Saturday December 28th 2024

 Saturday December 28th 2024

Being in the period between Christmas and New Year, this was a relatively quiet session…

However…

Motobi Moped…

Jose’s 1960s Motobi moped still refused to run continuously and the engine insisted on dying for no apparent reason, then refused to re-start.  Graham helped Jose by inspecting the contact points and checking the points gap through the holes in the generator rotor.  With a gap between 0.3mm and 0.4mm the setting seemed to be about right.  A mark on the outside of the rota indicated that the points were opening in roughly the right place.  Of course checking visually isn’t accurate enough for good running in the long term.  This could only be achieved through the use of a timing buzzer and a dial gauge to determine the precise position before dead centre (BTDC) that the points were opening.  However our rough test would be sufficient to ensure the bike should run.  The fact that the feeler gauges slipped easily through the contact points indicated that the points hadn’t become pitted.  Pitting most commonly occurs when a spark regularly jumps between the contact points when opening or closing.  Bright sparks across the points are most commonly a sign that the condenser is failing.  Condenser is an automotive name for a capacitor.  The condenser acts as a temporary battery storing current that would otherwise jump the points making them spark,  until the points close again. Storing the current in the condenser not only eliminates arcing across the points, it also causes the magnetic field in the ignition coil to build up much more suddenly thus ensuring a high voltage  is generated causing a healthy spark at the spark plug. So the absence of pitting on the points accompanied by the absence of arcing across them indicated that our condenser was quite sound.  The fact that the bike had continued running with the plug cap held about an inch away from the plug also indicated that we had a very healthy ignition system. So what on earth was causing the issue.  It was just about possible that loose carbon deposits in the exhaust were moving from time to time and blocking the outlet. However when it was running it revved quite freely, so this probably wasn’t the cause. The carburetor had previously been cleaned but Graham checked it again just in case some debris was blocking a jet or causing the float needle to jam.  However soon after the reassembled carb was put back on the engine, the engine stopped yet again and refused to start. Graham had fitted a plug cap to the HT lead with a clear body and a spark gap inside it.  When the engine was running well a vivid white spark could be seen in the plug cap.  However, although this spark could still be seen when it started to die, the strength of the spark in the plug cap was much weaker. Although the spark plug appeared new and was an NGK (Graham’s preferred brand) Graham went and found another spark plug to try in place of the one fitted to the moped. Much to our surprise the little moped started and ran faultlessly for the next 10 or so minutes until it was manually brought to a halt. I think the moral of this story is never to assume anything about any component within a system that is under suspicion and try a few component swaps before delving deeper.

Motobi Moped running to perfection - eventually

Jialing 125 Trials bike project…

It was great to see ace fabricator Phil J back at the workshop.  Especially as he had first hand knowledge of disassembling the Jialing and was best placed to start the process of putting it back together.  During the course of the session Phil J installed new steering head bearings, the lower steering stem and the engine back into the frame.  Phil J then explained how some of the remaining assemblies could be fitted back onto the bike to help turn it into a running project fit to be used as a trials bike.  In the meantime Phil H worked to respray some of the rough looking components that had been removed from it.  The wheels would need to be extensively cleaned up before they were refitted to the bike and exhaust gaskets and seals would have to be located.  Still it was good to see some progress made.

Jialing 125 starting to take shape.

MZ150.

Mal, Keiran and others worked to reassemble the trusty Eastern European bike which then started and ran with minimal effort or fuss.

Annabelle's MZ150 running once more.

Piaggio Velofax…

In a surprising development the little Piaggio made its way onto a work bench where Graham worked to try and establish where its spark had gone.  After a search Graham eventually found the club’s factory workshop manual for the bike which of course provided wiring diagrams.  This confirmed that the ignition did not depend on the presence of a battery.   AC voltage was found to be coming from the coil feeding the ignition system in the generator and AC voltage was coming from the trigger coil. So in theory the bike should have produced a spark when the engine was turned over on the kick start but didn’t.  Perhaps a new ignition control unit would have to be obtained.  The ignition control unit comprised not only the ignition coil but also the electronics to control it. Another possible issue was that the resistance through the trigger coil was a little high.  The manual suggested this should be 88 ohms +/- 5% but ours produced a reading of 111 ohms.   The resistance through the general coil devoted to the ignition within the generator was well within specification. Once we have located the spark the bike should run as it has a new piston, new piston rings and purpose made carbon reed valve. It had been a long time coming but it wouldn’t take a lot more to get the engine running.  Some puzzling would then follow as the rest of the bike had been in pieces for over four years.

So this is what a Piaggio Velofax looks like.