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Cam Sprocket Position


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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

 

As Found Cam Sprocket Position.jpg

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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.

20210309_145225.jpg

20210309_145521.jpg

Edited by siteunseen
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Posted (edited)

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

 

 

Edited by Jughead
clarification
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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.

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Posted (edited)

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 by Jughead
clarification
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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.

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Thanks Zed Head. 

The pointer is pointing to the first notch in this picture - the furthest to the left, when they're on top & you're viewing from the front of car.  There are 5 notches.  At idle, under the timing light, it points to the 3rd notch from to the left (10 degrees).  

And I've double / tripled checked the wires. 

However, I haven't started the car since adjusting the valve clearances. Do you think that might make a difference? 

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An open valve might cause intake popping.  The valve lash gets tighter also as the engine warms up.  So it seems possible that a valve or two might not seat fully as the engine warms up.

Just for clarity, these engines often get what people call "front-fire" or intake system popping.  Backfire usually means popping out of the exhaust pipe.  I'm guessing the noise is from the intake manifolds/carbs?

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Thanks for that clarification.  Actually,  I've experienced both backfire & front-fire, depending on mixture setting. Just couldn't get the right mixture, which is why I yanked the valve cover.  In order to get it to run right i've had to enrich the mixture to the point where it fouls the plugs.  Then, when I lean it out, it starts to front-fire.

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 What wt. oil are you using in the carbs? Thicker oil will enrich the mixture under a load. 20 wt. was recommended by the factory. Were you using the nozzle screws to adjust the mixture? If the floats are at the proper ht. (primary mixture adjustment), 2 1/2 turns down on the nozzle screws should get you close. Do you know what needles you're running?

Edited by Mark Maras
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Posted (edited)

I'm running the needles I got as part of the Ztherapy rebuild kit.  And I used the "blow method" to set the floats.  I've tried SAE 20wt,  but Z- Therapy suggested ATF,  so that's what's I'm using now.  I need to screw the mixture "nuts" ~ 4.5 turns, otherwise it front-fires and runs like crap.  Around there I get a decent sounding idle, minimum smoke out of the exhaust & the lift pin test seems right.  I have NOT gotten a decent bunsen blue ColorTune reading yet.  It kinda goes from yellow to white-ish blue to white...

Steve @ Z Therapy has told me that since I live @ sea-level (Seattle), the 4.5 turns wasn't surprising. 

Edited by Jughead
typo
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4 1/2 turns down surprises me.  Seattle, Portland and Salem are all within a few hundred feet of sea level and to my knowledge, 2 1/2 turns down has been the standard for anything near sea level. I'd look into rechecking the float levels and identifying your needles.

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I just reset the floats.  Here's the procedure I used (which I got from Steve @ Z-Therapy):

1) Removed float chamber top and held with float on bottom.

2) Blew into fuel banjo fitting while slowly lifting float and holding 9/16" spacer between top of float and bottom of float chamber cover.

3) Adjusted (bent) the float adjustment tab so that airflow thru the banjo just stops at  9/16", and air flows with any downward movement of the float.  

After doing this, I checked the levels with Float Syncs. The levels are exactly the same in both carburetors: ~ 1/2" above the top of the metal bases of the Float Synchs.

Thoughts?

 

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22 hours ago, Jughead said:

I just reset the floats.  Here's the procedure I used (which I got from Steve @ Z-Therapy):

1) Removed float chamber top and held with float on bottom.

2) Blew into fuel banjo fitting while slowly lifting float and holding 9/16" spacer between top of float and bottom of float chamber cover.

3) Adjusted (bent) the float adjustment tab so that airflow thru the banjo just stops at  9/16", and air flows with any downward movement of the float.  

After doing this, I checked the levels with Float Syncs. The levels are exactly the same in both carburetors: ~ 1/2" above the top of the metal bases of the Float Synchs.

Thoughts?

 

Does Steve know you have a '72?  The '72 SU's have a different float height in the front vs rear carb,  The stanchion is higher in one than the other, so both will have a fuel height of 22mm from the top, but a different height from the bottom.

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  • 2 weeks later...

Here's my '72s and you can see the different heights. The DVD that came with my rebuild kit talks about stacking washers to make up the difference. The valves they sale now are the same length where the OEs are different.

I guess Steve is saying you can make up the difference bending the floats? I had a helluva time with mine and ended up using two shorter rear lids like the earlier 4 screw SUs. One of the best things I've done to my carbs. I read the reason for the newer difference in heights was to make up for acceleration and the fuel going to the back side of the front bowl. I've never had any problem with fuel starvation after I got the floats right.

float distance.png

 

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@Siteunseen:  I get it!  You're saying that the distances are NOT the same for front and rear!  (This is the first time I recall hearing this!!!).  At what distances should the respective  valves shut off (front and back)?

BTW: Since my last post I've adjusted the valves. (Some of the valves clearances were fairly tight.)  I also checked for timing chain stretch.    As you can see in my pics below, the "notch" is exactly aligned with the "groove".  I'm told that's the factory setting  - is that right?  

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I've always heard and seen it should be to the left just a hair, and found that to be true over the years.

As for the the valve gap I'm saying true TDC is equal for the front two. I don't remember the numbers but once they were the same it was at tdc on my car. So for instance and I mean JUST for instance they would measure .10 exhaust and .10 intake with feeler gauges. That's one of the few ways I was of "measure 3 times, cut once" mindset. It was winter and I was overthinking big time but it worked. My motor is nice and tight, dead on all the marks.

I'm sorry if I may have caused any confusion but it all worked for me in the end. 

I was overthinking the whole thing and got a lot of laughs but it was fun and I entertained a few of the engineers on here. That meant a lot to me at the time. Maybe gained some respect? LOL

Actually they are too nice to call me a doo-mass so we all got a laugh. Good guys, they are, no matter what I say.

 

 

Edited by siteunseen
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When you checked the mark on the cam sprocket at TDC - was the #1 mark on the sprocket where it should be ?

Id say your valve timing is retarded by the pic , but doesn’t explain running issues . Don’t be shy about advancing the sprocket - it will help low end torque .

4.5 turns seems way too much . Sure you have the needle jets seated correctly ?

Verified good strong spark from the plugs ?

Verified no vacuum leaks ? Taken a vacuum reading ? 
 

Compression test ? 
 

 

Was Pertronix set up properly based on ballast resistor used or not ? 

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