightly, added a shim under the needles and adjusted the air/fuel mixture screw. still stumbles before it goes from idle to mid.

 

[TrialsRider]

my issue is going from idle to mid range the engine stumbles some. Top end it screams rpms. velocity stacks look sweat but prefer pods for air intake quality, ingesting dust and gravel would be bad for the engine, velocity stacks have none to minimal filtering. i increased my pilot jet slightly, added a shim under the needles and adjusted the air/fuel mixture screw. still stumbles before it goes from idle to mid.

Yep, it's called flat spots. Put some stacks on it and it will tune.
Velocity stack has nothing to do with air filtration, that's a different part, the manufacturer did not put them on there for looks or because they wanted to increase the production cost, they were on there because they are necessary.

 

[Classic250]

my issue is going from idle to mid range the engine stumbles some. Top end it screams rpms. velocity stacks look sweat but prefer pods for air intake quality, ingesting dust and gravel would be bad for the engine, velocity stacks have none to minimal filtering. i increased my pilot jet slightly, added a shim under the needles and adjusted the air/fuel mixture screw. still stumbles before it goes from idle to mid.

When we are talking velocity stacks, it's not the open stacks seen on some carbs like you mentioned - it's an extension from the carb to the intake, like a long intake manifold....at least that's my understanding of the term.

 

[TrialsRider]

Look at the parts you took off, you will find stack shaped intakes.

 

[jsmith800]

Ok now i understand wat velocity stacks are, yep they were the black inlets between carb and airbox. sorry

 

[TrialsRider]

Ok now i understand wat velocity stacks are, yep they were the black inlets between carb and airbox. sorry

Put them back on and your tuning problem will go away.

4-stroke piston reciprocating engines All have piston blow-by and the crankcase is vented to atmosphere so water ends up in there too. The crankcase vent needs to vent somewhere and the airbox was designed to collect the mung that comes out of that vent. You need something that does all that and does not end up restricting air movement ever, because the crankcase needs to remain at atmospheric pressure.


going riding on my go fast motorcycle now, it has stacks too

 

[DesmoDog]

You need something that does all that and does not end up restricting air movement ever, because the crankcase needs to remain at atmospheric pressure.

Now explain PCV valves and vacuum pump kits that keep crankcase pressures negative?

Engines were running bare/filtered tubes on crank vents for ages before they began routing the hoses to the airbox. Every narrowcase Ducati ever made for example.

 

[TrialsRider]

Now explain PCV valves and vacuum pump kits that keep crankcase pressures negative?
...

Sure; the lower the forces are below the piston, the higher the forces are pushing the piston down.
You planning on building a dragster that has a lot of blow-by it might yield you an extra horse or two.
Just as with a PCV the unburnt fuel and oil residue is returned to the engines intake path, where it can go through the combustion process for a second time and potentially reduce hydro carbon emissions released into the atmosphere.

Your old Ducati crankcase ran at atmospheric pressure and spewed unburnt combustion products directly into the atmosphere. The crankcase is still operating at atmospheric pressure. If you want to destroy the engine just plug that crankcase vent and watch your slowly engine self destruct.

... do you know the symptoms of a clogged PCV valve?

 

[DesmoDog]

Sure; the lower the forces are below the piston, the higher the forces are pushing the piston down.
You planning on building a dragster that has a lot of blow-by it might yield you an extra horse or two.
Just as with a PCV the unburnt fuel and oil residue is returned to the engines intake path, where it can go through the combustion process for a second time and potentially reduce hydro carbon emissions released into the atmosphere.

Your old Ducati crankcase ran at atmospheric pressure and spewed unburnt combustion products directly into the atmosphere. The crankcase is still operating at atmospheric pressure. If you want to destroy the engine just plug that crankcase vent and watch your slowly engine self destruct.

... do you know the symptoms of a clogged PCV valve?

Yeah... I was commenting more towards a PCV restricting flow in one direction and a vacuum pump keeping pressures below atmospheric, and how neither of these plays well with this statement:

You need something that does all that and does not end up restricting air movement ever, because the crankcase needs to remain at atmospheric pressure.

BTW the exhaust pipe on an old Ducati spews more unburnt combustion products directly into the atmosphere than the crankcase vent ever will. Pretty much typical for a bike running carbs that isn't jetted EPA lean.

 

[TrialsRider]

Crankcase vents to the airbox by convention and the intake creates a vacuum pulse that is in perfect time with the crankcase vent timing. Keep the airbox and there you have your less then atmospheric pressure, 4-stroke engines create a vacuum.
We are talking about 4-stoke engine basics here and if you put a stupid little paper pod filter on the crankcase vent you are asking for problems, if you just dump it on the road well good luck with that, it's illegal at the track and it's illegal everywhere else.

 

[DesmoDog]

I really don't mean to draw this out into a big deal so my last comment.

Your other diagram/post made me realize a distinction I was overlooking; the PCV connects after the throttle, where manifiold vacuum is present. The vent connects before the throttle, where there is no manifold vacuum.

On a stock 900ss from the mid 90s there is a valve that sits on the crankcase, then a hose that runs to a couple of collector boxes/expansion tanks, then another hose that runs to the airbox. It is not uncommon for people to connect a K&N filter on the end of the hose out of the valve and ditch the tanks/airbox connection.

With that configuration, you have a valve in the system that restricts flow in one direction, and a filter on the hose that never gets "mucked up". My mistake was considering that valve to be a PCV valve, it's not.

Illegal? I'm no lawyer but the chances of getting into trouble because my '74 Ducati had a filter on the end of the hose instead of being run to one of the air cleaners was slim to none. Illegal with regard to racing rules? Sure. You have to pass tech. Replace that filter with an old olive oil can and you're good. Use a beer can if it's not an Italian bike. If you want to get really fancy you can build a can with the hose on one fitting and a filter on the another. Best of both worlds.

 

[TrialsRider]

.... It is not uncommon for people to connect a K&N filter on the end of the hose out of the valve and ditch the tanks/airbox connection. ...

Not an uncommon but silly idea. If that could work every engine manufacturer would be doing it that way to optimize profits.

Have you ever tried breathing through a oil soaked felt cloth? You can't.
Keep the air box, keep the velocity stack, keep the working crankcase vent and dump the silly pods. Your engines will thank you for it.