Air Intake And Fuel System

STAGE 1:

Air Filter:

The Problem - The first component that requires modification is the current style air filter. This differs from the original in that the air box has been sealed and does not contain the cleaner element. It is now just for show while the actual air cleaner element is housed in the right hand tool box.

The two downsides of this design are the loss of storage in the tool box and the tortuous path that incoming air must now take on it's way to the carburettor.

The Solution - Actually there are two possible solutions. The first is to get a hold of one of the older style air boxes that actually house the air cleaner element. This looks the same as the newer one except that it has an opening at the bottom for the incoming air. Into this box you fit a K&N high-flow, reusable air cleaner element. The fitting of this requires a little modification of the air box but this is fairly straight forward. Click here to go to the instructions for doing this. These instructions come from Thomas and are posted on the Royalenfield Yahoo discussion group.

The second solution is to install a K&N (or similar) "pod" filter. This completely replaces the standard air box and clamps directly onto the carburettor. The unit pictured below incorporates a half chrome cover.

With the first solution you retain the original look but allow better breathing of the engine. The second solution does not look stock but allows even greater air flow, it does however increase the induction noise of the bike. Some people find this offensive, personally I like it.

Carburettor Jets:

The Problem - Increasing the airflow into the engine will cause the air/fuel ratio to be out of balance as there is now more air. This causes the engine to run in a lean condition which is detrimental to your engine and needs to be rectified. As it happens when the bikes leave the factory they are set fairly lean anyway.

The Solution - Replace the main jet and pilot jet with larger ones and move the needle jet position. You need to do this if you replace the muffler (silencer) with a more free flow one as well. If you are also going to do this then it pays to carry out both the air filter and muffler modifications first before re-jetting.

As this is not an exact science some experimenting may be necessary due to different climate conditions, fuel make up and octane ratings. Here is what I have found works for me.

With a clamp-on K&N filter and short muffler:

Main - 125   Pilot - 27.5   (500cc)

Main - 110   Pilot - 27      (350cc)

As this carburettor is a copy of the Mikuni VM28 you can use standard Mikuni jets. Take your old ones along to whoever you are buying the new ones from to use for comparison as there are a couple of different types of jet.

                                                                Main Jet                    Pilot Jet

You may also want to move the circlip which holds the needle jet down one notch. Try it in the original position first and move if necessary.

Once you have replaced the jets you will need to keep an eye on your engine to ensure that the bike is not running too rich (too much fuel) or too lean (not enough fuel). To test how the engine is running you need to do a "plug chop" test.

The following is an excerpt from Pete Snidal's excellent Bullet manual which is a must for any serious Bullet enthusiast and can be purchased online from www.enfield.20m.com/

"Mid-Range Carburetion Check - the Plug Chop

This is a road-test exercise. Put a plug wrench in your pocket or toolbox. Put on your hat and gloves, start the bike, and take it for a warm-up cruise to your test site. This will be a place where you can ride at half throttle for 30 seconds or more before suddenly cutting off the ignition as you pull in the clutch and come to a full stop at the roadside, in a place safe to do a little work. (You will likely need a slight uphill in order to be able to maintain half throttle for 30 seconds.) Be VERY Careful! If any other motorists are present, there is heavy danger they won't understand your reason for the sudden stop, and will run over you. Also, you must be in a safe place for working on your engine when stopped. Be well off the shoulder and stay aware of traffic coming toward you at all times!

Then, pull the plug and look at the colour. If it's white, you're too lean, and the needle must be raised a notch (the clip lowered.) If it's black, you're too rich, and the needle must be lowered (the clip raised.)"

STAGE 2:

Replacing the Standard Carburettor:

For everyday running around the standard 28mm Micarb is adequate however if you wish to gain greater performance then 28mm is not enough.

Although there are several options in carburettor the most straight forward is to replace the Micarb with a 30mm AMAL. These carbs have long been attached to British bikes and in fact the AMAL company in Britain is still running. Most Enfield dealers stock the AMAL jetted specifically for either the 350 or 500.

Below is a picture of the "kit" as sold by Hitchcocks in the U.K. or DRS Cycle and Classic Motorworks in the USA:

While fitting this straight onto the bike will improve performance, starting and idling the most benefit is to be gained by porting the head as mentioned in the section under head work.