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Modifications - Porting

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The following cannot guarantee success for your ETZ 250, but there is a book by A. G. Bell, "Two Stroke Performance Tuning" which gives the general theory.

General cleaning up of all the ports improves the gas flow. This helps at high revs as well as reducing the 3000 rpm bottom limit.

Smoothing the inlet manifold, inlet port, trimming gaskets (so as to not protrude into the ports) increases the bottom end response. Bevelling the cast iron barrel around the ports reduces the risk of the piston clashing with the ports.

Messing with the transfer ports is rather risky, so changing where they emerge onto the barrel inside is not advisable.

At The Barrel-Engine Case Interface

What can be done?

Ensure that your new paper/card gasket does not cover the transfer port at the edges. Smooth the ports - small fingers are necessary. Ensure you do not allow anything to fall into the big end/con rod hole, even dust from abrasive paper. Cover it!

 

The Inlet Port
  1. Gaskets must not protrude into the inlet tube.
  2. Very slight bevel inside of port, about 0.5 to 1.0mm.
  3. Barrel, aluminium casting is usually very abrupt, smooth this with very coarse carborunum paper then fine paper (or file).
  4. Smooth inlet manifold with wet and dry paper.
  5. Carburettor and inlet manifold need to be matched properly, file and paper down. Clean carburettor and reset.
Inlet Port Side View

 

The Exhaust Port

This has been opened up from the TS to the ETZ range, but you will still find room for improvement.
Enlarging the port improves top end power but it is quite near to the limit already.

Some modification to the piston can be made, but remember that metal is a lot more difficult to replace than to remove. So only take off small amounts then try the engine out.

 
Internal view of cylinder, showing exhaust port areas to remove
  1. Very slight bevelling of all ports makes the likelihood of piston ring/barrel conflict much smaller. This is important on the largest ports, i.e. the exhaust and inlets.
  2. Enlarging the port vertically by 0.5mm steps (+ slight bevel) increases the time exhaust gases can escape per stroke, enlarging the edges increases the amount of gas that can escape.

Do Not Enlarge Sideways Past
The Original Width.
  1. Slight bevel on port.
  2. Major filing/grinding and smoothing required on both aluminium and iron to give a smooth exit path for waste gases.
Side view of exhaust port showing areas to be removed

 

The Piston

The piston can be altered, instead of the exhaust port, by removing 0.5 - 1.0mm on the front of the piston - gentle gradient - very smooth - this is the hottest part of the piston in use.

 
Piston, showing beveling and area to be removed
  1. 0.5 - 1.0mm removed for increased top end power.
  2. Bevelled piston 0.5 - 1.0mm up the side to ensure no fouling with the ports and to allow petroil mix to slide up the barrel, reducing friction.

No Responsibility Can Be Taken For Missing/Untrue Information or Incompetence.

Remember,

 

  • Put every thing back - CLEAN - with proper gaskets (squish band 0.7 - 1.2mm)
  • If you remove too much from the top of the piston or exhaust port you get a reduction in power.
  • The carburettor settings/jetting may require adjustment for a leaner mix due to improved flow.

Many thanks to Kevin Holder.

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