The Knowledge - Velocette Owners Club

A Diagnosing Troubles
A.1 Hard Starting and Poor Running -
L #223 At last year's Stanford Hall Rally my Venom did not want to start. After I had exhausted
myself a volunteer from the inevitable crowd that were watching finally managed to get it going. On
kicking it over, it had now developed a habit of spitting back enough neat petrol for it to be seen
running down the oil tank. It got me home OK and ran well at higher revs. After repeating this
performance a couple of times I tackled all the obvious things, plug, BTH manual magneto points,
pickup etc. with no joy. I then changed the carburettor, from monoblock to concentric. This was easier
to start but ran lumpily at low revs and this time spat a small quantity of neat petrol back through the
carburettor on each firing stroke; again it seemed to be OK at higher revs. Getting more desperate I
have removed the cylinder head to find no problem with the inlet valve or seat. I have taken off the
timing cover and found the cams, followers and gears all to be in apparently excellent condition. What
next?
#225 I had the same problem on my 58 Venom, after fitting a new Monobloc. Spitting back at low revs
indicates richness, which is controlled for the first quarter of throttle movement by the Pilot Air Screw.
Try screwing it out a little at a time (to weaken the mixture) until the spitting stops. You may have to
adjust the throttle stop afterwards to obtain a satisfactory tickover.
#228 One of the major reasons for a Venom spitting back is due to the large amount of valve overlap
that a M17/8 cam gives. If you check out most Venoms after a run, you will find petrol and oil debris on
the oil tank that has been jettisoned from the carbs. A MSS with a softer M17/7 doesn’t suffer from this.
Check your valve timing, remembering to use the increase valve clearances. You will find that the
timings will not be spot on mine were 65–39–50–55 rather than the 65-35-45-55 as specified. If yours
are out you can spread the error by moving the cam a tooth either way to see if it helps.
L #431 Since the main bearings failed in my `54 MAC after getting back from Bavaria last year I
haven't been able to get the bike to run properly. I've had a number of problems including nipping up a
piston since, possibly unrelated. I have discovered, in the course of investigation, that the bike has an
M17/7 cam and followers as fitted to MSS and early Viper. Has anyone else got an MAC with this cam?
If so is the cam set up according to the marks on the wheels? I've checked the cam timing with the
valve clearances set to 30 thou inlet, 35 thou exhaust (as specified for the MAC). The results are as
follows (M17/7 spec in brackets) [Inlet opens 25deg BTDC (19deg) - opens early Inlet closes 50deg
ABDC (49deg) - closes late Exh Opens 50deg BBDC (49deg)] opens early Exh Closes 16deg ATDC
(19deg) - closes early I've tried taking cam lobe centre readings but as I haven't got a dial gauge it
proved to be difficult to be accurate enough to perform the check. Can anyone comment on whether I
have the valve timing right or not? Should I get a dial gauge and do the cam lobe centers? This is
driving me mad. My MAC used to be an easy starting good performer. Now it doesn't start well and
coughs back through the carbs.
#431b Just a further thought - the MSS with a M17/7 cam has ignition timing 36deg BTDC fully
advanced. Does the ignition timing follow the type of cam i.e. if I'm using an M17/7 cam (MSS type) in
my MAC in place of the M17/5 (MAC type), should I change the ignition timing from 38degrees to 36
degrees? I have a sneaking suspicion that I should. This is most perplexing. Before the recent
problems the bike ignition timing was set by the roadside, after yet another ATD spat its fibre teeth,
using a piece of stick found at the roadside with a rough estimate of about 11mm for the piston crown
distance BTDC. The bike has been running fine ever since:-)) I've never used a degree wheel until
recently. Precision measurements ain't what they are cracked up to be. Going to get a dial gauge and
check the cam lobe centers.
#434 You might check your ATD to make sure that the springs are still okay, I had one break once, the
ATD went into full advance. Velos are very sensitive to ignition timing, less so to cam timing. But the
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