Read below for a partial diagnosis and fix but not a solution yet.
History:
1950 Comet, Amal always flooded on and off since purchased in 1969. 6v electrics, Lucas mag.
2004 upgrade 12v Alton generator.
2020 (June?) fitted Pazon Smart-fire and fitted battery key-switch. Converted all lamps to LEDs. Ran like a dream except flooding carb.
2021 Then, September, suddenly started a misfire/power loss about 55 mph (88kph). Drop back speed and correct power restored. Serviced Amal carb. retimed ignition, checked all wiring, plug (gapped 30 thou), valve clearances. No improvement.
2021 November, fitted Mikuni VM28-49. Carburation brilliant but misfire/power loss continued.
Symptoms:
Slightly rough running at tickover but acceleration great through the gears but between 50 and 55mph the power drops way back and not exactly a misfire but certainly seems ignition related. Bring the throttle back to let it settle and it seems fine until trying to accelerate through again. I discovered with the bike electrics disconnected so only the battery, key-switch and ignition in circuit it performed better than ever but as soon as the alton and its regulator were re-instated the symptoms returned. Changed the regulator. No change to symptoms.
Output from Alton >25ACv (unloaded). Regulated output 11.3-13.03DCv with less than 0.5ACv ripple (loaded with battery).
Always, remove Alton and symptoms disappear, replace with either regulator and they return. AC output leads from generator good condition externally.
So, folks, where next? Any ideas? Clearly plug and ignition per se is OK because I get normal performance when the generator is out of the picture. The fact that the problem disappears rules out the rev-limitting function of the Pazon. It's not the regulator because swapping makes no difference. Is the minimal ripple at higher revs upsetting the ignition? When first fitted, there was an intermittency in the circuit and that upset it but fixing the connector removed that problem and, until September it ran perfectly. (I wonder about 30thou gap itself - any comments? - but that's not relevant to the problem because it runs properly without the generator.)
As an aside, each of the upgrades alone was well-worth it and the bike is a first- or second-kick starter every time. The lights are brilliant and the carburettor flood-free (and easy to fit - thanks Steve, Motocarb, UK).
History:
1950 Comet, Amal always flooded on and off since purchased in 1969. 6v electrics, Lucas mag.
2004 upgrade 12v Alton generator.
2020 (June?) fitted Pazon Smart-fire and fitted battery key-switch. Converted all lamps to LEDs. Ran like a dream except flooding carb.
2021 Then, September, suddenly started a misfire/power loss about 55 mph (88kph). Drop back speed and correct power restored. Serviced Amal carb. retimed ignition, checked all wiring, plug (gapped 30 thou), valve clearances. No improvement.
2021 November, fitted Mikuni VM28-49. Carburation brilliant but misfire/power loss continued.
Symptoms:
Slightly rough running at tickover but acceleration great through the gears but between 50 and 55mph the power drops way back and not exactly a misfire but certainly seems ignition related. Bring the throttle back to let it settle and it seems fine until trying to accelerate through again. I discovered with the bike electrics disconnected so only the battery, key-switch and ignition in circuit it performed better than ever but as soon as the alton and its regulator were re-instated the symptoms returned. Changed the regulator. No change to symptoms.
Output from Alton >25ACv (unloaded). Regulated output 11.3-13.03DCv with less than 0.5ACv ripple (loaded with battery).
Always, remove Alton and symptoms disappear, replace with either regulator and they return. AC output leads from generator good condition externally.
So, folks, where next? Any ideas? Clearly plug and ignition per se is OK because I get normal performance when the generator is out of the picture. The fact that the problem disappears rules out the rev-limitting function of the Pazon. It's not the regulator because swapping makes no difference. Is the minimal ripple at higher revs upsetting the ignition? When first fitted, there was an intermittency in the circuit and that upset it but fixing the connector removed that problem and, until September it ran perfectly. (I wonder about 30thou gap itself - any comments? - but that's not relevant to the problem because it runs properly without the generator.)
As an aside, each of the upgrades alone was well-worth it and the bike is a first- or second-kick starter every time. The lights are brilliant and the carburettor flood-free (and easy to fit - thanks Steve, Motocarb, UK).
Last edited:
