Trying To Mantain My Idle In 1000 Rpm But Not Possible

[jalexquijano]

Recently installed a Shneider 274F, changed the timing belt, adjusted the time, and installed SU Carburetors fully rebuilt by Ztherapy. I decided that 1000 RPM as idle was my target but if i get stucked in a traffic jam or leave the engine on for more than 10 minutes, the idle starts to lower to 900 or 800 rpm. If i turn the lights on it lowers more. How can i install an AC System in this car if the idle is not stable.

 

Am i missing something here?  is it normal to have this unstable idle with SU CARBS?

 

 

Edited March 21, 2015 by jalexquijano

[Mark Maras]

 I don't recall an unstable idle with my SUs. Why the 1000 rpm idle as a target. I assume it has something to do with the cam.

 How hot is the engine when the idle drops? From what you've described, it sounds like it may be temperature related, Are you sure the engine was at full temp. when you set the idle?

[jalexquijano]

Well mine is unstable. I set my carbs at 3.5 turns and rpm at 1000. If i turn the lights on it lowers to 900 RPM.

 

However, I did another test, i left the engine on with the lights turned on for 15 minutes. It drops to 600RPM and raises back to 850 RPM. Is this normal? THe mechanics i have taken this car here in Panama say this is old technology and i should learn to cope with it!

 

I really need to make it stable before deciding on installing an AC System. Having spent 3,000 amongts Labour and parts and still cannot get the car to idle steadily its a big problem!!!

[Patcon]

Look for vacuum leaks. A weak idle is sometimes cause by vacuum leaks. The technology is old but it does work. If you have serious idle fluctuations there is something wrong.

Charles

[siteunseen]

Put a vacuum gauge on it.  They're very helpful diagnosing problems.

 

http://atlanticz.ca/zclub/techtips/vacuum/

[jalexquijano]

Look for vacuum leaks. A weak idle is sometimes cause by vacuum leaks. The technology is old but it does work. If you have serious idle fluctuations there is something wrong.

Charles

OK. Reason why i adjusted to 1000 RPM:

 

1. The camshaft installed is a Schneider 274-F,with new valve springs, retainers and .175 Lash Pads. I also changed to new nissan oem rocker arms. The specifications of this cam are the following:

 

 

Intake Duration (gross): 274 Exhaust Duration (gross): 274 Intake Duration (.050”): 215 Exhaust Duration (.050”): 215 Intake Valve Lift*: .480" Exhaust Valve Lift*: .480"     Intake Valve Lash: .008" Exhaust Valve Lash:  .010" RPM Range: 1800-6000

 

2. My car has a  L26 crank which makes the motor 79 mm stroke instead of the stock 73.7 mm

stock bore is 83 mm for the L24 and the L26 motors. My block has been bored out 1 mm to compensate for wear

that makes my  motor a 2.627 liter motor and has a  E88 head from 1972 will give about 8.7 to 1 compression

 

I need find out why my idle is not completely steady. I did the carb cleaner spray test and no rpm increase was noticed. What could be affecting my Idle? Still in the nutshell!!!

Edited March 23, 2015 by jalexquijano

[Stanley]

If the idle drops when you turn on the lights, that might suggest an electrical problem.

[jalexquijano]

Thanks Stanley.

 

I have spent so much money in parts and  mechanical engineers here in Panama and nobody seems to solve the idle issue. I want my idle totally stable before installing the Air Conditioner System to the car. My car has a Bosch 60 Amp alternator:

 

http://www.thezstore.com/page/TZS/PROD/classic12h02d/12-4068

[Patcon]

Maybe someone with a similar displacement and a similar cam could tell you what their idle rpm's are and how stable it is. I would be surprised if it was a cam problem. I guess you could try to contact Schneider and see what they suggest for an idle rpm. Having to rely on mechanics to sort your antique car can be a problem, unless your have some money to spend. I wouldn't rule out ignition or electrical issues contributing to your problem.

Charles

[jalexquijano]

Maybe someone with a similar displacement and a similar cam could tell you what their idle rpm's are and how stable it is. I would be surprised if it was a cam problem. I guess you could try to contact Schneider and see what they suggest for an idle rpm. Having to rely on mechanics to sort your antique car can be a problem, unless your have some money to spend. I wouldn't rule out ignition or electrical issues contributing to your problem.

Charles

Charles,

 

Maybe you could be right! I do not think its either a cam or carburertor tuning problem. The car runs just perfectly. The problem is mantaining the idle. I set it at 1,000 RPM it goes up to 1,100 and down to 900 on traffic or stop lights. If i leave the car parked with the headlights on then it starts lowers to 900 and after 8 to 10 mins goes down to 550 or 600 RPM. Really weird. I even sprayed carb cleaner in case there was a vacuum leak somewhere and there aint none. How do i check if it is the ignition, or any other electrical issues? What should i be checking here?

[rossiz]

i think that when you turn on the headlights you are asking the alternator to provide more power, which takes energy and slows down your idle. electricity isn't free - requires work to generate.  mine does that exact thing and it's a 78 280 with efi - no carbs or timing issues. i can turn the lights on and idle drops 100 rpm, turn 'em off and it goes right back up again.

[Chickenman]

i think that when you turn on the headlights you are asking the alternator to provide more power, which takes energy and slows down your idle. electricity isn't free - requires work to generate.  mine does that exact thing and it's a 78 280 with efi - no carbs or timing issues. i can turn the lights on and idle drops 100 rpm, turn 'em off and it goes right back up again.

 That is entirely correct. This is old technology and it does not have a a modern Digital feed- back system to control idle speed . There is no way for the analog ECU to compensate for extra load from imposed by things like alternator drag etc.

 

The factory AC system uses an Vacuum " kicker " pot that mechanically raises the idle via a simple on/off switch. It is an archaic design... but that's what you have.

 

Add to that a wider overlap cam, that may be slightly fouling plugs due to the " Tune " not being optimized for the Cam... and basically you have a Car that is affectionately known as having " Character ". It's going to have " quirks " and will never run like a box stock Honda... so don't expect it to.

 

In other words... you may have to live with it. Now a sharp.... REALLY sharp... tuner may be able to stabilize the idle a bit more. But those type of guys are few and far between these days. Most of the really sharp Crab tuning guys are in their 60's now. It's a dieing breed...literally!!

 

BTW.. I have a 1976 280Z running factory ECU and Factory AC. I have the equivalent of a Shneider 280 cam in it. The car idles pretty consistent at 1,000 rpm when warm... but it took a lot of tweaking. When the AC kicks in it's another matter. Still haven't got the stoopid AC vacuum kicker adjusted right. It's an archaic device. I just live with it.... until I get my MegaSquirt Pro system, with WB O2 sensor, and the Option for a modern GM style Idle air valve.