Tube Exhaust

Tube Exhaust

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Why would I be getting suction from the exhaust? Timing appears to be correct.?

Using the kick start to turn the engine, by putting my hand over the exhaust tubes I get suction for a moment. Bought bike as a non-runner, but timing belts appear to be in the correct position. Ducati Desmodromic valve system. Cagiva Elefant 650

Maybe because ,

The Exhaust Valve is OPEN while the Piston is Moving Down and away from TDC?

Exhaust Valves OPEN before Bottom Dead Center(BBDC) so that Valve has time to be Fully Open when piston goes UP on Exh Stroke.

If there's NO Combustion Pressure during "Power Stroke",,,
Cylinder WILL be under Vacuum when Exhaust Valve Opens.

RIGHT?
Piston is moving Down the Bore while all valves are Closed.
If there's NO Combustion,,,then there Will Be Vaccum.

It VENTS when Exh Valve begins it's cycle,,
Which Begins before piston hits bottom.

ALSO,,A Secondary Vacuum Pulse in Exhaust Tract occurs very briefly at TDC on INTAKE Stroke.
It's Variable.

Occurs During what's called OVERLAP Period.

If Throttle is CLOSED .
while Exhaust Valve is still open during Overlap period,,,
A Vac Pulse will occur in Exhaust Tract.

Piston is Going DOWN the Hole,,,
Closed Throttle is Not Air Tight,,,but compared to the Lesser Restriction of an open exhaust,,,
The Piston will aspirate thru the Exhaust.

It wont pull any vacuum on Carb Inlet until EXHAUST becomes the restriction.

Exhaust remains path of least resistance during Overlap,,,until Throttle is opened.

Airflow dynamics thru an engine
During Starting or Cranking
are entirely different than when Running.

You get a consistent Exhaust suction pulse at end of Power Stroke,,,
and a variable exh suction during Overlap.

Think about it.
Piston falling in a Dead & Sealed Cylinder.
Before piston reaches BOTTOM--
then the Exhaust Valve opens.

If ya DONT have a momentary Vacuum Pulse in exhaust tract,,
THEN ya have a problem.