Valve Operation On BSA A65

[Kev59]

Hi,

I'm having trouble getting a newly rebuilt 1971 A65L to run with the choke off.
I've tried all carb alterations to enrich the mixture to no effect.
The carbs are off at the moment and I've noticed that on compression stroke some of the cylinder contents are blown back through the inlet port.
The valve shuts before ignition I'm sure as there is no firing in the carbs.
Someone said to me it's valve overlap and perfectly normal.
I'm not so sure and wonder if this is the cause of the weak running with the choke off.
Any ideas?

 

[atomsplitter]

Overlap is the time both intake and exhaust valves are open and is necessary for efficient engine performance. During the exhaust stroke the rising piston accelerates the exhaust gasses out the exhasut port into the exhaust pipe. At Top Dead Center the exhaust gasses are at maximum velocity and the piston begins the downstroke for intake. As the exhaust gasses reach the collector in the exhasut system the radiply expanding volume rapidly decreases exhasut pressure, creating a vacuum effect in the combustion chamber. With the exhaust valve open the intake valve opens and the in-rushing air helps sweep out the remaining exhaust gasses left behind into the vacuum in the collector. The vacuum in the collector is what promotes the efficiency of getting a fresh air charge for every intake stroke. As the piston nears bottom dead center the exhaust valve shuts and then the intake valve shuts for the compression stroke. On carbureted motors there is always some fresh fuel going down the exhaust pipe becuase it's mixed in the carb before being sent to the cylinder. (Fuel injection not so much as the fuel can be injected after the exhaust valve shuts).

As to your needing choke to run the bike, that is a function of the idle mix screws. There are two types of mix screw, one that meters fuel (like the CV Mikuni's on my old XS 1100) and one that meters air (my Trophy 1200 uses this type of CV carb). In the fuel mix type the fuel flow is adjusted to meet the idle requirements of the motor. As RPM is increased then the various jets are employed to increase fueling. In the case of the air mix screw, the idle fuel flow is fixed and the amount of air allowed into the combustion chambers is regulated. The best way to determine if your motor is running too lean is using a "Colortune" spark plug. This allows you to see the flame color under operating conditions (i.e. motor is running). The idle mix screws can be adjusted while the motor is running so you get the correct flame color (hence color tune) and that should be the ideal setting for throttle response.


View: https://youtu.be/9mKQvWM0nMg

 

[Kev59]

Thanks atom,
"As the piston nears bottom dead center the exhaust valve shuts"
Mine are open at BDC and only start to close as the piston rises.
Valve clearances are fine.
Is this a problem with the valve timing?

 

[atomsplitter]

Not necessarily. The cam timing marks need to be verified they are indexed properly. If so, then it's fine. If you have a service manual it should have the information on camshaft timing.

 

[proturn1]

Hi,

I'm having trouble getting a newly rebuilt 1971 A65L to run with the choke off.
I've tried all carb alterations to enrich the mixture to no effect.
The carbs are off at the moment and I've noticed that on compression stroke some of the cylinder contents are blown back through the inlet port.
The valve shuts before ignition I'm sure as there is no firing in the carbs.
Someone said to me it's valve overlap and perfectly normal.
I'm not so sure and wonder if this is the cause of the weak running with the choke off.
Any ideas?

Hi
Have you double checked the cam timing with a degree dial on the crank to check the valves are closing at the correct degree on the dial? On the compression strokes.