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Discussion Starter · #1 ·
I’m posting this with a friendly invitation to blow holes in my thought process in trying to solve a problem I’ve been having with my wee bike.
Lets start with the specs:
1974 CB360G with 6750 (maybe) miles on the clock.
Recent rebuild of top half of engine. All new gaskets. Valves cleaned and lapped. New stem oil seals and no play in valve guides.
Valves are adjusted and rechecked. Cam chain tension is adjusted and correct.
New OEM contact breakers and condenser. Timing set statically and then dynamically with timing gun. Ignition advances as it should when revs rise. Battery hold a good charge and a new rectifier regulator unit is outputting 13.6-13.8 volts.
Carbs are cleaned, floats set to spec and balanced. Fuel is flowing freely carbs in sufficient quantity. The petrol cap is clean as a whistle and venting correctly.
Compression in both cylinders is above 150psi cold.
The bike starts pretty much on the button from cold and runs very well on the idle circuit.
At ¼ to ¾ throttle she pulls well up to about 70mph and that’s when the problem starts.
Above ¾ throttle the bike begins to be very hesitant and can bog before taking up again. It will not go over 75-80 mph in any gear. If I slow down the symptoms begin to disappear. On coming to a halt the bike has a tendency to stall on take up.
I checked the oil and found a ‘slight’ smell of petrol (it will get changed as soon as).
Left plug is a nice light brown colour. Right plug is slightly darker. They are dry, not oily.
Deduction
I’m suspecting the coils. They perform well until put under pressure at higher revs when I think they may be breaking down. I think that when this happens, it is causing the cylinders to be washed with unburned fuel which is making its way down into the crankcase. They are getting sufficient power from the battery. The right coil is my primary suspect as the right plug is darker than id like it which leads me to think that WHEN it fires the unburned fuel is causing a slightly rich condition in that cylinder.
I am open to any other sensible suggestions.
Many thanks in advance
 

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So is it a fuel or ign problem...

Does it seem to misfire at the higher speeds ?

Dwell correct and equal for each set of points and new condensers fitted ?

Red neon plug caps are a great visual aid for diagnosis as they flash when HT passes thru them but will need and adapter to fit to bike.

They will be easily visible at night, so "missing" HT can be seen when the problem starts...

They are inexpensive...

So, rule in or rule out an ignition problem but first check dwell.

A scope ( or neons ) will show up ign problems...

Can't remember if your HT coil has removable HT leads. If non removable, do they wobble/seem a bit loose in the coil HT tower ?

I assume the oe plug caps are all good.
 

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Discussion Starter · #5 ·
So is it a fuel or ign problem...

Does it seem to misfire at the higher speeds ?

Dwell correct and equal for each set of points and new condensers fitted ?

Red neon plug caps are a great visual aid for diagnosis as they flash when HT passes thru them but will need and adapter to fit to bike.

They will be easily visible at night, so "missing" HT can be seen when the problem starts...

They are inexpensive...

So, rule in or rule out an ignition problem but first check dwell.

A scope ( or neons ) will show up ign problems...

Can't remember if your HT coil has removable HT leads. If non removable, do they wobble/seem a bit loose in the coil HT tower ?

I assume the oe plug caps are all good.
Coil leads are not loose. New OEM plug caps. Points are OEM new and set to correct gap. Ignition timing set with strobe gun at the crank and is dead on. Ignition advances as expected when throttle is opened.
At high RPM & Load, the bike is 'missing' as you describe and will not gain power past 7000 rpm.
 

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At high rpm and load the engine is misfiring etc....

Ideal for using the red neon plug caps to help in diagnosis...missing flashes when problem starts...or not, as the case may be...

Rule in or rule out "missing HT "...

You could of course use a Dyno and scope etc., A rolling road...
 

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I’m posting this with a friendly invitation to blow holes in my thought process in trying to solve a problem I’ve been having with my wee bike.
Lets start with the specs:
1974 CB360G with 6750 (maybe) miles on the clock.
Recent rebuild of top half of engine. All new gaskets. Valves cleaned and lapped. New stem oil seals and no play in valve guides.
Valves are adjusted and rechecked. Cam chain tension is adjusted and correct.
New OEM contact breakers and condenser. Timing set statically and then dynamically with timing gun. Ignition advances as it should when revs rise. Battery hold a good charge and a new rectifier regulator unit is outputting 13.6-13.8 volts.
Carbs are cleaned, floats set to spec and balanced. Fuel is flowing freely carbs in sufficient quantity. The petrol cap is clean as a whistle and venting correctly.
Compression in both cylinders is above 150psi cold.
The bike starts pretty much on the button from cold and runs very well on the idle circuit.
At ¼ to ¾ throttle she pulls well up to about 70mph and that’s when the problem starts.
Above ¾ throttle the bike begins to be very hesitant and can bog before taking up again. It will not go over 75-80 mph in any gear. If I slow down the symptoms begin to disappear. On coming to a halt the bike has a tendency to stall on take up.
I checked the oil and found a ‘slight’ smell of petrol (it will get changed as soon as).
Left plug is a nice light brown colour. Right plug is slightly darker. They are dry, not oily.
Deduction
I’m suspecting the coils. They perform well until put under pressure at higher revs when I think they may be breaking down. I think that when this happens, it is causing the cylinders to be washed with unburned fuel which is making its way down into the crankcase. They are getting sufficient power from the battery. The right coil is my primary suspect as the right plug is darker than id like it which leads me to think that WHEN it fires the unburned fuel is causing a slightly rich condition in that cylinder.
I am open to any other sensible suggestions.
Many thanks in advance
Sounds like it is simply starving for fuel trying to pull in 5th gear at high speed. What jets are you running? Just speculation though.
 

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I'd do a full throttle plug chop, sounds like running too rich on main jet. Did you just clean the carbs or put a carb kit through them, it's not uncommon to have incorrectly sized jets in aftermarket kits. And double check you diaphragms are seated properly and don't have any holes, but I'd expect them to cause problems at half throttle as well.
 
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