Cam Sprocket Position

[Jughead]

Zedheads,

Background:

I’ve been having trouble getting my carbs adjusted and eliminating backfire.  I’ve rebuilt both carbs with Z-Therapy kits, adjusted both floats, used a Colortune & Float Syncs to set up carbs, and a Uni-Syn to balance them. Ignition is new Pertronix electronic setup from MotorSport with a 3.0 Ohm coil. 

Still the car backfires under load when driving.

So, I checked my valve clearances.  Most of them were too tight, and I’ve adjusted them all to Haynes’ specs.

When I checked the cam sprocket position, it appears that I have a “textbook” example of a stretched timing chain (see picture below), with the notch on the sprocket to the left of the engraved line on the locating plate.  

I’ve read the Haynes manual and watched the Z-Therapy video on how to adjust the position of the sprocket, and I’m planning to do so next weekend. 

In Preparation, I have a few questions for the experts:

1)      Would this sprocket position (and the tight valve clearances) explain why I was unable to set my carbs?  Between the clearances and the sprocket position, which would be more likely to cause issues?

2)      Would it make sense to try to adjust carbs now with only the valves clearances adjusted?  Or is it a no-brainer to reposition the sprocket?

3)      How will repositioning the sprocket affect the timing light reading?

4)      Based on the picture below,  does it appear that the sprocket has been already repositioned by a previous owner?

5)      Anyone been through this procedure, and have any advice (e.g. potential “gotchas”) etc.?

Thanks much in advance – I’ll report back with results!!! 

Jughead

 

[siteunseen]

That doesn't look right to me. The line should be counter clockwise before the top bolt on the cams thrust plate. And it should be a little to the left of the sprocket notch.

Edited March 14, 2021 by siteunseen

[siteunseen]

I run a ZX distributor fully advanced . All the way clockwise. 17 degrees and 34 at 2,500 rpm. Try turning yours all the way and see how it acts.

[Jughead]

Thanks siteunseen.  Mine is a fairly original 1972 240Z.  Does anyone have one that looks like mine?

 

 

Edited March 14, 2021 by Jughead
clarification

[Zed Head]

Do you have a picture of the timing mark on the damper?  Like site said your timing would have to be way way off to have the notch over there, if you're on TDC.

[Jughead]

I'll provide a picture.  Dan you clarify where/what the "damper" is?

 

[siteunseen]

The pulley it's called sometimes, many names.

The wheel with the the alternator and water pump belt on the front of the motor.

[Zed Head]

 

[siteunseen]

[Jughead]

[Jughead]

OK,  hopefully the 7 pictures above are worth 7,000 words!.  I think I found the mark @siteunseen indicated above.  It looks to be right above the notch.  All these pictures were taken without turning the engine.  Does this indicate the need to reposition the cam sprocket?  The damper pic is blurry, but the 2 white marks are (from right TDC & 10 degrees).  Let me know if a clearer pic would be helpful

Edited March 14, 2021 by Jughead
clarification

[Zed Head]

3 hours ago, Jughead said:

 

3)      How will repositioning the sprocket affect the timing light reading?

 

It won't affect it at all.  But it's not looking like you need to move it.  And the engine should still run well even with cam timing off.

Have you set ignition timing?  Can't really tell which notch on your damper the pointer is pointing at but, actually, everything looks pretty good.  

Backfiring could be lean mixture, or incorrect ignition timing, or flattened cam lobe.

First thing I'd do is get a timing light and set timing to 10 degrees.  Also, make sure that your firing order is correct on the distributor cap, since you've been working there.