1974 260z timing issue

[siteunseen]

4 hours ago, Dolfinz said:

Can you explain the diagram further?  I'm going to remove the fuel lines and provide a clearer straight on picture of the cam sprocket.  Perhaps that will help.

The thrust plate has a small indented line, about 1/4" long.

the cam sprocket has 3 V shaped cuts on the backside for the 3 timing chain adjustments.

the line should be a frog hair to the left of the V for proper chain tightness. This is also another indicator of TDC.

hope this helps.

 

 

 

[siteunseen]

I just noticed something. Your cam sprocket "window" should show the top bolt of the thrust plate. Your's needs to be farther clockwise I think. The bolt is not visible in the photo you posted.

Edited March 9, 2021 by siteunseen

[siteunseen]

I think you have the cam sprocket on wrong somehow. There's a couple of things that aren't right. The small dot shouldn't be above the TDC window. Looking at a sprocket I have the V notch isn't in the same spot as the dots on the front side. I'm sorry if I confuse but that's what I'm seeing. Good luck, I'm gonna hush up now.

 

[Zed Head]

Here's an old thread about the notch and groove.  Some familiar names in it.  Not sure that the windows would need to be at top if there's no notch.

 

Edited March 10, 2021 by Zed Head

[siteunseen]

I'm remembering the ocd things I did to get tdc. With the 1st and 2nd cam lobes kind of bunny eared or the smaller ends up, I used a feeler gauge on the 1st and 2nd rocker lash pads and the cam lobes to get the same gap (thickness/thinness). When they were equal to each other that was TDC.

Smarter people correct me please but that's what I rember. 

Edited March 9, 2021 by siteunseen

[Dolfinz]

I think the question isn't the cam alignment but the crankshaft and it's associated pulley.  With the crank at TDC as shown in the picture and the fact that the pulley can't install any other way due to the keyshaft doesn't that mean the the timing mark has to be in the wrong place?

[Dolfinz]

My question would also be that since the lower end was rebuilt couldn't the keyway have been placed 180 degrees out?

[Zed Head]

There's only one keyway.  It's relation to piston #1 will not change.

35 minutes ago, Dolfinz said:

that mean the the timing mark has to be in the wrong place?

Maybe.  That's what I meant when I said the damper pully might have slipped.  They tend to slip before they come apart completely.  You can pry on it and see if it moves.

Here's what they look like in two pieces.

 

[Patcon]

So I have a question...

How have you confirmed TDC? Not by marks or some other, but true TDC?

[Dolfinz]

What other method is there?  We turned the motor to verify compression stroke and the piston was verified at TDC by using a screwdriver inserted through the spark plug hole to insure it didn't fall once it rose to it's maximum height.  The lobes are flat and the crank is where the service manual timing mark shows it should be.  i still question that since the lower end was rebuilt by one shop and the head was installed afterwards if the lower end wasn't 180 degrees out with the new head possibly putting the timing mark on the pulley out of alignment?  Perhaps the lower end was on the compression stroke and the head was on the exhaust?  I'm not a mechanic and this is confusing.  I have personally rebuilt this engine 3 times and this has always been the most difficult part.  Having others involved has only clouded the situation regarding the work that was done.  I don't believe the damper has slipped on the pulley as I have two pulleys that are exactly the same.  I really appreciate the help y'all are providing and I think we'll figure this out.  Keep the ideas coming and thanks.

[jonbill]

at the bottom end, there isn't a difference between compression stroke or exhaust stroke. theres just pistons going up and down. the cam timing determines whether a piston up stroke is compression or exhaust. 

so if the piston #1 is at TDC (determined by your screwdriver) and the cam dowel is correctly engaged with the cam pulley and the marks line up on the cam pulley and the thrust plate, then #1 is at TDC on compression stroke. 

Then if the distributor drive spindle is aligned 11:25 with the tang offset to the correct side (which side? can't remember!) then the spark timing is in the right ball park and the engine should attempt to run if you have sparks and fuel. 

[Patcon]

Yes, what Jonbill said