HOW TO CONVERT / TRANSFORM OLD THUNDERBIRD INTO THUNDERBIRD TWINSPARK

For starters just knock in an additional Spark Plug and you already have something worthwhile. And no, you don’t have to go for the Royal Enfield’s new unit engine, of the Thunderbird TBTS to have the Twin Spark Plug advantage. As I have been riding one for the past eight years.

A second Spark stands for cleaner and faster rate of combustion translating both into higher RPMs, better power and leaner burning. In fact far leaner and Fuel efficient then the Enfield’s AVL engine.

Coming with a laterally set Spark Plug, it doesn’t take much to realise that the combustion efficiency of an Enfield Bullet , both the 350cc & 500cc models, can be greatly improved by fitting another one, on the other side. An addition that seems almost essential when taking into account the position of the Decompressor valve, which is located on the right side of the Cylinder Head, almost mirroring the Spark Plug and even has same threading as the Spark Plug socket. Furthermore, the only technical requirement for such a conversion is limited to just a bit of innovative machining.

This piece is vital because the AVL engine like most Thumpers, utilises the Exhaust Valve for Cylinder decompression. Here a little cable-actuated lever on the Tappet Cover turns the Cam that lifts the Exhaust Pushrod slightly which in turn depresses the Exhaust Valve a little bit, just enough to effect a little air leak. Coming with same dimensions as the one from the Std. Bullet, the AVL unit is a simple bolt on fit.

Having completed the job successfully and being in possession of a genuine Twin Spark Plug Bullet Cylinder head, you have now elevated yourself into the coveted realm of the owners of an Alfa Romeo Twin Spark Roadsters, a Norcroft Royal Enfield or just an owner of a Bajaj Pulsar DTSI(whatever the acronym means). But wait; to actually get the conversion to work, the AVL Decompressor needs to work on your std. Bullet.

Insert the push rod back into the tunnel and seating it on the cam follower, check for clearances so that the washer does not touch the sidewalls. Next refit the rocket arm and fix the AVL tappet cover. After attaching the cable, activating the handle mounted Decompressor lever should have the push rod rising slightly. If at first you fail to detect any movement or it occurs to you as too minute to be of any effect, don’t press the panic button. This arrangement works only when it is needed, i.e. when the exhaust valve is closed and the push rod is at its bottom-most position. And because decompression is caused by a slight leak of the exhaust valve, a near microscopic gap suffices. Slowly kick the machine so that the rod depresses and try again. If there is still no movement try adjusting the cable for zero play. A bit of adjustments and tinkering would have Decompressor working.

The Ignition System : 

Care must be taken when linking the new Ignition Coil to the existing circuit as a parallel connection results in very weak sparks and a tremendous load on the battery. The connections must hence be made in series where the wires attached to the +ve junction of the existing coil should be detached and wired to the +ve point of the new coil. Meanwhile the new Coil’s -ve point should be wired to the +ve point of the original coil.

However, as I did my conversion long before eBay motors, I developed another practical solution to this one. Talking two single dry type 12v Ignition coils, I joined them by bolting together their outer metallic field plates and using the terminal of one for +ve and the other for -ve. The only drawback of this setup is a little difficulty in mounting it on the bike as the central exposed metallic part becomes a live part of the ignition coils’ circuit and needs to be heavily insulated. Thus, several feet of fabric dunked in insulating varnish were warped around the metallic part and then covered by soft foam strip and the entire unit was mounted beneath the fame’s backbone under the Fuel tank, attached by two adjustable clamp. A neat compact arrangement, functional as well for it has very short HT leads and is always protected from water.

It pays to start the Engine once before fitting the Rocker covers back, that way one can can hear any untoward sounds. If everything is normal, just close the engine and refit the Tank.

To get the first impression of the Twin Spark Plug conversion, start the engine and then remove one of the leads and notice how the RMP falls , refit it back and see how it rises. Once on the road the engine would seem torquier and acceleration more linear and faster. However the real benefit of this conversion with today’s high gas prices is at the refill station.