376/097 on 1958 SR

[Slymo]

It's an Super Rocket so has manual advance and retard so I did knock it back and it did remove the pinking on takeoff. It's a tool in terms arsenal for certain. My next step though is to try a new 106 jet to see if the old an probably original one is maybe a bit worn.

[Slymo]

Ok interesting. There appears visually to be a bit of difference between the original 106 needle jet and the new one so I used the digital calipers to give me a comparison and found that the old jet was nearer 107 thou than 106. swapped the jets and had a similar result as with the bigger cut away although the plugs are now a lovely even mushroom colour. I think with a richer idle jet setting I may get rid of the initial pinking and it will be interesting to see if I get better fuel economy. Always a bit irritating having to put in one extra litre than my mate with the Velocette Venom every time we stop for gas!

[mikeb]

do you run 98 fuel in your SR Slymo?
I find 98 (mobil or BP) the best for my super rocket for reducing pinking, 95 ok and 91 not great. I avoid those with ethanol tho that may just be superstition.

[Triton Thrasher]

It is no surprise that any 650 twin uses more petrol than a Venom, but...

A worn or too-big needle jet is bad for fuel consumption. 

You can tune round it a bit, with cutaway and pilot air screw, but that means they are likely to be wrong when you renew the needle jet.  That’s why a new needle jet is the first step in any old carburettor, not an afterthought.

Just a note- Trying to compensate for a worn needle jet by dropping the needle position leaves it dangerously weak at bigger throttle openings.

[Slymo]

Yep big improvements with the new needle jet. The Air Screw needed winding in and that solved the pinking at takeoff and the idle on retard is now lovely and even. Plugs are mushroom and I'll await a decent ride to see what the new fuel economy is like.
S:)

[Slymo]

Oh and yes always 95 octane with 9:1 pistons and no squish bands I never risk 91.

[Black Sheep]

When I was doing big mileages I kept logs of petrol stops. When the mpg dropped below 70, it was time for a new needle jet. Once the jet started to wear, mpg would drop fairly quickly to 50. You shold see a noticeable improvement. 

[Slymo]

Did 167 miles the other day and the bike went beautifully. I downloaded an app for the iPhone called Ride with GPS for free and it told me the millage and that my top speed reached was 78mph! I didn't see anything fast than 70 on the Chrono so will have to mount the iPhone on the bars next time to see the comparison. The Chrono is one from a RGS or a Gold Star I think as the turns per mile are wrong and the matching Rev counter had a drive in the reverse direction. I have a mate who without doing anything un-reversible changed the drive direction of the rev counter and re calibrated the speedo so that it reads spot on at the appropriate revs. However the odometer is still being driven too slowly so the millage is way too low. With the GPS odometer the MPG seemed to work out at 57 which is a substantial improvement from the 50 I used to get but no where near 70. Still the plugs look good and the performance is lovely so I might be satisfied with that and call it job done! Love to talk with anyone with a 1440 revs per mile speedo cluttering up their workshop by the by.