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MG MGA - New timing light

Just got myself a new timing light a Sealey TL93 that does dwell, tach, advance and makes the tea. Now of course I want to check the timing on my car. Looking at the archive posts it seems that the general concensus is that timing should be set at around 32 degrees btdc at 4000 rpm with the vacuum pipe taken off the distributor. Does the vacuum opening in the distributor need to be blocked? I seem to recall reading this somewhere. Also I read in a previous post that some timing lights do not like copper wire type plug leads - they can give erratic readings. This seemed to be the case when I tried mine, I got a reading but it kept flashing up other numbers in between. It was suggested in the post to keep another cap with a set of silicone leads just to use with the timing light - can you fit these leads to a side entry cap where the leads are fixed in with screws?

Many thanks,
P Parmenter

Hi Phil are you still in touch with Steve

g c pugh

The vac pipe to the dizy does not need to be closed in fact its better open but block the vacuum source. Solid HT wires do throw a lot of stray signals around that can effect electronic equipment. You are far better off with a set of good quality silicon wires on the car.
32* is good and you only need to rev it until the advance reaches its peak or a little past just to be sure. Some times with these more complex timing lights you get a more steady signal from number four cylinder I think because of the shorter more direct lead. The timing is the same.


I agree with all Denis says, although I do not bother to block the vacuum diaphragm hole after disconnecting the pipe. The only pressure affecting that side of the diaphragm is static (ambient) pressure and this does not vary during the tune up process. The purists may argue that air blast past the orifice may vary during fan speed-up, causing a slight variation to the static pressure. But, IMHO, this would be so minute that it can be ignored. It would only have some relevance if the hole was pointing forward towards the fan. In this case the pressure would be dynamic + static on the hole side of the diaphragm and static on the other. This would then have the reverse effect on the vacuum unit, making it retard the ignition, not advancing it. But here again, what with all the disturbance to airflow in the engine compartment we are talking trifling amounts.

I am not trying to make an issue here, just suggesting that it does not matter whether you seal the hole or leave open.

Steve Gyles


Denis says to block the vacuum source i.e. the manifold. He says to leave the Distributor tube open.
This is exaclty what the MG factory said when advising all mga twin cam owners to disconnect their vacuum advance permanently.
Obviously you cannot allow excess air to enter the inlet manifold.

M F Anderson

Thanks guys, so do I need to block the end of the pipe that I take out of the distributor? Is this what you mean by blocking the source?

Gordon, yes saw Steve a couple of weeks ago. He's got a couple of new toys, a very nice early mgb roadster and a BGT - he just can't stop buying them. He sometimes wishes that he'd kept the A - have you seen it recently?

P Parmenter

Yes Phil, you are absolutely correct!
Neil McGurk

Hi Phil, glad he is ok will you pass on my regards when you next see him, haven't seen the A for a couple of months but I know where it is

g c pugh

Thanks Mick. Good call.

Steve Gyles

This thread was discussed between 11/03/2010 and 12/03/2010

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This thread is from the archive. The Live MG MGA BBS is active now.