I always check the spark with a timing light and the plugs under
compression. Compression makes it harder for the spark to jump the gap, the
HT voltage rises, and it can rise above the breakdown point of various
components. Use a timing light with an inductive pickup on the coil lead
and each plug lead. If only the coil lead flashes the rotor and/or cap are
breaking down. Even if they all flash the plugs could be breaking down so
if they are an unknown quantity change them. Whilst on No.1 lead check the
timing is about right. While cranking see how far you can pull the HT lead
out of the coil before losing the spark at a plug. Should be at least 1/4"
and can be as much as 1/2" or more. Less than this could mean the condenser
has failed.
After cranking for a bit remove the plugs and look at/sniff them. If they
are wet they are flooded - with SUs and a manual choke push the choke home
and crank with a wide-open throttle, be ready to release the throttle and
pull the choke half-way or so when it fires as it should. No fuel smell
means no fuel is getting through, so check the fuel pump, float chambers,
jets (by blowing gently in the overflow pipe) etc. Check the choke
mechanism as best you can - easy on HSs, less so on HIFs. If SU make sure
the jets are set to their initial mixture of two full turns of either the
jet adjusting nut (HS) or screw (HIF) down from being flush with the bridge.
If both fuel and spark are OK check the firing order, and confirm that the
rotor is pointing to No.1 plug lead when No.1 piston is at the top of its
compression stroke which is both valves closed throughout the compression
stroke, or if you haven't got the valve cover off the compression stroke
should lift your thumb from the plug hole.
Check the voltage at the fusebox while cranking, should be about 10v with a
good battery. A combination of slightly weak battery, old plugs, poor
mixture etc. can be enough to prevent firing-up whereas just one of them
wouldn't.
PaulH.
----- Original Message -----
> Can you shed some advice on a non starting, fresh engine non MG?
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