The Knowledge - Velocette Owners Club

#916 Quite right. The top grade should be used in Vipers and Venoms. Lower grades are OK for MACs
and MSSs in standard trim, but their use with the higher CRs will cause pinking and eventually holed
pistons. Using the wrong spark plug can do this too, by causing pre-ignition. NGK B8ES or harder is
necessary for VM etc.
#919 I always run Sunoco grade 94 (they claim is the highest available in Canada) Is Aviation fuel an
option?
#926 Personally I wouldn't recommend using Avgas. Although the normal rating is 100LL (100 Octane
Low Lead) my experience with an Aprilia 250 GP bike demonstrated that this is not a cure all for
detonation problems. This machine was bought new direct from the factory in '90 for use in the British
and European championships and came with the usual amount of technical documentation and help
that one gets from an Italian factory - not a lot! Believing that we ought to be running high octane race
fuel which was unobtainable in the UK at the time and since the rules on fuel were being tightened up,
on advice from BP we began running the machine on Super unleaded 98 RON pump fuel (BP claimed
at the time that their pump average for this fuel was 99 RON). Since this was a two-stroke machine we
mistakenly believed that the lack of lead shouldn't be a problem. Within a dozen or so laps of Cadwell
on the first time out we had a holed piston. Consultation with people in the know suggested that
perhaps we ought to be running leaded fuel so the next time out we ran a 50/50 mix of 4* leaded fuel
and Avgas, a common brew in racing use at the time. This stopped us holing pistons but we continued
to have a small amount of detonation erosion on the top of the pistons and the rider always reported
poor carburation whatever jetting settings we ran. I gradually increased the proportion of Avgas until we
were eventually running 100% of the stuff but never completely got rid of the det damage and it
appeared that the carburation problem was getting worse. Eventually frustration led me to go to the
other extreme and I tried 100% 4* as a last resort and - hey presto! - no more detonation and perfect
carburation! This led me to believe that although Avgas is rated at a high octane it is not designed for
use at ground level but at several thousand feet up in the air where the oxygen content of the
atmosphere is lower. The "Low Lead" tag is misleading too, in fact Avgas has a higher lead content
than 4* leaded gasoline. However, as the regs regarding fuel have changed over the last few years,
most of the factories have produced racing machines which are designed to run on Avgas and have
combustion chamber shape and ignition advance curves altered to suit the burn characteristics of the
fuel. My conclusion was that in general Avgas was no help in tackling det problems in engines unless
those engines were designed for it in the first place. Better to alter the timing to suit the available fuel,
particularly when you can't get Avgas from your everyday petrol station. What would you do if you went
for a touring holiday on the bike - have someone follow with a tanker full of special fuel?
#932 In the USA Sunoco sells a variety of race fuels, their website is www.racegas.com There is a
dealer close to me that has 110 octane (R+M/2) leaded, they also offer unleaded up to 104. And
leaded up to 117! Also methanol if you are racing speedway or sprint cars. With a 50/50 mix of the 110
and the 91 octane U/L "premium" sold in our local gas stations, my VMT starts first kick, no pinking.
Since this bike is not intended for long-distance touring (rather for 100 - 200 mile rides on Sundays),
this works for me. All other Velos in our garage are timed at 36 - 37 degrees and run 8:1 CR or less.
30 years ago I thought my Venom had a rod knock, took it down to the crankshaft looking for a nonexistent
problem, realized years later it was pinking.
L #934 What about so called octane boosters? Is any one brand preferred over another?
#940 Octane boosters are a waste of space.
#942 Warning! Anything potent enough to raise octane significantly in small amounts will likely be so
poisonous and/or carcinogenic that you wouldn't want it in your garage, much less toolkit or tankbag.
Remember Dave Allcock.
#944 I have spoken in the past with a fuel chemist, and he recommended adding xylene at NO MORE
THAN 10% to gas if octane boost were needed. Toluene I would imagine is much the same, but I
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