CB750 Barn find project - First Timer

[sye]

Could be  vacuum in the tank caused by a blocked breather. Take the fuel cap off when it conks out and if air rushes in it could be the culprit.

[Underdog1]

Conking out after 15 mins then not starting till cooled down sounds like an electrical component failing at higher temperature... coil maybe. Or it could be fuel starvation but unlikely....

Could be the condensers, they are the original ones.

Didn’t have any tools with me to check the spark, was thinking at the time it would be ideal to have a spare spark plug in my pocket as I could do it with no tools

[Underdog1]

Could be  vacuum in the tank caused by a blocked breather. Take the fuel cap off when it conks out and if air rushes in it could be the culprit.

Tried this at the time, didn’t make a difference

[K2-K6]

Not too rich is it? Just chokes it when warm possibly.
What colour are the plugs?  Do they indicate rich setting or the opposite?

If rich,  crank it with throttle wide open to get more air in and it may go.

There are breathers into the float chambers of each carb that allow the fuel level to stabilise,  you can run into problems with filling if these are not all clear.

As a means of diagnosis,  when it stops you can open the throttle and spray some easy start into the carbs via the filters to see if it'll fire again straight away.

[Underdog1]

Not too rich is it? Just chokes it when warm possibly.
What colour are the plugs?  Do they indicate rich setting or the opposite?

If rich,  crank it with throttle wide open to get more air in and it may go.

There are breathers into the float chambers of each carb that allow the fuel level to stabilise,  you can run into problems with filling if these are not all clear.

As a means of diagnosis,  when it stops you can open the throttle and spray some easy start into the carbs via the filters to see if it'll fire again straight away.

Just popped back out to ensure the battery was charging properly and it roared to life on the button no problems.

I did try cranking it with open throttle when I was stranded earlier and anything it seemed even more reluctant to start some how. By breathers into each float chamber do you mean the little tubes that join each carb bowl pair together (connected by a rubber tube)? I was meticulous in cleaning the carbs - soaked in carb cleaner, scrubbed, wire pushed through holes, multiple passes fully stripped down in a US cleaner, blew through with compressed air rebuilt with all new bits etc..

I’ll check the plugs out tomorrow, jetting is probably out and they haven’t been synced yet.

What’s a symptom of a dodgy condenser?

[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?

[Underdog1]

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?

It dies gradually, seems like it’s not pulling properly and then stops acellerating but will maintain speed if I stay on the throttle, as soon as the throttle let off it dies.

I’m going to order new condensors, I assume the DS repro ones are ok? What about his repro coils?

[Rob62]

The gradual thing doesn’t sound electrical to me..... I have new ds condensers on my k4, one of them failed after 50 miles (missfiring and spaarking like hell under the points cover) so I can’t recommend those... I’m wondering if its fuel or worse still mechanicsl. Have you checked valve clearances and compression?

[Underdog1]

The gradual thing doesn’t sound electrical to me..... I have new ds condensers on my k4, one of them failed after 50 miles (missfiring and spaarking like hell under the points cover) so I can’t recommend those... I’m wondering if its fuel or worse still mechanicsl. Have you checked valve clearances and compression?

Set the valve clearance when I rebuilt, haven’t checked compression but everything was within spec,barrels were honed and rings replaced.

Could be fuel flow as I’m getting random carb bowl overflows.. will have time tomorrow to check it all.

I’ve got a hondaman ignition module to go on but I want to have it running sweet before I add more things into the mix

[Rob62]

Ps if you want repro coils try the ones from wemoto......much cheaper and work well i can vouch for those!

[Rob62]

Check compression! Burned or leaky valves could be an issue.... build up of carbon on the stems can cause the valves to stay open once warmed up  (i had this issue back in the 80’s)... a re-lap of the valve seats transformed the motor. A compression test will help, when cold and also at running temp... keep going, you will get there.