Cam timing question

markjenn · Feb 16, 2009

Hi, I'm getting ready to do my first valve check on my 2005.

I've read most of the tutorials and have done this before on several other bikes, so I'm reasonably familiar with the process and pitfalls. One thing caught my eye in the (very poorly written) service manual. Before timing the cams, it says "when piston #1 is at TDC on the compression stroke, align the T mark...", then it goes through the process of aligning the cams. My question is this: If the cams are out of the engine, how do you know whether you're at TDC after the compression stroke or TDC after the exhaust stroke? Doesn't timing the cams establish the difference? Is there anything on the bottom end of the engine which knows the difference between the two TDC's? (I assume the FJR is a "waste spark" engine that fires the ignition every time a piston is near TDC, not just after the compression stroke.) I see there is a "cylinder identification sensor" in the head so I would assume this is what tells the ECU the difference between the strokes, but this senses something on the cam, right?

My plan is to leave the crank alone and only work on one cam at a time if I have to reshim, but I still was wondering about this.

- Mark

ionbeam · Feb 16, 2009

If you follow the (******) manual... When cyl #1 is at TDC the lobes of the intake & exhaust cams will be at 10:00 & 2:00. You will know that you are on the compression stroke because the coil pickup rotor will have the T on the left side at 9:00 and aligned with the seam in the case. If the T is in any other position cyl #1 is not on the compression stroke. Have I under thought your question/contemplation?

charismaticmegafauna · Feb 16, 2009

markjenn said:
My question is this: If the cams are out of the engine, how do you know whether you're at TDC after the compression stroke or TDC after the exhaust stroke? Doesn't timing the cams establish the difference? Is there anything on the bottom end of the engine which knows the difference between the two TDC's? (I assume the FJR is a "waste spark" engine that fires the ignition every time a piston is near TDC, not just after the compression stroke.) I see there is a "cylinder identification sensor" in the head so I would assume this is what tells the ECU the difference between the strokes, but this senses something on the cam, right?

Right...., "physically", there are 2 positions of the crankshaft for TDC #1 -- you determine it (which position) when you install the cams.

But, there are ancillary (ignition/computer) items that need to be considered.

markjenn said:
My plan is to leave the crank alone and only work on one cam at a time if I have to reshim, but I still was wondering about this.

I'd recco that you don't worry about it that much. You're smart enough and have a good understanding of what's going on here -- to just dis-assemble all of what you need to. You'll work-through how the cams need to be installed.

It's the best way, I think. Short-cuts (leaving camshhafts in -- leaving chains attatched to sprockets, etc. -- just add to the difficulty of the operation.

I have confidence you'll prevail....

Brodie · Feb 16, 2009

Mark

Keep it simple, when you pull the cam shafts do not rotate the crank shaft. Since the cam shafts were at 10:00 and 2:00 for cylinder 1 when you pulled them, when you reinstall them things will be right.

As far as I can tell, the manual doesn't instruct you on how to find the TDC for compression as opposed to TDC for exhaust after an engine tear down. So logic dictates it must be a wasted spark design. Imagine what kind of power we'd be dealing with if that wasted spark was actually used with another compression stroke. :hyper:

One more thing I may add, take lots of pictures with your digital camera through every step of disassembly. Since you paid for developing when you bought that high capacity memory card the pictures are free. My pictures save me a lot of grief when attempting reassembly, they answer a ton of questions!

Hope it helps.

Brodie

UselessPickles · Feb 16, 2009

Brodie said:
So logic dictates it must be a wasted spark design.

There's also only 2 ignition coils on the bike. One is shared by cylinders 1 & 4, the other shared by cylinders 2 & 3. There's only 2 ignition wires coming from the ECU; one for each coil. Maybe that "wasted" spark helps burn excess fuel during the exhaust stroke (in conjunction with the fresh air provided by the air injection system).

wfooshee · Feb 16, 2009

Wasted spark is something all modern electronic fuel injection systems use. The spark is triggered by the crankshaft, and there's no need to determine if compression or exhaust just happened. It's not left there to do anything, it's just easier than timing spark off the valve gear.

If the cams are out there's only one TDC for #1. It becomes the compression stroke when you put the cams in. Ensuring the crank is on the compression stroke would be for checking the cams if you suspect a timing problem, like a jumped tooth or something.

markjenn · Feb 16, 2009

wfooshee said:
If the cams are out there's only one TDC for #1. It becomes the compression stroke when you put the cams in. Ensuring the crank is on the compression stroke would be for checking the cams if you suspect a timing problem, like a jumped tooth or something.

Thanks for everyone's replies.

Yes, this is the crux of it - if the cams are out, then there doesn't appear to be any way to distinguish between the two TDC's: after compression and before power vs. after exhaust and before intake. So really, all you need to do is find put the motor at the one and only TDC and you make it TDC after compression by installing/timing the cams. The manual basically confuses things by indicating there is a need to ID TDC after compression with the cams out.

I was just wondering if there was some sensor somewhere else in the engine irrespective of the cams that was timed every other rotation in time with the cams which needed to be at the proper position. Doesn't sound like there is.

I bet the "cylinder ID sensor" in the head is there to tell the ECU which TDC is which. Most of today's modern FI engines have a "camshaft position sensor" which does this and perhaps Yamaha just called there sensor something else. While the ignition system doesn't necessarily need to have this info in a wasted spark design, the FI system probably needs to know when to squirt fuel and they may only want to squirt fuel in time with the actual engine intake which would need the camshaft position, not the crank position.

Have I said yet that the shop manual really sucks? Thanks again,

- Mark

ionbeam · Feb 16, 2009

ionbeam said:
...When cyl #1 is at TDC...You will know that you are on the compression stroke because the coil pickup rotor will have the T on the left side at 9:00 and aligned with the seam in the case. If the T is in any other position cyl #1 is not on the compression stroke...

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Just wonderin'

wfooshee · Feb 16, 2009

If you're trying to emphasize the rotor timing mark, I don't see what you're saying. Unless you've pulled the rotor, it will be aligned at both compression and exhaust TDC, because it's on the crankshaft, not the camshaft. It can't get out of phase with the piston. That extra tooth tells the sensor that it's #1 coming to TDC, but it can't tell it if it's compression or exhaust. It just can't.

Until the valve gear is connected to the crank, there's just no such thing as compression stroke vs exhaust stroke.

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