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CB750 Barn find project - First Timer
K2-K6:
Condenser is there to in effect "buffer" the the electric load being switched by the points.
As the points open to break the earth contact to the coils (this starts the HT spark travel from the coils to plugs) the voltage wants to jump across the points gap as it's very small at the start of opening. The condenser absorbs this affect (stops the sparking) to give a clean switching of the points circuit.
If the condenser is inefficient, then the spark across the points will increase. If you got such a bad jump across there it would not be switching the coils as they would still be earthed.
Unlikely to get both failed together but I guess possible.
Rob62:
In other words, if you take the points cover off and run the motor and the points are visibly sparking like mad then the condenser is knackered, if they aren’t its ok. :)
Underdog1:
Nice one for thebasgice on checking points guys.
I’d like to get the bike back to the same state to see if it stops getting a spark. I’m aware these bikes get very hot when idling and I don’t want to damage anything, but I’m thinking about leaving the bike idle away until It cuts out so I can test the spark at home rather than take it for a ride and brake down somewhere..
If the sparks good when it dies then I guess that rules out electrical stuff. I could try easy start around the carb boots too when it heats up to see if there are any leaks
K2-K6:
Rob's right, loads of sparks they need to by changed.
To add, when very bad they are reluctant to start very well which you don't initially have.
Float bowl breathers, you're right that it's the paired vents into the chambers that allow pressure neutralising. You should be able to blow into them to see if they aren't clear as a quick and dirty check.
I wouldn't worry about heat build up at tickover on these, I've been in bike jams into race circuit on a hot day for much longer than that. They don't really overheat as such but probably abit hotter than ideal.
It still looks like fuel to me unless you can eliminate something else. They'll run close to 5 miles on full float bowls with fuel tap turned off if going gently.
To progress that theory of slow or restricted filling, when it stops, turn off fuel and remove one outer float bowl to see if you have fuel there or not. Think you can just get them off with a stubby screwdriver.
That was what I was aiming at with the easy start test...... No fuel scenario, add flammable to carb throats, if you get response then it would confirm....I wouldn't put easy start around the outside of a hot engine as you may set it alight!
Have you got a good flow of fuel from tank outlet? Again, if you can get a float bowl off you can see how quick the fuel is flowing through the entire delivery system from tank to destination.
Rob62:
This kind of fault can be difficult to diagnose.... if its fuel starvation and you go through all the various components to ensure they aren’t blocked or damaged you will eventually eliminate it. For an electrical fault it will also be a process of eliminating the faulty component. If an electrical component is damaged, cracked, broken etc as it warms up it can lose connection then as it cools it reconnects.... so the only real way to diagnose it is to replace components one at a time until you solve the problem. The condensers are so cheap they should be changed anyway, if that works then its a cheap result... i would suspect the coils but it could be anything. Does it stop dead or begin to missfire and die gradually?
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