Even though the distributors all look the same, there are differences in the
mechanical advance curves, maximum advance, and so on. Static advance (set
with the engine not running) can vary from as little as 2 deg BTDC to 8 or 9
deg BTDC. When running at 1000 rpm, the mechanical advance would almost
certainly be operating, adding 4 to 5 deg advance. Since the distributors
vary, in terms of weights, springs and maximum mechanical advance (which is
stamped on the metal plate down inside the distributor that the weights
mount on) the only way to really get the advance right is to try it and see.
So - the first great rule of ignition timing is don't over advance to the
point of detonation. With A series engines, the total advance, measured at
5000 RPM (where most distributors reach their maximum advance, although some
Cooper S units keep advancing to 7000 RPM) you should see 30 to 34 deg. Use
a dial back timing light, or degree the crank pulley.
Next, you need to be able to start the car. So advance at idle (1000 rpm)
should be less than 15 deg - probably less than 12 or so in most cases.
After you can start the car, and not blow it up racing that Volkswagen
Beetle on the "autobahn", you need to be able to drive it. Vacuum advance
was invented so that the car would get better gas mileage under normal, low
throttle operating conditions. The engine can tolerate a lot more advance
when working very lightly, as in cruising about the town. So just make sure
that the car seems to run well after you've set it as suggested above, and
in particular make sure that it isn't detonating (or pre-igniting too much)
under low to mid RPM, high load conditions like driving up a hill at 40 mph
in third gear with full throttle.
Check the plugs. Assuming the heat range is correct (Champion N9Y for
pretty stock engines) the plugs should look good - coffee with milk colour
for the faint of heart, all the way to almost new looking dead white for the
racers with big wallets. Detonation is shown by little black or aluminium
coloured balls on the plug insulator - those are little bits of your
pistons, so caution is the word!
What I would do? Set the timing at idle to factory, find a hill that loads
the engine, and run up it, advancing the timing a bit at a time until I
heard pre-ignition (tinkling sounds), then back off 2 Deg. Or just set it
and drive the thing, which is simpler. As long as it's running well, the
exact timing doesn't really matter much - you can meaure it on a dyno, but
you can't feel it in the seat of your pants!
Cheers, Brian
At 04:26 AM 15/11/97 -0500, you wrote:
>I posted this question to the MG list and didn't get much of a response,
>perhaps someone here could clear this question up for me:
>
>someone else wrote:
>
>> Set the timing at whatever the book said, plus 5 degrees for modern
>>fuel, then go driving and have a good time.
>
>I've never heard this one before, which leads me to ask: Just what are
>Midget owners with stock 1275s setting their stroboscopic timing to?
>My bentley book says 13 degrees BTDC at 1000 rpm with the vacume line
>disconnected.
>
>In light of the above statement should the timing now be set to 18 deg BTDC?
>
>If anyone could clear my confusion I'd greatly appreciate it.
>
>TIA,
>
>Robert Weeks
>Durham NC
>1969 Midget
>http://www.woozy.com/midget/
>
>
>
|