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Another SU high idle thread


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Ok, I got the 240z running after several years.  I cleaned up and went through the SUs.  I used the ZTherapy video as my guide.  I initially chased some vaccum leaks, and got the idle down from 1250 to 1050, which also took care of the lean condition.  Both carbs are synced and pulling 10 on the sync tool, but I can't get the idle any lower.  The idle screw is completely backed out on the front carb.  The choke and linkage is disconnected.  The AFR gauge is in the 12s at 1050, so it's not a lean condition.  I put a vaccum gauge on the intake, and its pulling 18 which is in spec I believe, so I don't think I have any major vaccum leaks at this point.  My timing is 10 degrees advanced.  On start up it idles nice at 850 but as soon as she warms up and fuel is burning more efficiently, the idles goes to 1050.  I suspect the issue is the front carb.  I took the front carb off to see if the throttle plates are closing all the way.  They look closed, but if you hold the carb up to the light, you can see a ring of light on the bottom half of the plates.  I don't know if this little bit of air is enough to cause the idle issue.  I will check for lose throttle bearing again, with some carb cleaner but I couldn't find the problem the first time around.  I don't know where to go from here.

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So there's the idle screws on each carb, and also a fast idle screw somewhere between them on the throttle linkage. The fast idle screw isn't screwed down is it? 

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Just a couple of ideas to check.

Are you sure the throttle linkage is allowing the butterfly valves to fully close?

Any chance there is dirt / crud / mice nests restricting the butterfly valves?

Not sure I am using the right term...butterfly valves. I am talking about the round brass plates that move 90 degrees and allow air / fuel in to intake.

May need to remove carbs and see that the valves fully close.

 

J

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1 hour ago, Tweeds said:

So there's the idle screws on each carb, and also a fast idle screw somewhere between them on the throttle linkage. The fast idle screw isn't screwed down is it? 

Nope, all the other screws are completely out of the equation.

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58 minutes ago, jonathanrussell said:

Just a couple of ideas to check.

Are you sure the throttle linkage is allowing the butterfly valves to fully close?

Any chance there is dirt / crud / mice nests restricting the butterfly valves?

Not sure I am using the right term...butterfly valves. I am talking about the round brass plates that move 90 degrees and allow air / fuel in to intake.

May need to remove carbs and see that the valves fully close.

 

J

I did that and noticed a slight ring of light on the bottom half of the butterflys.  The only thing I can think of is that they need to be re-centered so there is no light shining through, if that's possible.

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Follow up to this.  I removed the front carb and recentered the butterfly ever so slightly to make it close completely.  There was a very slight gap on the bottom half of the butterfly plate.  You really couldn't see it unless you put the carb up to the light and looked through.  I didn't think it would make a difference, but sure enough it did.  The car will now idle as low as I like.

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50 minutes ago, Patcon said:

Are the throttle shaft screws staked? It is possible to ingest those if they were to back out, but staking must be done with care...

I don't believe the screws were the original.  They were 2.5mm hex, and I don't think that is the original screw that was used.  The carbs were sent to Rebello many years ago, so they may have put the hex screws in.  I hope to god they don't back out!

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