The Knowledge - Velocette Owners Club

position. What can be the problem? The ignition should be a little higher, I believe. What is the correct
setting (timing). What is the correct position for the air control level on a hot engine? It seems that the
handle in forward position is giving the best result.
#3861 You may find that the carbs is weak not rich. If the air slide is closed (forward) this indicates
weak mixture. Is the slide worn? Ignition should be 38 degrees BTDC as book, probably 36 degs.
would be better on current fuel. Your carbs settings are as book. Check the float chamber level
carefully, this has a big effect.
#3866 When supplied new in Sydney Australia in March 1967, VMT457 and VMT458 were both fitted
with no3 slides. Dennis Quinlan, first owner of 458 soon changed to a No4 slide and the running rich
problem was solved. My Thruxton, VMT457 has run a No4 for the past 30 years of my ownership.
Dennis wrote to the factory and advised them of the problem but I don't know if they corrected it in later
bikes supplied with a GP carbs, or in any updates to the red book - apparently not. So I don't
think the carbs is weak - change the slide to a No4 by filing away the leading edge by 1/16" (I think! can
someone in authority please confirm) and the problem should be solved.
#3867 I agree it is 1/16, so a 4 slide has 1/4 inch cut away. My VMT would never run with a 3 slide
either, 4 and even 5 needed. Also too rich with the rich needle (has about 3/4 inch of taper) even on the
leanest setting and new jet. I used the lean needle which is straight for all but the last 1/4 inch. This on
US petrol.
L #4140 I am thinking of fitting a Mikuni carbs to my Venom. Information I received from the
supplier stated that the 3 gallon petrol tank needs to be raised by an inch and a half at the rear to fit it.
Has anyone carried out this modification and could they detail how the tank was raised and did it spoil
the symmetry of the bike ? Also was there a noticeable improvement in performance ?
#4146 I have used a 30mm Mikuni kit for 6 years on my Venom Clubman with the big tank. There's no
need to change the tank in any way if a standard Venom head is used. I had a problem with the airflow
at the inlet, it's a known fact I think that the performance is dependant on the driver holding his right
knee to the tank. I fixed this with a homemade inlet extension of about an inch which is not conical but
cylindrical. The top end performance was not improved but it's always a first kick starter and the idle is
reliable as a steamboat.
#4149 I have just fitted a Mikuni, supplied by Allens, to my Thruxton. The bloody thing doesn't really fit
at all. I have had to squeeze it in and it presses with great force on the oil tank and underside of the
Thruxton petrol tank. It WOULD fit if the finned aluminium inlet spacer was not the full 4 inches long
that it is. Does anyone know how I can get hold of a slightly shorter inlet piece (say 3 inches?) Also, at
a standstill the petrol pours out of the little plastic breather pipes. Oh woe is me!!
#4158 4 inches - thats ½ inch longer than standard. (size matters) Not sure of the mikuni fitting but I
have a 3.5 inch inlet manifold going spare for a stub fitting Mk2 Concentric (i.e. rubber mounted as I
expect the Mikuni is) for a Thruxton. Its not an original cast item but a 'machined from solid alloy'
modern thing.
#4143 Now, raising the rear of a small tank and inch and a half would be atrocious! Fortunately that's
not necessary. My Venom had a 38mm Mikuni on it for some time that fit (albeit snugly) under the small
tank without raising. I have a 36mm (1036) Amal Concentric on it now which also fits fine.
L #4430 I recently bought a 1936 rigid mac now need to set the carbs (a new concentric) up as it
did seem to be running lean going by the colour of the plug, I had replaced the old mk1 concentric fitted
with a new one and need to set it up correctly this time!! jet size is 170 ,the new needle seemed to be
too short so replaced it with the original one but I do not know what the correct length should be any
advice on setting up would be appreciated thanks.
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