XJ650RJ Seca Timing Question

Hi everyone... I'm rebuilding the top end of my 1982 XJ650RJ Seca and I have a timing question.

As we know, the way this engine is set up is with two camshafts on the top, obviously intake and exhaust and a timing chain. For every revolution of the crankshaft, the camshaft spins twice. There are markings on the crankshaft and camshafts to set the timing with the chain per the manual instructions. Seems pretty straight forward.

I put the crankshaft on TDC (and I verified that #1 and #4 are TDC) I've lined up the camshaft correctly and set the chain tensioner per the instructions in the manual...

I hand crank the crankshaft 1 full turn and the camshaft timing marks are 180 degrees off. I continue and turn the crankshaft another turn and the camshaft timing marks are back in alignment.

My question is... Is there a difference of which position of the crankshaft is in when I align the camshaft marks???

I have scoured my workshop manual for this answer and in the timing section, they don't mention this difference. I've attached a pic of the page...

 

Stop and think about this for a sec. It's a four stroke motor, which means two revolutions of the crank are required for one complete ignition cycle.

But, during those two crankshaft revolutions (four "strokes") the cams only have to open/close their respective valves... once.

My point being, the camshafts are not turning 2X crank speed; as you've discovered they are turning at 1/2 crank speed. The pistons have no way of "knowing" if they are in the midst of a compression/power crankshaft cycle, or an exhaust/intake cycle. They just go up and down once every time the crank shaft revolves. So let's consider one pair of pistons, one and four. They are going to go past TDC once every crank revolution. The cams, on the other hand, only need to do their thing once every other crank revolution. There is no "which position" for the crank, TDC is TDC it only happens once a revolution. If you bring pistons 2 and 3 to TDC, your TDC mark on the crank will be 180 degrees "out." (Or thereabouts; for discussion, this assumes a 180 degree crankshaft.)

So, to summarize, what you re seeing is perfectly normal, the cams will only be in the "proper position" once every other crankshaft revolution; and if they keep coming back to all lined up, you done good.

Hopefully this makes sense, I do need more coffee...

 

+1, but

TDC for cylinder #1 is the baseline or setting valve timing.
For the purpose of setting the cams the #1 cylinder needs to be at TDC so the marks on the cams line up with the marks on the head. This is to ensure that spark timing and valve timing do not end up out of phase.

 

Yes I just installed my cams a few days ago on my 900f. Same thing the dots on the cams that are visible through the right hand end cap appear once every other revolution. I wondered about that and then realised what was happening. So when a piston moves from top dead centre to bottom dead centre 180 degrees the cams turn 90 degrees i think?

 

Correct.

 

Thanks everyone for the 4 stroke lesson... It makes sense to me.

So both #1 and #4 are TDC at the same time, but when I check their lobes on the cam shaft, they are 180 degrees different which tells me when one is in it's power stroke, the other is in it's exhaust stroke... Correct?

My new question is how do the electronics know which plug to fire? Is this what is meant when the system is out of phase?

 

With cam and ignition timing set correctly one pickup fires cylinders 1 & 4 and the other 2 & 3. It is a waste spark system so on the power stroke on cylinder 1 cylinder 4 gets a spark too.

The timing marks on the cams and pickup backplate and rotor ensure the spark happens at the correct time in the engine cycle. If the cams were out of phase valves would be bent and the spark would happen at the wrong time.

 

Not quite, when #1 is on the power stroke, #4 in on the intake stroke

 

That makes sense... I thought about a wasted spark, but didn't know that is how they designed it. Simplifies the design I guess, but I have never heard of it before. Looking at the pickups, I see how it works now.

Thanks all for the lessons!!!

 

Without the waste spark you would need four coils.

 

Or a distributor, but then you have a car!

 

It can be argued the crankshaft rotation and pickups act like a distributor in sending the pulses to the TCI. But traditional distributors are different they have a cam and points like my 1974 Honda CB 750 SOHC. Apart from a distributor making the spark happen at the correct time the difference is that the points on a distributor cut off the low voltage to the primary coil as they are opened by the cam therefore inducing high voltage in the secondary coil both of these being inside the ignition coil. On our bikes the transistors in the TCI units do the same job as points did in a distributor and that means the ignition timing is controlled electronically rather than mechanically. There are two main transistors in the TCI that are cutting the power at the right time in the engine cycle to create a spark at the plugs on the waste spark system.

 

Quite a few early multi-cylinder motorcycles have distributors. The Indian-4 is one example.

 

I think the NSU Mammut is the same but that had a car engine.

 

while this thread has bounced all over the motorcycle spectrum, don't forget the Amazonis

 

That old Beetle motor has been used in countless chassis.

 

And valve overlap had just happened on 4.

 

The mistake here is calling tdc a stroke, ultimately it can't be.
You have tdc at the top of the compression stroke (the squeeze-bang). You have tdc at the top of the exhaust stroke (the blow).
Someone said the ignition is wasted spark - it is, the spark for squeeze-bang and the wasted one where the spent gases have just been blown out. Well you hope they were spent, if there was a misfire previously you could have another in the exhaust, technically.

 

i have wasted day ignition on my push mower

 

Yes indeed that makes sense.