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old motorbike question

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Old Jun 22nd, 2020, 11:01   #1
heckflosse
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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 11:30.
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Old Jun 22nd, 2020, 11:49   #2
john.wigley
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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.
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Old Jun 22nd, 2020, 12:07   #3
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When it conks out release the fuel cap and try again. Possible vacuum lock if the fuel tank breather is blocked.
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Old Jun 22nd, 2020, 12:27   #4
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Quote:
Originally Posted by reggit View Post
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 🤔
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Old Jun 22nd, 2020, 12:49   #5
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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?
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Old Jun 22nd, 2020, 13:05   #6
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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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Old Jun 22nd, 2020, 13:53   #7
heckflosse
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Thanks everyone!
It is probably an HT lead fault.
I have ordered coil(which has a built in HT lead}, a new plug, points and condenser. Also the special puller tool to get the flywheel off.
Bits are refreshingly cheap but all that grovelling around on the floor to work on it is a bit like my day job(I'm a chimney sweep).
Regarding electronic ign, I looked into that and it needs a 12v conversion etc which is a bit overkill for an old bike.Otherwise I'd have gone that route without hesitation. Had an Accuspark kit on my old Westminster and that was brilliant. My Rocket 88 is still on points but with MSD and all our 240s are electronic.
The fun we have with our old junk, eh?
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Old Jun 22nd, 2020, 14:35   #8
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Another guess for coil. My CX500 engined JZR wouldn’t start when hot, unless I changed one of the plugs. A bit inconvenient if stalled in traffic.....

I tried replacing plugs, leads, coils, fitted lecky ignition, no better.

Then I realised all the spares I’d been using were 20+ years old, so I bought a pair of new £8.50 Far Eastern coils via eBay.

Sorted.
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Old Jun 22nd, 2020, 18:17   #9
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I have had condenser break down when hot on a 1600 cortina, intermittent fault that would stop the car and often 5 or 10 minuets of standing would see it start and run perhaps for a couple of days. Finally broke down at a traffic lights and rather than potch just fitted the new spare condenser, never had a problem after. Back in the days of points they were changed every service and a spare condenser was always in the box just in case. Pitted points could be taken out and rubbed on the kerb stones to get you home if needed and if done regularly .015" could be accurately judged by eye. A dicky condenser the only option was to change it, a loose condenser could also cause intermittent miss fire.

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Old Jun 22nd, 2020, 19:22   #10
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Hi jim,

cb50.. good one to get used to riding on 2 wheels 😁

could it be fuel starvation?, i think theres a mesh or nylon strainer in the fuel on-off lever assy, might be its full of crud and limiting the flow of fuel.. or perhaps float in the carb related sticking or not moving enough maintian the fuel level ? always worth trying something different if your replacing isnt helping much...
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