Idle dying when warm

[Geoff's 240z]

Hi team!

I'm a new Z owner. First carbureted car and I'm here to learn (wife thinks it is a midlife crisis). My issue is the car starts (with choke) and idles fine as it warms up (choke off). I have a somewhat lumpy cam, which seems to like the idle at 1,000 rpm. Once fully warm after driving around, the car will slowly lose idle and fade to a stall when stopped at a light. Otherwise, runs well other than a minor amount of dieseling when I turn off the ignition.

I'm stumped. I've tinkered with the carbs and have used a Colourtune to try to perfect fuel mixture. The only issue noticed during tuning is this: when doing the standard lifting of a piston to shut one carb off during the tuning process, the remaining cylinders operating sound like a front load washer on spin dry or a diesel engine. Other carb seems to perform well with slight increase in idle, then fall to more normal operations on the remaining cylinders.

Any help would be appreciated. Here's my setup, as inherited from the PO:

• L28
• E-88 milled to bring compression to 10:1
• Camshaft: 280 single pattern camshaft on 109 lobe
  • 480 lift/31-69 to 69-31
  • Not sure what all this means, taken from PO notes. Is this similar to Stage 3 offered by most companies?
• Round top SU's
• Nismo Headers
• 5 speed from '79 280zx

And a pic, just to keep you guys interested.

Thanks in advance!

[Zed Head]

16 minutes ago, Geoff's 240z said:

Once fully warm after driving around, the car will slowly lose idle and fade to a stall when stopped at a light. Otherwise, runs well other than a minor amount of dieseling when I turn off the ignition.

The only issue noticed during tuning is this: when doing the standard lifting of a piston to shut one carb off during the tuning process, the remaining cylinders operating sound like a front load washer on spin dry or a diesel engine. Other carb seems to perform well with slight increase in idle, then fall to more normal operations on the remaining cylinders.

Nice car.  Are you saying that one carb behaves differently than the other carb, when tuning?  Like the front is different than rear?  Are you sure that all six cylinders are fully functional?

Just clarifying.  I'm not a carb guy.

[Geoff's 240z]

Hard to say if it is the carb behaving differently, but there certainly is a different outcome when tuning. Yes, it appears that all 6 cylinders are functioning but not sure how to easily tell if we have a material fault.

[Zed Head]

I meant that the actions taken on one carb should produce the same outcome for the other carb.  If you had your eyes closed and somebody else tested the carbs you shouldn't be able to tell that the carbs were acting differently.  

Think of it as two 3 cylinder engines connected in the middle.  If one carb gives a different result than the other that's not right.  Could be the cylinders or it could be the carbs.

A weak cylinder might explain the lumpy idle also.  Just a possibility.  Doesn't mean that there's damage.  Could be valve lash adjustment, for example.

[siteunseen]

A timing light that shows rpms is very helpful. You want both carbs separately running the same rpms.  Use the nozzle knobs to get them close. Front may be at 2.5 turns down while the rear could be 3 or 2 turns?  Get them both idling the same rpms.  google "quick and dirty su carb adjusting classiczcars.com".

Thanks to @240260280 for the info.

[Geoff's 240z]

Yes, I've reviewed the quick and dirty SU tuning, that's what I've done. Are we getting away from the main issue of the idle dying at normal temp?

[Patcon]

14 minutes ago, Geoff's 240z said:

Yes, I've reviewed the quick and dirty SU tuning, that's what I've done. Are we getting away from the main issue of the idle dying at normal temp?

Possibly, but maybe not. My first suspect would be fueling, so starting with the carbs is where I would start. Next would be ignition

[Zed Head]

3 hours ago, Geoff's 240z said:

 My issue is the car starts (with choke) and idles fine as it warms up (choke off). I have a somewhat lumpy cam, which seems to like the idle at 1,000 rpm. Once fully warm after driving around, the car will slowly lose idle and fade to a stall when stopped at a light. 

Have you tried just turning up the idle speed?  You could add a lot more detail about engine temperature and time.  Does it happen immediately or after a long drive?

Maybe you have the simple percolation problem, from a hot engine.  Are your heat shields installed?  Nissan wrote a whole book about the problem.

Anyway, it's all in here.  Good luck. 

 

[Geoff's 240z]

Thanks for the idea, short period of warming up and yes I've got heat shields installed. This is a relatively new issue.

[Mark Maras]

 ZH is correct. The carbs should act the same. Some more tuning needed there. What were the results using the color tune? Were you able to get the proper color on all six and what color are the plugs? Pics of the plugs would be good. I'm betting at least three of them are dark. Carbon in the cylinders may be the cause of the dieseling. 

[Geoff's 240z]

I pulled the plugs before Colourtune and they were maybe just a little rich. I'll pull again and take pics tomorrow. The good news is that it sounds like we're working with a carb issue, not something harder to track like a vacuum leak.

I appreciate the ideas, guys. 

[Geoff's 240z]

Hi guys, this is the best tuning I can get out of the carb that sounds funny. Whatcha think?