Mark Tanner Racing.

As far back as the late 1980's I have been involved with gearbox kart racing. In those days it was the KTM 250 SX engine that powered us along. Today's favoured power plant seems to be the Honda CR250, whether it be the pre 2001 barrel induction engine or the more recent crankcase induction variant.

Experience of the large inlet port in the rear of the cylinder and unsupported piston skirt led me towards a move to the crankcase induction motor.

While looking around for the correct after market race engine components it became apparent that a proper 1 piece carburetor inlet manifold was not available and I was not too impressed with some of the 2 piece 'screwed together, sealed with silicon' variations that I had seen.

Having all the necessary design and manufacturing equipment to hand that I use in my 'day job' I decided to design and make my own.

So here you have it:-

Gold anodized 6mm 6082 (HE30) Aluminium mounting plate,
65 Shore Nitrile PVC rubber,transfer moulded and chemically bonded to the mounting plate,
Upstanding 'O' ring profile to optimizes the sealing against reed block.

cr250 inlet manifold, rubber inlet, 2002 crankcase engine,V Force 2 holder

The next component to come under scrutiny was the V Force 2 Reed Valve.

Having been told the horror stories of the petal retaining screws coming loose and dropping into the engine to damage the piston, head and cylinder I designed an alternative mounting plate to captivate the screws.

The new mounting plate is designed to captivate the petal retaining screws, alter the valve angle and incorporates a 'splitter' upstream of the VF2 moulding. With accurate measurement of the crankcases to take casting draft tapers into consideration a very close fitting design was achieved.

There is NO modification to the cases and the original Honda gasket is still used.
The close fitment and design shape of the mounting plate also serves as a 'stuffer' to reduce crankcase volumes and therefore increase pressures.
The original flange to valve body fixing screws are still used.

cr250 inlet manifold, rubber inlet, 2002 crankcase engine,V Force 2 holder cr250 inlet manifold, rubber inlet, 2002 crankcase engine,V Force 2 holder

         Cylinder modification

One of the problems with the crankcase induction engine is the cracking of the exhaust bridge in the barrel. The bridge cracks at the radius where the bridge intersects the exhaust port at the cylinder wall and then has to be repaired, by welding and re-plating of the cylinder bore.

To overcome this problem I designed (with CAD information gathered from the original cylinder dimensions ) a bridged exhaust port 'tube' that was machined from solid. The cylinder bore coating is removed and original exhaust port is machined away. The new port is inserted and welded into place and machined to the required port shape from the CAD model. 

The video below shows a short sequence from the final stages of machining the exhaust port. The cylinder is then re - plated, head studs fitted and ready for use. This modification removes the power valve flaps and the voids in the port surface giving an improved, less turbulent gas flow and more power.


In order to reduce the angle of the carburettor a 12 degree manifold for the crankcase induction engine is available. This brings the floatbowl on the Kehein carburettor down close to the crankcases. These are available to suit 39 and 41mm diameter carburettor spigots.

Straight manifolds for the Honda barrel inducted engine and the WIWA GASGAS engine are also available as are manifolds for Pavesi ICC and Open engine styles. Pictures and design spec's to follow shortly.

If you have any questions to do with these products plese do not hesitate to contact me.

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