----- Original Message -----
From Dick Davis <x856 at email.com>
To: rob thomas <rob@thomasr.greatxscape.net>
Sent: 26 May 2000 02:14
Subject: RE: Re: Gen Q about ignition timing
Reading a 1957 copy of "Practical Motorist and Motor Cyclist" magazine on
the subject of Vac tuning...........
"To avoid over advance, retard the ignition till the needle shows a drop of
3/4 in. Hg. and tighten the clamp. Then reset the carburettor to a normal
tickover and re-adjust the mixture control. You now have an ignition
setting which takes into account the overall engine condition - a really
'individual' setting"
Over advance? Not sure of the reasoning behind this but suspect that it
prevents the engine from finding aa RPM where the ignition fires too, early
such that the piston is still rising on the compression stroke when the
shockwave hits to push it back down. (Wild assed guess with a little merit?)
Other useful info:-
Idle should be about 18=19 in HG
Stab throttle. Should drop to 0 then rise to 24-25. If not, engine
poor
Vac 16-17 and wavering slowly. Air leak in intake system/weak mix
Vac 15-18 and floating. Too rich/float level too high/blocked
filter/punctured float.
Regular beat with lower vac indication. Burnt valve
Vac low and needle steady. ignition retarded
Vac slightly low and needle tremoring. Tappets mis adjusted
Vac normal (19-19) but irregular kick. Points/bob weights/plugs
Vac normal but occasional drops. Plugs or sticking valves.
Vac high (19+) and irregular kick. Over advanced/loose timing chain
tensioner (need A+ tensioner???)
Short each plug. Normal for 2 in. Hg. each
Switch off fuel/ignition. close throttle to idle. Run starter. Should
get steadyish at 15 in Hg. If not, compressions uneven/head gasket gone.
Setting mixture. Go for max vacuum reading possible AFTER setting
points/timing/valve backlash etc.
Hope this helps
Rob Thomas
(some financial interest......<G>)
> > Hi Rob,
>
> Thanks for the reply. Do you have any idea why the drop
> of .5 inches mercury?
>
> Thanks again,
>
> Warm the engine.
> Set 1000 rpm.
> Turn dizzy in opposite direction to the rotor arm until max vacuum is
> reached.
> Turn it back until it drops by 0.5 inches of mercury.
>
> Maybe this is where the error occurs on your car?
>
> This is the ONLY way to tune the ignition on my pre-war as there are no
> timing marks. How do we know that the timing marks on a Spridget are
> correctly positioned anyway? They are right in the way for being bent
every
> time the engine is taken out and are really hard to read from under the
car.
> It's hard to understand why it took so long for BLMC to convert to the A+
> covers with the marks at the top where they can be read whilst working on
> the distributor. My 1275 is so low to the ground that I couldn't get
> underneath to read old style timing marks (and it is a 1500 front chassis
> anyhow, so its shaped differently on the front crossmember.) The
> contact-breakerless 65DM4 electronic ignition doesn't have points so it
> isn't possible to set the timing without the engine running meaning it
would
> be a real pain in the A$$ to re-tune the car every few months when the
> points get replaced.
>
> Rob Thomas
> No tuning required for about 4 years on a daily driver, and still running
> perfectly!
>
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