Forum

[vince]

best to ignore original jettings as petrol definitely different. In the shed is an exworks 250 road racer , the original factory notes from 1963 stated 300 main jet. it ran to rich on modern petrol so now runs on a 170! on the original it only pulled 6000rpm now pulls the correct 8500rpm.

[Tim NZ]

'new' unleaded is denser than leaded petrol, and a small (2-5%) leaner jet change is often called for...

[Scalyback]

Thanks guys,

I have a couple of other possibilities to try out, but have ordered a 210 and a 170 jet to experiment with.



WIll let you know how it goes when I get back IF there is not a metre of snow by then!



I wonder if Santa has a red Royal Enfield?

[Dennis C]

210 170 is probably a bit extreme, you should only need to drop one or two sizes.

[Scalyback]

oh, ok, well I have not completed the order yet, the original as per book is 250 (Monobloc 376) so maybe a 220 and a 190? would that be more sensible?

[simon]

I agree that the stoichiometry of modern unleaded fuel is very different to old leaded juice. I found that in order to tune my Ducati and the Bullet with the big carb it was a trial and error process. Sooty plugs are a definite no no but finding out when they get sooty is vital. On a ride around town with a limited throttle you are unlikely to involve the main jet in any substantive fashion. With the monoblocs there is a wide range of adjustment in all the various carb circuits. As yours starts and idles well I would start with the main jet. A short strop at wide throttle on a country road will let you switch off the bike whilst going full tilt. A quick look at the plugs will tell you if they are too lean or rich on the main jet. You can then either replace with a smaller or larger jet or do what I did which was to solder up the Jet and ream with a fine drill. You need to buy or preferably borrow a set of 0.1mm increment twist drills from someone although you can generally buy them individually from a good engineers supplies. Obviously if it's too lean you simply replace with a bigger jet and work backwards. I never drilled out brass as it ment a loss of datum. For a long time with the Bullet I had a situation where the bike went beautifully at all speeds but the idle was unreliable and adjustment on the air screw either made it too lean or rich with no sweet spot. I replaced the slow running jet with a smaller one and bingo a good idle. There was then a flat spot between idle and reving, slight but annoying. Replacing the 3 1/2 cutaway slide with a 3 sorted that. Of couse if you've got sticky Bob weights on the distributor or partial condenser failure or one of a host of other potential electrical faults messing with the carb is a waste of time. Ah the joys of old machinery!

[Dennis C]

Scalyback, the amount of adjustment needed will probably be quite small, I would suggest 240 and 230 should be all you will need.