Valve timing

[chrisx1]

When my engine is at top dead centre on compression stroke the left hand exhaust valve is slightly open. To get both exhaust valves closed it doesn't happen until 5mm after TDC. It's the same with the R/H ex. Valve. And then the R/H inlet valve is also closed.

[idie]

Sounds like valve overlap most engines that are a bit sporty are like that.

[DAVE BRADY]

Hi,

TDC is compression stoke on one cylinder and is the end of the exhaust stroke on the other.  If you take the plugs out and look in the cylinder you should just see the valves as they reach fully open .  Watch one of the pistons and you will see the inlet valve open and then close as the piston descends and then rises.  This will be the induction and compression stroke.  When the piston reaches TDC both valves will be closed.  As you rotate the engine further and past BDC simulating the power stroke and bring the piston up again you will see the exhaust valve open.
I will not try to explain the 'overlap' and 'scavenge' but there some good videos on youtube.

Dave.

[chrisx1]

So, as I said the exhaust valves don't both close until the piston has descended by 5mm so as Idie said could this be valve overlap and if so is that ok?

[DAVE BRADY]

Hi,

A65 and A10 manuals have the valve timing and the amount of overlap can be worked out so some over lap is built in so it is OK.  It is also that each piston is on a different stroke.

Dave.

[chrisx1]

I'm not the most technical person Dave but when I look at the manual it says that the valves close 37 before tdc I don't know what that is but clearly it should be closed before tdc not after it. Otherwise it would have squeezed fuel and air out into the exhaust, wouldn't it?

[V500]

Yep.
To follow the simplest  "suck, squeeze, bang, blow" sequence, the exhaust valve shouldn't be opening until the end of the bang stroke/ beginning of the blow stroke.

[DAVE BRADY]

Hi,

Think of each cylinder separately.  At tdc on the compression stroke both valves are closed.  At TDC on the exhaust stroke the exhaust valve will  be open.
Now with a BSA twin the cylinders run parallel at 360º so when one cylinder is at TDC on the compression stroke the valves are closed whilst  the other cylinder is at tdc on the exhaust stroke so the exhaust valve is open.
The BSAOC has a cutaway A10 engine that sometimes is taken to shows and as it is cranked the process can be seen.  I do not know where or when it will be on display next.

Dave.

[chrisx1]

Of course, I was thinking that they went up and down opposite to each other but having just had the head off I know that's not so. It's old age Dave.😂