Idle dying when warm

[Geoff's 240z]

Here are the plugs from that side of the engine.

[Mark Maras]

 Those plugs should be near white to a light tan if it was running with the choke off. I'm going to assume a few things like; there's a good hot spark at the plugs, compression is good, the ignition timing is OK. Let's start with the easy stuff. When the choke is operated, the cables pull the nozzles down in both carbs. The nozzles are known to occasionally stick in the down (choke) position when the choke knob is pushed forward (off). You can reach under the carbs and feel the nozzles come down with the help of a friend operating the choke knob. When the choke knob is pushed forward, manually push up the nozzles to see if they're sticking in the down position.

 Do your round tops have 3 or 4 screws and is there a drain fitting on the float bowls? Ztherapy has a video called just SUs that explains the workings of SUs and how to tune them. SUs are really quite simple carbs IMO.

[siteunseen]

I agree. They look rich from possibly the choke cables aren't adjusted right.  Have you changed the length of the cable wires?  Maybe the hold down clamps have loosened resulting in the nozzles not pulling up completely.  I had one with stiff fuel line instead of the pliable hoses that kept mine from completely closing.  

I may be way off as you describe this as a sudden problem.  Cold weather?

[Geoff's 240z]

No luck on the choke, I checked both and are functioning properly.

[Mark Maras]

 How many turns down are the nozzles?

[Zed Head]

I would give the engine a full tuneup before getting too far in to the carbs.  Valve lash, ignition timing, etc.  Many tears have been shed and much time wasted just to find out that a simple tuneup did the trick.  Plus, since you have a modified engine, I would check the engine basics, like cam timing and compression ratio.  People often go off-spec. when they install their new performance cams.  You don't know that the PO did things right.  Your engine does sound odd in that short sound clip.  Could be cam and/or ignition timing.

Also still looks like you have two issues - dying at the lights when the engine gets hot, and getting the carbs synchronized.

The tuneup procedure seems lengthy but it's not too bad.  When you're done you'll know a lot more about what you're working with.

 

[Stanley]

Do you have a points distributor? If so check dwell, condition of points cap and rotor. Also timing as above. Using premium gas right? Raising idle will usually make overrun worse. Car can't run right with fouled plugs. What carb needles? You need a synchrometer or flowmeter to do the carb tuneup per FSM or "quick and dirty" (or an expert's ear).

20 hours ago, Mark Maras said:

 The nozzles are known to occasionally stick in the down (choke) position

 

I would change "occasionally" to "frequently". Hit linkage with carb cleaner and follow with lithium spray grease to fix. Maybe good to do that at least once a year.

Car looks good BTW. Nismo headers wow, let's see them.

[Mark Maras]

 In an attempt to consolidate the help in the previous threads, Ignore (for now) all advice that doesn't pertain to a complete tune-up. We need to eliminate all the gremlins that are eliminated in a tune-up before we can get into the finer points of the carbs. You may want to review Jalexquiano's threads titled "1972 float adjustment" and "Misfiring while cruising". 

[w3wilkes]

4 hours ago, Mark Maras said:

You may want to review Jalexquiano's threads titled "1972 float adjustment" and "Misfiring while cruising". 

Is there an abridged version?  ?

[AK260]

I whole heartedly agree with Mark. Do all the tuning stuff first.

BTW I had a VERY similar problem - the car would humble modern cars on the go but struggled to idle and would eventually stall at the lights.

Spent HOURS AND HOURS playing with carbs. Turns out I had a vacuum leak where the carb joins the manifold on the rear carb, the nuts "looked" tight but had loosened off. When idling, the butterflies are near shut and manifold vac high, so any leak becomes material. Operating the choke lever not only drops the chokes but also opens the butterflies a little, therefore would hide a vac leak.

So in short I would advise checking all your manifold nuts, carb to manifold nuts, checking for carb spindle bearing leaks, top hat to carb body sealing, manifold gasket leaks, vac pipes to brake booster etc.

FWIW my 270 deg cam with .46" lift likes 1000rpm and given I use a fidanza (half stock weight) flywheel, it does have a gruff lumpy idle - kinda race car like ;)

BTW, that's a stunning car!!

[Geoff's 240z]

Guys, really appreciate the help. Going to check for vacuum leaks and complete a compression test today. 

AK260, I hope it is as easy as me reliving your experience. Thanks for comments on the cam, too. I asked MSA about how aggressive my cam was, and the guy said it was equivalent to a Stage II. I know they aren't mechanics and it isn't their exact product, so not most confident on his opinion.

Anyone else's opinion on the cam given the numbers I've provided would be appreciated.

 

[heyitsrama]

Hey Geoff what area do you live in, maybe someone from the club can lend a hand?


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