timing questions? -clyinder firing

[william550]

Hi,
I'm rebuilding my 1979 550 above the crankcase.
Just to help me understand, When re-installing the pistons, cams etc.
I have read how to put the engine at TDC, and then how to line up the cams and cam chain.
When putting it back together, if I put #1 cyl. on my 550 at TDC can I be sure #1 will fire and not #4?
#1 and #4 pistons are both at the top of their stroke at the same time.
While #1 is firing at the top of its compression stroke, is #4 at the top of its exhaust stroke?
With cars I know that the distributer can be 180 degrees out even though the engine is at TDC. and the crankshaft turns twice for every 1 camshaft revolution. Does the crank turn twice for every camshaft rev. in the 550 engine?
Sorry-I don't know how clear or muddy I am on this question.
Bill

[Big T]

Both cylinders for each coils fire together - thus #1 and #4 both spark at the same time, on is on compression and the other on exhaust

So, you can't be "out"

Just follow the cam timing procedure and it should fire right up

[Suzuki_Don]

What BIGT said +1. It is the camshafts that open and close the valves, so until the cams are in place you cannot have the pistons on compression or exhaust stroke. When the cams go on the motor and are timed correctly then you have #1 piston on compression stroke because both valves are closed in that position and #4 is on exhaust stroke because the lobe on the exhaust cam has opened the exhaust valve as you tightened the camshaft caps down to specified torque.

is that as clear as MUD.

[william550]

Thanks guys

So, #1 and #4 will fire at the same time, then 180 degrees later #2 and #3 will fire, and 180 degrees after that, #1 and #4 will fire again etc.
So the firing order would be 1, 2, 4, 3.

So I guess we have a "wasted spark". There are 2 coils and they each spark 2 cylinders every 360 degrees. A cylinder will spark once to ignite the air/fuel mixture, and also 360 degrees later when the exhaust gases are being pumped from the cylinder. The second spark does nothing good, and nothing bad. It just makes things simpler because there are less coil-packs. right?

makes sense now-different altogether than my Mopars from 30yrs ago.
Bill

[bwringer]

That's it exactly. It's a wasted spark system.

The main benefit is that it's simpler and saves the space that would be needed for two more coils.