I do not doubt you know more about magnetos than I ever will, or care to. What I do have is practical experience built up over 55 years and hundreds of classic bikes. Invariably when they run fine up until being stopped when hot and then refusing to start, a mag rebuild cures the problem. I could be wrong and have been many times before, but that is my experience. Theory is useful, but practical experience tops theory.
E: Engine Comet will not start when hot
- Thread starter everiman
- Start date
- Status
I'll add my vote to the mag pile.
As far as magnetos always causing a misfire when the old condenser is kaput - this wasn't the case with my BSA Super Rocket.
It always started easily when cold, ran smoothly at all times but could not be restarted when hot.
After a thirty minute roadside cooldown it would restart and off you go, repeat cycle.
If you listened carefully you could hear a slight change in the exhaust note when sufficient heat got into the mag. It would take on a flat sound and after that it would not restart if allowed to stall.
I'm not sure one could hear that exhaust note change on a Comet. To my worn out ears the Comets tend to have a flat sounding exhaust note at all times.
A club friend who is skilled at magneto and dynamo repair fitted an Easyspark ceramic condenser to the BSA mag.
This involves disassembly of the mag and removing the old condenser. It looked to be a delicate job so I was glad to have someone with the right skills do the work.
End result, perfect starting hot , cold or tepid now. Also, the ceramic condenser is easy to change should it ever fail. It is located right under the end cap.
The man who developed the Easycap is on the A10 forum and I have chatted with him.
He tells me that they have only had one failure in about 2500 that are out there working.
Glen
As far as magnetos always causing a misfire when the old condenser is kaput - this wasn't the case with my BSA Super Rocket.
It always started easily when cold, ran smoothly at all times but could not be restarted when hot.
After a thirty minute roadside cooldown it would restart and off you go, repeat cycle.
If you listened carefully you could hear a slight change in the exhaust note when sufficient heat got into the mag. It would take on a flat sound and after that it would not restart if allowed to stall.
I'm not sure one could hear that exhaust note change on a Comet. To my worn out ears the Comets tend to have a flat sounding exhaust note at all times.
A club friend who is skilled at magneto and dynamo repair fitted an Easyspark ceramic condenser to the BSA mag.
This involves disassembly of the mag and removing the old condenser. It looked to be a delicate job so I was glad to have someone with the right skills do the work.
End result, perfect starting hot , cold or tepid now. Also, the ceramic condenser is easy to change should it ever fail. It is located right under the end cap.
The man who developed the Easycap is on the A10 forum and I have chatted with him.
He tells me that they have only had one failure in about 2500 that are out there working.
Glen
Last edited:
I also have over 55 years of practical experience with classic bikes (56 to be precise...), but only with scores of them, not hundreds. Well, that's not quite true. I also have made notes each year of the failures I saw at the Irish Rally during the more than 15 years I've attended, so with ~150 bikes covering ~500 miles ea. per year, my observations were of failures (and solutions) during ~100,000 classic bike miles.
However, irrespective of individual experience, all of us only have the limited information the original poster gave us to go on. Since at this point he has been given more opinions than he could have hoped for (albeit, contradictory), he's on his own to decide what to do.
Fit a BTH and enjoy riding the bike without the worry of it not starting.
Well my first Comet suffered the same issue........good starting from cold but a right bitch when hot..........Carby was a Mk 1 1/2 concentric in overall very good condition, magneto was rebuilt by the best mag re-builder in Australia at that time. The engine was in good mechanical condition with good compression and all that......... Well on a closer inspection with the timing cover off.......You guessed it........the cam timing was out. I actually replaced the cam and followers with new and set the timing per the book.......Happy days thereafter. Sometimes you need to look outside the square..........However, if indeed the mag armature is in original condition, then this would be a good start........Winding failures are more common than condensers.
Here is an easy thing to ckeck first before trying the expensive stuff.
My Velocette Venom had similar problems. I assumed it could be caused by a too rich mixture. Therefore I tried to restart the bike when hot with the throttle more and more open and voila: With the throttle about half open it starts first kick whilst with only 1/16 to 1/8 it wont start at all. If cold, its vice versa.
My Velocette Venom had similar problems. I assumed it could be caused by a too rich mixture. Therefore I tried to restart the bike when hot with the throttle more and more open and voila: With the throttle about half open it starts first kick whilst with only 1/16 to 1/8 it wont start at all. If cold, its vice versa.
Last edited:
Some of those fail too.
I know of two failed BTH units out of a fairly small total number of installations done locally.
Of course some install them and never have an issue, just as many rebuilt Lucas mags work well.
Glen
So far. after reading the post from magneto man I took his suggestion, removed the float chamber, polished up the float needle and put it back together. Now it will not start at all, hot or cold, so I suppose that is progress of a sort.... I was aware of the need to keep the float vertical, it always dies immediately on the side stand or even slightly leaned over, another reason for considering a different carburetor. I can see the level with the float cover removed, not sure what is supposed to be. It does not appear to leak if the fuel tap is open and the tickler is not touched. Tank is full, cap is vented. No problems getting gas to flow.
Re the magneto, it has a spark hot or cold. I opened up a spark plug about 3/16" it sparks when I kick it over with the plug removed.
Spark plugs I tried among others, new Champion N7YC, also NGK BPR7E and BP7E and some NGK platinum tipped. All the spark plugs except the Champions have resistors, other than Champion I can't find spark plugs that do not have resistors. Spark plug resistance checked with ohmeter.
The HT wire and cap do not have resistors, checked with ohmeter.
Since the no start at all I did a rough timing check, the points are opening at about TDC on the compression stroke, ignition retarded, so that is good.
Re my starting technique, since I played with the carburetor settings, I start with gas tap open, kick it a few times, close the choke, kick it a few more times, one stab at the tickler, a few more kicks, another stab and so on, until finally there is gas all over the place and still no start.
Pull the plug and it is still dry. Kicking it through with the compression release pulled in I will get the occasional gun shot bang as unburned mixture fires in the exhaust pipe. I also try various throttle openings, including WOT when I figure it should be flooded.
As far as my technique for starting singles goes, I also have a Yamaha 500 XT which also has magneto ignition, and a reputation for being a bitch to start when hot. I can usually fire it up with one kick when it is hot, two or three when cold. There is a muscle memory thing happening there I am not totally aware of. It did take time to learn how to start the Yamaha, and it was hard at first but I can't really say what I am doing differently other than following the owners manual technique exactly and not over using the enricher thingy.
I have been chasing the comet starting problem since I got the bike a couple of years ago. Along the way I found the cam and followers badly worn, and replaced them. Cam timing was set using a degree wheel, not the the marks on the timing gears. As I posted earlier, when it does run it runs very well.https://www.youtube.com/watch?v=qAoWlDEmGo4
See for yourself,
Comet Running
(Corrected the link. BigEd)
I am leaning towards magneto replacement-rebuilding, guy has a rebuilt one on ebay uk for sale, he also sells dresses and ladies shoes, not that there is anything wrong with that
I would be a lot more confident about mag replacement if the effing thing did not have any sparks, but I also get that any one who thinks they know how electricity works is delusional.
Re the magneto, it has a spark hot or cold. I opened up a spark plug about 3/16" it sparks when I kick it over with the plug removed.
Spark plugs I tried among others, new Champion N7YC, also NGK BPR7E and BP7E and some NGK platinum tipped. All the spark plugs except the Champions have resistors, other than Champion I can't find spark plugs that do not have resistors. Spark plug resistance checked with ohmeter.
The HT wire and cap do not have resistors, checked with ohmeter.
Since the no start at all I did a rough timing check, the points are opening at about TDC on the compression stroke, ignition retarded, so that is good.
Re my starting technique, since I played with the carburetor settings, I start with gas tap open, kick it a few times, close the choke, kick it a few more times, one stab at the tickler, a few more kicks, another stab and so on, until finally there is gas all over the place and still no start.
Pull the plug and it is still dry. Kicking it through with the compression release pulled in I will get the occasional gun shot bang as unburned mixture fires in the exhaust pipe. I also try various throttle openings, including WOT when I figure it should be flooded.
As far as my technique for starting singles goes, I also have a Yamaha 500 XT which also has magneto ignition, and a reputation for being a bitch to start when hot. I can usually fire it up with one kick when it is hot, two or three when cold. There is a muscle memory thing happening there I am not totally aware of. It did take time to learn how to start the Yamaha, and it was hard at first but I can't really say what I am doing differently other than following the owners manual technique exactly and not over using the enricher thingy.
I have been chasing the comet starting problem since I got the bike a couple of years ago. Along the way I found the cam and followers badly worn, and replaced them. Cam timing was set using a degree wheel, not the the marks on the timing gears. As I posted earlier, when it does run it runs very well.
See for yourself,
Comet Running
(Corrected the link. BigEd)
I am leaning towards magneto replacement-rebuilding, guy has a rebuilt one on ebay uk for sale, he also sells dresses and ladies shoes, not that there is anything wrong with that
I would be a lot more confident about mag replacement if the effing thing did not have any sparks, but I also get that any one who thinks they know how electricity works is delusional.
Last edited:
- Status