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25d Aldon Yellow Distributor W/vac Advance Unit

25d Aldon Yellow Distributor W/vac Advance Unit

25d Aldon Yellow Distributor W/vac Advance Unit
Aldon Yellow distributor, vacuum advance, for non-A+ performance road engines.
Once the engine has been modified, it will invariably require a different ignition curve-especially when compression ratios have been raised and camshafts changed. There is no substitute for a tailor-made distributor determined by an engine or chassis dyno session, however this can be prohibitively expensive! We offer the Aldon Automotive range of distributors for A Series engines. For road use it is advisable to use the vacuum advance for economy. Many say, incorrectly, that this over-advances ignition, causing detonation in performance engines. That is not true as the vacuum advance only works at partial throttle openings, not under load with full throttle when no manifold vacuum exists!
Technical Information:
DISTRIBUTOR- Initial ignition timing set-up

Terminology-

Dizzy- Distributor

TDC- Top Dead Centre

BTDC- Before Top Dead Centre

ATDC- After Top Dead Centre

CR- Compression Ratio

Tacho- Tachometer, rev counter (not to be confused with Mexican food delicacy!)

Vac- vacuum (pipe, unit, etc.)

Anydeviation away from the original standard engine specification, or where a dizzyfrom another source is used in the engine you have, the ignition timing willgenerally be different from that set by the manufacturer.

Dizziesthat have had their advance curves altered to suit a particular specification bya specialist should come with an initial setting of some sort - either static orstrobed. Even having a strobe figure stated has its pitfalls - least of allhaving no reliable or accurately set TDC mark/pointer on the crankshaft pulley!To get the engine started for running in or to a rolling road for initial set-upor where no rolling road exists where no such information is given, it'sessential to get the ignition timing in the right operating envelope to avoiddamage caused by incorrect ignition timing. Results of which can be disastrous.Terminal even.

Thereare no hard and fast rules or methods carved in stone with guarantees on howthis should be done. But clearly a starting point is needed, so the following isthe method I use where information is a little thin on the ground!

Thereare two stages to this method. The first to get the engine started and warmed up- a static setting, the second to get the best from an unknown quantity withoutcostly damage - a running setting.

Thefirst is very quick to sort out, as it is merely to ensure easy starting forwarm-up. Whatever distributor you have, setting the ignition statically to haveno more than 5 or 6 degrees advance (firing BTDC) is the way to go. Youcertainly don't want it set to fire ATDC - it causes all sorts of problemsincluding exhaust pipes glowing red hot at idle! So just run through thestandard method for setting ignition statically (see relevant separate article).

Oncesatisfied the static ignition is set, start the engine and run it until itreaches as close as you can get to normal running temperature or at least untilthe thermostat opens where one is fitted. Easily detected by the sudden increasein temperature of the top rad hose to the touch. Switch the ignition off,holding the throttle wide open to avoid any possibility of running on. Slackenthe dizzy clamp bolt off slightly so the dizzy can be reasonably easily turnedby hand. Connect a tacho up so it's easily read, or get an accomplice to adviseyou what the dash mounted one says. Also disconnect the vac pipe if applicable.

Restartthe engine. Using the idle adjustment screw on the carb, increase the idle speedto near-enough 2,000rpm. Now slowly and carefully advance the ignition timing byturning the dizzy clockwise whilst keeping your eyes or your assistant's gluedto the tacho. As the dizzy is turned, the revs should start to rise. Keepadvancing the ignition until the revs stop rising, then retard the ignition byturning the dizzy anti-clockwise until the revs drop by around 250rpm. If therevs don't rise, retard the ignition until you get a marked decrease in the rpmshown, then progress as previously outlined. Once satisfied, turn the ignitionoff, and nip the dizzy clamp bolt up. Try to do this in a reasonably swiftmanner to prevent any possible over-heating. Let the engine cool for a while,then re-start and re-set the idle speed. Don't forget - for cams with verysporty profiles that cause rough idling - DO NOT set the idle speed as standard(750-800rpm). This will cause premature valve train damage, let alone cause amechanical cacophony! An idle speed of 1,000 to 1,100rpm should be your goal.

Todouble check you're not running into detonation problems, drive the car aroundusing minimum loading (part throttle and use the gearbox) to get the engine upto running temperature. Then drive the car at about 25mph/40kph in third gearand slowly apply the handbrake to two-thirds operation, then accelerate swiftly(i.e. don't just floor it). If any rattling/pinking (detonation) can be heard,back the ignition timing off by a very small amount statically and try againuntil detonation is eradicated. Alternatively, if no detonation occurs, you canadvance the static ignition timing until detonation registers then back it off.The idea is to test the engine at it's most critical rpm range for detonation -around 2,500-3,500rpm (dependent on cam type). If the engine just bogs down, trya slightly higher speed with the aforementioned rpm envelope in mind.

Oncehappy you've achieved the required goals, check what ignition advance you haveat 2,000rpm using a strobe, engine hot, vac-pipe disconnected, using thestandard timing marks/pointers if there are any. If not - contrive a pointerthat's easy to use. Doesn't have to be exactly at TDC, it's just a referenceshould you need to disturb any of the ignition components in the future forwhatever reason. Make a note of the reading some place safe.

ForAldon distributors only, the manufacturer's recommendations are asfollows -

CAM SPEC. 9-1 CR 10-1 CR 11-1 CR 12-1+CR

Fast road upto

731, 276,270

'Y'

8-12 deg

'Y'

8-10 deg

Rally/Race up to 643, 286, 285

'Y'

6-10 deg

'Y'

6-8deg

'Y'

4-6deg

Race

649,296, 300 and up

'R'

4-6deg or

17-19deg @ 2,000rpm BTDC

'Y'- is Aldon 'Yellow' spec dizzy

'R'- is Aldon 'Red' spec dizzy (note - this dizzy is only suitable for race use).

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