Oracle Tip of the Week

k2fmag1

The previous TIP of the WEEK concerned ‘Adjusting for the Perfect Clutch’, here we discuss magneto ignition timing.

The Basics of setting timing on a Pre-Unit Triumph Twin.

Setting the timing on a Lucas / BTH mag to be exact on both pots is virtually impossible. It’s a balance of compromise between the two pots.

Use this guide in conjunction with your Oracle.

It’s important you follow the Triumph Workshop Manual &/or Lucas instructions for basic technique of backlash reduction, positioning and tightening up. Here, we concentrate on commissioning of the timing and assume all is good with your mag.

Checks.

The carbon pickups are clean, in good contact with no cracks. The points backing plate is ‘timed’, with an undamaged locator into the armature notch. The pickup closest to the cylinder is to the RHS plug. Ensure you are on the compression stroke with both valves closed on the LHS cylinder. The points should be ‘rising’ on the LHS of their travel.

Variables.

One discrepancy on it’s own may be of little consequence, several may be disastrous to your Triumph.

The cam ring will always be a variable degree out between the two lobes, even with new cam rings they may not be found to be exact. This will be addressed further.

Additionally, most timings are set that ignore the dwell. This will be several degrees, (depending on what wear your engine has) but will always exist on the ‘rollover’, between the rise and fall of the piston. For accuracy, this dwell should be split, so say a 5 degree (@2mm) dwell, a 2.5 / degree (1mm) split is actually ‘*True’ TDC. This variation may not become apparent until you do a final check. But whatever you do, continue to follow your same setting methodology throughout all adjustments, otherwise you can not be sure any change of technique is not affecting the settings.

In practice this is simply ‘feeling’ for the middle of the rise & fall when the crank is still moving, but NO measurement is evident at the piston. If using a TDC plug guage or degree disc, allow for this. So if the dwell is say 2mm, (1mm BTDC & 1mm ATDC) the actual TDC is from where the piston stops, plus 1mm. Ignoring this will result in a little extra advance, in this example 1mm, you should always er on the side of retard, rather than advance for a road going machine, a holed piston is worse than poor fuel economy.

How to Set, Triumph magneto Timing

The LHS is always the reference pot! This is important, because this is the reference point for all other measurements.(LHS when sat on the bike)

This true*TDC then, is your starting point. Set your points gap first, again this may require balance on the two cam lobes. If one points opening measures 0.012″ and the other 0.008″ balance to say 0.014″ and 0.010″. If you have a large discrepancy between cam lobes, check the cam ring is not being squeezed or fouled in the Mag Cap housing. Any ‘misshape’ can distort the cam ring.

A properly fixed degree disc is easier to read. You can get a little light bulb kit, which is far more accurate than any ciggy paper that can not account for pitted points faces. A normal loop bulb will not work. You can also adapt a digital vernier gauge for your TDC spark plug tool. If using a plug gauge, don’t ‘jamb’ it on the piston. If you can hold it and feel for the piston contacting it during rotation, do so.

As for the cam ring, the timing should be measured (checked) on the opposite pot (RHS) fully advanced,(after first setting on the LHS fully advanced).You will find that this RHS pot is invariably out by several degrees / mm. The difference between the two then, requires splitting.

So, if you have set the LHS reference pot correctly set for your model, in this example say, points open at 9.5mm BTDC(*Before top dead centre) fully advanced (FA), ie, (Cam ring fully anti clockwise for manual advance, Auto advance fully wedged open) and the RHS pot then reads on checking, say, 11mm BTDC, this is 1.5mm too far advanced and you must split the difference to give a maximum advance of 9.5mm on any pot.

The difference in this case is (11mm – 9.5mm) = 1.5mm. So by re-setting the LHS at 1.5mm less, to 8mm BTDCFA (9.5 -1.5mm) the RHS pot should then measure 9.5mm BTDC FA. Despite one pot being retarded, this gives you the correct and optimum timing. It is important to check again (on both pots) when completed and make no assumptions. A max tolerance of 10% is acceptable.

It may sound a pain, but once you understand the principle and have set it once, you can then be sure that setting to the same figure on the LHS pot at any time will give the right result for both pots. Also, be aware that any subsequent adjustment of the contact breaker points gap will affect the timing!

Oracle Classic Triumph Feature Tip

The Triumph Auto Advance mechanism springs can have a profound effect on the slow running / tick over if they are incorrect or weak. Renew these if in doubt.

 

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