old motorbike question

heckflosse · Jun 22nd, 2020, 10:01

Ok, not very Volvo I suppose, but are there any experienced motorcycle people out there?
We have bought a 1978 50cc Honda CB50J for our son to ride to college in the autumn.Its four stroke.
It needs fettling after being laid up. Ive cleaned out the tank and carb plus fresh fuel. Im having ignition problems.
The bike starts from cold, runs for a while then conks out after which it wont restart.
Connections are good and it has a new 6v battery. kickstart of course.
I can get a lovely fat spark for two or three kicks then nothing.
Thoughts are coil/points and condenser.
Its all a bit unfamiliar to those who prefer four wheels!
Any ideas? thanks, Jim

john.wigley · Jun 22nd, 2020, 10:49

1978 is a little new for me, 'heckflosse', but I'll have a go. Firstly, you're definitely looking in the right area. If you have eliminated fuel problems, then the fault is almost certainly electrical.

The points should be relatively easy to check, clean and gap, unlike the BSA Dandy, where you had to split the engine to access them(!), so that is where I'd start. In my experience, condensers are not normally problematic - they either work or they don't. Along with the coil, they are best checked by substitution with a known good one.

If I were to make an educated guess from what you describe, I would suspect the coil. You say the bike will start and run from cold, then stops and refuses to restart? For approximately how long does it run before stopping, and how long before it will once again start. I think that your coil may be breaking down under load when hot. If it'ii run for 15 minutes or so and restart after about an hour or two, that's where I'd lay my money. I think you can rule out timing, assuming that the bike runs well when it is running.

Couple of additional thoughts. Is there an owners' club and / or forum similar to this one that you could consult for model specific information, and is it possible to fit an electronic ignition system in place of the old points-based one?

Regards, John.

reggit · Jun 22nd, 2020, 11:07

When it conks out release the fuel cap and try again. Possible vacuum lock if the fuel tank breather is blocked.

Whippy · Jun 22nd, 2020, 11:27

Quote:

Originally Posted by reggit

When it conks out release the fuel cap and try again. Possible vacuum lock if the fuel tank breather is blocked.

This. Or possibly coil. I had it on my Healey, get it five miles and it would die, checked everything. New coil sorted it. Would like the electronic conversion but want to keep it as original as poss. Now, if only I could find out how my twenty six DKW went back together 🤔

37 RUBY · Jun 22nd, 2020, 11:49

If that sporadic spark is with plug out & earthed to engine or frame then it is most likely condenser or coil failing, high chance it's the former. Possibly poor earth.

If that spark is in the cylinder and gets snuffed out, ie wet plug, then stale fuel with little or no octane.

You have described a sporadic spark so unlikely to be vapour lock.

Has it got points or cdi conversion?

HankScorpio · Jun 22nd, 2020, 12:05

If the coil doesn't come with a ht lead, I'd change that as well.

First bike I ever went on (and first bike I ever crashed) was a cb. For reference, never tell a 12 year old to 'just let go of the clutch'

Jun 22nd, 2020, 10:01	#1
heckflosse Premier Member Last Online: Oct 28th, 2023 12:30 Join Date: Feb 2009 Location: dereham	old motorbike question Ok, not very Volvo I suppose, but are there any experienced motorcycle people out there? We have bought a 1978 50cc Honda CB50J for our son to ride to college in the autumn.Its four stroke. It needs fettling after being laid up. Ive cleaned out the tank and carb plus fresh fuel. Im having ignition problems. The bike starts from cold, runs for a while then conks out after which it wont restart. Connections are good and it has a new 6v battery. kickstart of course. I can get a lovely fat spark for two or three kicks then nothing. Thoughts are coil/points and condenser. Its all a bit unfamiliar to those who prefer four wheels! Any ideas? thanks, Jim Last edited by heckflosse; Jun 22nd, 2020 at 10:30.

Jun 22nd, 2020, 10:49	#2
john.wigley VOC Member since 1986 Last Online: Yesterday 23:59 Join Date: Jan 2007 Location: Leicestershire	1978 is a little new for me, 'heckflosse', but I'll have a go. Firstly, you're definitely looking in the right area. If you have eliminated fuel problems, then the fault is almost certainly electrical. The points should be relatively easy to check, clean and gap, unlike the BSA Dandy, where you had to split the engine to access them(!), so that is where I'd start. In my experience, condensers are not normally problematic - they either work or they don't. Along with the coil, they are best checked by substitution with a known good one. If I were to make an educated guess from what you describe, I would suspect the coil. You say the bike will start and run from cold, then stops and refuses to restart? For approximately how long does it run before stopping, and how long before it will once again start. I think that your coil may be breaking down under load when hot. If it'ii run for 15 minutes or so and restart after about an hour or two, that's where I'd lay my money. I think you can rule out timing, assuming that the bike runs well when it is running. Couple of additional thoughts. Is there an owners' club and / or forum similar to this one that you could consult for model specific information, and is it possible to fit an electronic ignition system in place of the old points-based one? Regards, John. __________________ Time flies like an arrow, fruit flies like a banana .....

Jun 22nd, 2020, 11:07	#3
reggit Premier Member Last Online: Yesterday 21:45 Join Date: Feb 2012 Location: Preston	When it conks out release the fuel cap and try again. Possible vacuum lock if the fuel tank breather is blocked. __________________ 2009 Volvo V70 D5 (Polestar 225bhp) r-design 180,000 miles

Jun 22nd, 2020, 11:49	#5
37 RUBY Master Member Last Online: Apr 24th, 2024 20:45 Join Date: Jun 2017 Location: Chesterfield	If that sporadic spark is with plug out & earthed to engine or frame then it is most likely condenser or coil failing, high chance it's the former. Possibly poor earth. If that spark is in the cylinder and gets snuffed out, ie wet plug, then stale fuel with little or no octane. You have described a sporadic spark so unlikely to be vapour lock. Has it got points or cdi conversion?

Jun 22nd, 2020, 12:05	#6
HankScorpio Floppy member Last Online: Jun 17th, 2021 22:13 Join Date: Jun 2019 Location: So far in the countryside, I can hear banjos	If the coil doesn't come with a ht lead, I'd change that as well. First bike I ever went on (and first bike I ever crashed) was a cb. For reference, never tell a 12 year old to 'just let go of the clutch'

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