Hi
I have recently become the proud owner of a 1946 B31. It ran well at first but started misfiring. I checked the timing and it was too retarded so I have set it so the points open 7/16 BTDC when fully advanced. It starts easily and and runs well at first, but over a short period of time (say 50 miles) it starts misfiring again. I think the timing is right but the plug becomes very sooty and I wonder whether this is the issue. I have never had a bike before with manual advance / retard and I tend to just put it onto full advance when riding regardless of the revs, except when stationary. Should I be adjusting the advance retard constantly to match the engine revs? I'm thinking maybe if it's too advanced when at low revs, this could result in the mixture not being burnt efficiently and cause the residue on the plug. Or maybe its the carb jetting and it's too rich? Or maybe something else completely different! And returning to 7/16, i read somewhere there is a bolt on the top of the head you can remove to put the gauge in but there's not on mine so I measure through the plug aperture which is at an angle to the bore so it isn't actually 7/16 but less - is that ok or do I need to increase the measurement to account for the angle? Any advice welcomed. Many thanks Ian
Take off the mag pinion and have a close look at the back where the spigot with the taper is. They can crack down the taper and will keep coming loose once the engine heats up. I have played this game with my B33
It sounds to me like the magneto is slowly giving up.
My B33 started misfiring as you have described, but I continued operating the bike until one day I rolled into my final destination under the power of gravity. I trailered the bike home and had the magneto rebuilt.
If the bike starts easily and runs ok at first, you are probably correctly setting the advance.
I'm in agreement with Jim. Most likely the magneto is failing when it gets hot.
It does sound like the magneto is starting to suffer from a deteriorating condenser, particularly if it becomes hard to start when hot.
If you are confident to work on the mag yourself you can get an armature rewound with condenser for around €80-100 and rebuild yourself. There is a lot of help in the lucas instructions and on-line.
Having the mag overhauled is a bit more expensive and do ask around for recommendations and likely cost. (check out forum at http://www.wdbsa.nl/)
You could also look into the solution from Brightsparks (http://www.brightsparkmagnetos.com/), a number of friends have recommended these.
I recently fitted a Thorsparx solution as a temporay fix for a duff mag but I may well keep it as it works really well (you just have to be sure your battery gets a charge). https://www.sussexmotorcycles.com/thorsparksecureonlineshop/
And you are doing it right with the advance / retard. Retard to start and then full advance. Theoretically you could need some retard if slogging up a hill but then you should change down anyway. You can always retard at the lights to annoy any nearby HD riders with a tickover you can count. :)
I hope you get it sorted, a B31 is a lovely bike!!
Thanks very much for all your replies.
Bit more background / context in case it helps.
The bike was restored by the previous owner and looking at the receipts which came with the bike, there is one from Armoto Ltd which says 'magneto repair (Lucas M01), new coil and condenser, slip ring, bearings and Magnetize'. In addition 'E3L dynamo repair; Armature refurb (resin skim & undercut), Field Coil and DE Bearing'. This dates from 2017 and the bike has only done about 5k miles since then. Does that sound like the mag should therefore be ok or can they deteriorate again that quickly?
On the starting approach, in case this gives any clues, it starts really easily hot or cold, but it will only start (or at least I can only get it to go) on full advance.
It seems to me that the main issue is the plug getting so gunked up which suggests something is not right and this (ie a h=gunked up plug) might lead to the misfiring? When I change / clean up the plug it runs fine again until the deposits return.
The carb was also replaced during the restoration with an Amal monobloc 376 / 1616. jetting is main 260, needle 106 and pilot 30 - does that sound right?
Do you think it could be to do with the fuelling or am i barking up the wrong tree!?
thanks again
If it's not pinking then it's not too far advanced.
I wonder if you might try dropping the needle 1 notch? Or perhaps a hotter running plug to burn off the deposits?
Hi,
Carb specification is fine, needle position should be 2. Clean the carb, especially the pilot jet.
Try finding TDC by using a piston stop with an old plug.
• Turn the engine by using the back wheel.
• Use a timing degree disc.
• Fit the engine stop.
• Slowly turn the back wheel until you feel the piston gently touch the engine stop.
• Read the degree value on the disc.
• Turn the engine backwards by the wheel until the stop gently touches the piston again.
• Read the degree value on the disc again.
• The real TDC will be half way between the values.
• Take out the engine stop.
• Turn the engine half way between the values (which will be TDC) in its normal direction of travel.
• Without moving the engine set the disc vertically to TDC.
• Set the timing using the timing disc, not piston height.
Best wishes...
For starting, I normally set my advance at about 1/2 to 2/3 advanced. If I forget, and try to start the engine set at fully advanced, I get a really severe kickback. If you are not getting kickback when starting with the setting at fully advanced then something doesn't sound right.
I use Service Sheet 604 to set the advance. Bess's method above is more precise. Remember that for full advance, the face cam should be rotated fully counter clockwise.
If the magneto was rebuilt in 2017, it should be good however an easy thing to check on the magneto is the radial movement at the points end. Remove the points cover and try to push the points side to side. If there is excessive movement then maybe the non metallic shim at the points end of the magneto has deteriorated.
Jim
Check that the oil seal for the magneto inches timing case isn't a "nearest metric equivalent they are too deep and sometimes hold the sprocket off the taper just enough to allow it to move under load.
Quote from: imckenna on 28 Aug, 2022, 13:19
On the starting approach, in case this gives any clues, it starts really easily hot or cold, but it will only start (or at least I can only get it to go) on full advance.
thanks again
Please check what type of advance setup you have. HTH