Adjusting the Pugh-O-Spark magneto on the Wildebeeste line

This document describes the simple process required to adjust the Pugh-O-Spark magneto on the Wildebeeste line. This advice is appropriate for machines manufactured in Scunthorpe and those assembled in Argentina, although there are a couple of minor differences in the latter that will be mentioned in the text. Note that the Empire Wildebeeste Tropical (instantly recognisable by the bamboo primary chain-case) uses a modified Pugh-O-Spark to combat the engineer's fears of typhoons and tropical floods and so, before embarking on this maintenance procedure, you first need to melt off the 3lb of tar sealant.

Schedule

Magneto clearances should be checked at the regular 500 mile major service intervals.

Description

As is so typical of Hurley-Pugh, the design of the Pugh-O-Spark ignition system is a sight to behold. Young Alaric was unhappy about the usual system of connecting the magneto directly to the crank and then having some form of contact breaker arrangement to allow for timed sparks. The constant drag of the magneto introduces frictional losses, and the contact breakers available at the time had only limited life-spans. Therefore, the Pugh-O-Spark uses a different approach. The rearward facing side of the conrod's big end is toothed. As the conrod rotates around the crankshaft, every revolution its teeth mesh with the teeth on the gearwheel at the end of the magneto. Thus, the action of the crankshaft and conrod themselves provide a regular "kick" to the magneto; each "kick" generates power for the spark. The astute among you will realise that, as such an arrangement will provide a spark on every revolution of the crank, the Hurley-Pughs operate a "wasted spark" ignition system as, on every other revolution, the spark will occur while the exhaust port is open. This is not a problem, though, as due to the side-valve design of the engine it is common for uncombusted charge to be present in the inlet chamber that the additional spark ignites. This doesn't increase power output as there is no compression at that moment, but the extra flame does help to clear the exhaust of carbon. Truly, this is the work of genius.

One implication of the Hurley-Pugh magneto arrangement is that it is crucial that the magneto gear's teeth are in exactly the right position to mesh with the teeth on the conrod when it comes past. If this is not the case, splintering of the gear teeth will occur, the debris will fall into the crankcases and quickly cause the oil pump to seize - Hurley-Pugh did not fit an oil strainer in the sump as the total loss lubrication system means that the oil pump normally only sucks in fresh oil. To ensure the correct alignment of gear teeth, there is first a friction brake - in the form of a leather washer - on the magneto shaft to prevent excessive rotation of the gearwheel, and second the magneto itself is shimmed to locate it in precisely the correct position. Incidentally, it is also by use of these shims that adjustment of the ignition timing is performed.

Gaining Access

First remove any leg guards, fairing panels, engine bars and/or auxiliary fuel and oil tanks that may be blocking access to the engine. Such additions depend on the model. In the case of the Gentleman's Twin-Outrigger Combination, remove both sidecars. Remove the cylinder head and cylinder. Finally, remove the seat (remembering to "safe" the ejection charges if fitted), fuel tank, main oil tank, carburettor, spark plug, battery and starter motor.

Removal

Now that access has been gained, put a 3 3/14" socket on the exposed end of the crankshaft and rotate the crankshaft in a clockwise direction until the engine is at 12 degrees before Top Dead Centre. There aren't any alignment marks on the crankshaft to mark this position, so you'll need a timing disc to locate this point. The Hurley-Pugh Enthusiasts Club can supply one that will fit on the recessed crankshaft for 73 pounds sterling.

With the crankshaft in the correct position, remove the six bolts that secure the magneto to the top side of the crankcases. You will notice that all of the bolts are different lengths - it is crucial that these are replaced in the correct holes otherwise you will lock the gearbox solid. Underneath the magneto you will almost certainly find one or more thin shims, fitted by previous owners. Hurley-Pugh itself did not actually shim the magnetos when the engines were originally manufactured. Instead, Bill "Masher" Stubbings precisely adjusted the magneto's mounting points by the careful application of a large hammer. If you look closely, you'll probably still be able to see the dents in the top of the crankcase where "Masher" skilfully wielded his scarred hammer. As Hurley-Pugh did not shim the magneto it didn't produce any shims; again, the Hurley-Pugh Enthusiasts Club can oblige with a set of paper shims for a reasonable 280 pounds complete.

Inspection

With the magneto removed, examine the teeth on both the gearwheel and the conrod for signs of distress. Any evidence of chipping, bending, burring or missing teeth will require replacement of both the magneto gear and the conrod, as well as a complete engine strip to remove the debris.

The leather friction washer can be found between the gearwheel and the magneto. To inspect this, use a blowtorch to heat the gearwheel to cherry red and use a 400lb press to ease it off the shaft. The heating process usually destroys the leather washer so it will need to be replaced regardless. Note that the washer is made from treated rabbit skin; some unscrupulous suppliers are selling washers made from inferior cow leather but these parts will not impart the correct amount of friction so should be avoided. The easiest way to test if a washer is from cow or rabbit leather is to send it off for DNA testing.

Adjustment of Friction Washer

The precise amount of friction that the washer imparts on the magneto shaft is crucial to ensuring correct alignment of the gear teeth in operation. Adjustment is obtained by moving the gearwheel slightly in or out on the shaft. The friction is at the correct setting if, with the magneto at operating temperature (60 degrees centigrade for European models, 70 degrees for Empire models - pad the magneto in clean towels and put it in the family tumble dryer until it is at the correct temperature) you should be able to ease the ungummed edge of a Rizla Green King Size cigarette paper - note, NOT Rizla Red - between the magneto body and the washer. If there is insufficient clearance, allow the magneto to cool then prise the gearwheel away from the magneto with a large tyre iron. If there is excessive clearance, tap the gear with a hammer.

Adjustment of Shims

With the friction washer correctly adjusted, if you are re-using the original gearwheel and conrod, replace the magneto - with any original shims - and torque the bolts down to 60ft-lbs for those on the rear and 65ft-lbs for those at the front. If you have replaced the gear and conrod, use 12 paper shims as a starting point and, again, torque down the magneto bolts. As Hurley-Pugh used as many mounting points as possible to support the weight of the sizeable fuel and oil tanks, including the magneto, replace the battery, carburettor, oil tank and fuel tank. Fill the fuel tank half-full with four star and the oil tank with the normal grade of lubricant. It is vitally important that the fuel tank is replaced for this stage as the weight of it alters the clearances for the magneto. Now, bend a 3 thou feeler gauge (4 thou for the Argentinean models, as Johnny Foreigner wasn't as good at manufacturing tolerances) into a Z-shape to allow it to be inserted into the mouth of the crankcases and check the clearance between the teeth of the conrod and magneto gearwheel. If a slight drag is felt then all is well and the machine can be put back together. If not, remove the fuel and oil tanks, battery and carburettor, unbolt the magneto and re-shim. Repeat this process until the clearances are correct. Note, it is advised that a new feeler gauge is used each time as they tend to bend slightly too easily and so can give incorrect results.

Testing

With the motorcycle now in one piece, follow the usual starting procedure but with the main ignition off. Use a wooden wedge to prop the automatic exhaust valve lifter open, release the auxiliary starter clutch and listen carefully for any quiet knocking or grinding sounds from the magneto. Of course, the noise of the unsilenced two-stroke starter motor may make such low-level sounds difficult to ascertain; with experience, though, one can readily notice the sound of a maladjusted Pugh-O-Spark.

Once you just hear the normal sounds of a healthy Hurley-Pugh, the process is done. Happy motorcycling!