The Twin Plug Enfield Bullet

When Royal Enfield motorcycle’s designers in Redditch blueprinted the Bullet, their intent was to create a workhorse and so emphasis was placed on mechanical reliability and not performance. As a result, the traditional Bullet engine packs a solid construction, one that can endure colossal mechanical fatigue and stress. Thus it doesn’t take much to garner appreciably higher power output from the motorcycle and one doesn’t have to be either Fritz Egli or have a state-of-the-art workshop to do it.

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.

Here, I would advise anyone aspiring to undertake this conversion to first get hold of a Decompressor set ( the tappet cover plate, the Decompressor lever, the cam, o-ring, cir-clip & cable) from the Enfield’s AVL engine, the one fitted in their Thunderbird & Machismo models.

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.


The Cylinder Head : The first step in adding an extra spark plug entails removing the cylinder head from the block, unscrewing the Decompressor Valve assembly and heading to the nearest reliable engineering work shop. The objective is to machine off the un-threaded top part of the Decompressor socket .
The easiest way to accomplish this is by taking an inch wide valve seat cuter and fitting it into a drill with the flat side out. Next, clamp the head into a vice and start boring. Make sure to go slow and occasionally withdraw the assembly to blow away the aluminum filings and monitor progress and stop when the threaded part is reached. Inspecting the finished work you will notice a tiny opening on the inner side of the newly drilled orifice. This is the passage through which the earlier Decompressor expelled air into the exhaust tract. Of no use now, it should be sealed tight for if left open it would pass out exhaust gases. A spot of aluminum welding would block the hole, so would a glob of epoxy putty, preferably the one mixed with aluminum.

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.

The Decompressor: Remove the Exhaust Rocket set and take out the exhaust push rod and unscrew the concave headed adjusting bolt at the bottom. Now you need to create a reciprocal surface for the Decompressor Cam that would be instrumental in lifting the rod, which in turn would, depresses the Exhaust Valve. For which you have to get a 2-3 mm thick HT steel plain washer with a 20 mm OD with the inner diameter corresponding to the threaded part of the adjuster bolt.

HT Steel Washer fitted as a lift ring for the
 AVL Engine  Tappet cover fitted  Decompressor cam.

Fixing the washer on the pushrod adjuster requires another nut, one with the same thread as the adjuster bolt. Trying getting the slimmest one, otherwise grind it into half its thickness. Check photograph to get the idea. As the new nut reduces the gap between the pushrod and the adjuster bolt, leaving little space for the locking nut, it makes sense to grind off about 2mm of the pushrod’s bottom.

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 : For the Twin Spark Plug Ignition system to work properly, we have to add another ignition coil. But, why another Coil, instead of just adding another HT lead to the existing Ignition Coil? The answer to this is actually quite simple. Yes, both the plugs would fire, but only under normal atmospheric pressure. Once fitted in the cylinder head and subjected to the high pressures inside, the electrical resistance of the air mixture increases many fold and only the plug facing lesser resistance in its gap would fire. A factor that becomes progressively more pronounced in higher RPMs. Having a second Ignition Coil to fire the second plug does away with this problem and in-fact improves combustion exponentially during higher RPMs.

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.

A Dual Output Ignition Coil.

The easiest way out is to head for eBay Motors and get a cheap Dual Output Ignition Coil the kind used in almost all modern in-line four motorcycles or in Harley Davidson and just retrofit it in place of the existing 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.

A home made twin output coil made by joining two 12v Vespa Ignition Coils.

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.

Want to retain the points??

In response to a couple of readers of this blog who queried me about the finner details of this modification, but expressed a desire to retains the CB Points instead of changing over to Eureka's electronic ignition, I recently carried out such a change on my 1992 Machismo. So here's the deal, you can carry out such a mod and all you need is another Lucas Bottle coil ( the small dry units just don't produce a strong enough spark) and a length of wire and a couple lugs and a mounting braket. The following diagram from a 70's era BMW Boxer twin would give you an idea about how the wiring needs to be done.

BMW Boxer's Twin Plug Single Points Diagram 

The rest of the modifications like machining the head and fitting a lift-ring on the exhaust pushrod are the same as with the electronic conversion.

Ungainly but practical location for the second Ignition Coil.

View from the Right Side, note the CV carb, a bitch to adjust,
but with the twin plug combo easily provides
an additional 5 km per litre fuel saving.

One word of warning thought, this setup seems to eat up points  and you better stock up on them. Though, so far I am not having any-high speed misfires  at slow speeds there are occasional misse due to sticking points. Adding a 2.2 uf 50 DC Volt condenser should do away with this problem, but thats for my next visit.

ALL MODS AT YOUR OWN RISK. THIS CONVERSION IS INTENDED FOR DIY-ERS WHO HAVE EXPERIENCE OF DISMANTLING AND ASSEMBLING AN ENFIELD CYLINDER HEAD AND PISTON AT LEAST COUPLE OF TIMES.

 THOSE WHO ARE UNSURE OF THEIR MECHANICAL SKILLS OR ABILITY ARE ADVISED TO TRY EASIER STUFF FIRST.