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Conquering the Himalaya(s) Part 4 - Needle jet

Now that the idle mixture screw is responding correctly and with the idle mixture set, it’s time to move onto the mid-range mixture which is a function of the needle and needle jet. The Himalayan is fitted with a vacuum style CV carburettor which was the mainstay of motorcycle carburettors prior to the emergence of fuel injection. The design is simple and reliable and has the advantage of a degree of automatic mixture compensation as engine revs and load vary.

The system uses three complementary jetting systems, the pilot jet (for idle and low speed mixture), the needle jet (for mid-range mixture), and the main jet (for higher throttle opening at heavy engine load).

The mid-range throttle response on the Himalayan felt too lean and I found that the engine was harsh under slightly increased load. This area of the mixture range is controlled by the vacuum operated needle and needle jet.

Helpfully, the four screws which secure the carburettor’s top cover are easily accessible by removing only the seat and fuel tank, making it a painless process to remove the cover and lift out the vacuum diaphragm, slide and needle. The needle is secured into position inside the slide by a small three-legged clip which is released by turning it one third of a turn with a Philips head screwdriver. Once released, the needle and small retaining spring can be lifted out.

The needle used by Royal Enfield is a fixed type, i.e. it does not have the five grooves cut into the needle commonly found on many Japanese carburettors. Never mind, a dive into the jetting box produced a 0.5mm shim to place under the needle circlip (thus raising the needle higher out of the needle jet by 0.5mm). Then a simple reassembly of the carburettor, tank and seat and we’re ready for some more test miles.

The difference was instantly noticeable, with much smoother mid-range power (let’s face it, it now has mid-range power) and a further significant improvement on trailing throttle. However, not being a fan of snap tuning decisions, I rode the bike over the next few weeks before deciding that a further lift in the needle would be beneficial.

This time I added a further 0.3mm shim (thus lifting the needle a total of 0.8mm). Again, the mid-range was smoother. The maniacal popping is now gone from the trailing throttle and the whole feel of the bike is smoother, stronger and far more enjoyable to ride.

With just over 1,000km on the bike the engine feels more at peace with itself and, with the tuning adjustments to date, much smoother and stronger than it did initially. So now it’s time to consider a change in the gearing to facilitate some more open road time, and perhaps some changes to the main jet.

More on gearing to come…

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