Idle doesn't decrease-sometimes surges-when shifting

[882993md]

Hi:

I've been driving my car and adjusting the jets and checking the spark plugs after my first attempt at tuning the 3 screw SU's. When shifting gears the RPM will sometimes not drop and I have to blip the throttle to get it to go down. I have my idle set at ~750 rpm and timing at 10 degrees. My plugs show I'm running very rich and I'm four turns down on the mixture for each carb.

Would the stuck/surging idle be a mixture issue, or would that be a mechanical possibility somewhere in the throttle chain? I did sync the carburetors at idle and at 3,000 rpm. Thank you.

Howard

'74 260z

Edited March 16, 2018 by 882993md

[7tooZ]

Could be your nozzles are sticking and if your are running rich I’m not sure you show be at four turns. See if you can replicate the high idle while sitting still Then push upon the nozzles up to see if they move up.


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[Patcon]

I would suspect the idle hanging up

[Mark Maras]

Will the rpms drop if you pull up on the pedal? I'd start raising the mixture screws in 1/2 turn increments to see if the dark plugs will clear up.

[882993md]

43 minutes ago, Mark Maras said:

Will the rpms drop if you pull up on the pedal? I'd start raising the mixture screws in 1/2 turn increments to see if the dark plugs will clear up.

I'll try it tomorrow and report bank

Howard

[882993md]

1 hour ago, 7tooZ said:

See if you can replicate the high idle while sitting still Then push upon the nozzles up to see if they move up.


Sent from my iPhone using Classic Zcar Club mobile

Thank you I'll try this tomorrow

Howard

[882993md]

Hi everyone:

 I haven’t had a chance to drive the car due to busy work schedule. However I did take some time to look at the mechanical parts of the throttle links. 

I checked  both throttle return spring‘s on the SU’s and they are new and are very strong.  The only thing I could think of is something is mechanically preventing the butterfly’s from returning to resting position, or unwanted air is entering the system.

I did find missing pieces on the throttle arm.  I believe there should be a spring,  washer, and cotter pin?

It was mentioned that a vacuum  leak may be the cause of the surge to 3000 or 4000 RPM. 

I don’t have that many vacuum lines on my system. I have new lines coming off the brake booster to the balance  pipe.  I double checked those connections and they are good and tight.

I have the vacuum line from the vacuum advance to the front carb. Hose is in great shape and connections are tight. 

Finally I have the heater control flap line also going to the front carb.  I checked that and found the end had split.  Apparently this tube is too small of a diameter to go on that carb. vacuum nipple. I have capped off that nipple for now. 

 I’m going to run to the store for a spring and washer and cotter pin then take her out for another test run. 

I also turned the mixture screws up back to 2 1/2 turns each. When I was trying to set the the mixture it was about 30° outside. Now that we have some 60° weather the car started easily at 2 1/2 turns.  I cleaned the spark plugs and once I have the above parts installed I’ll drive and report back with results in a few hours. 

Howard

 

Edited April 1, 2018 by 882993md

[882993md]

SOLVED!!! (And Happy Easter!)

I just came back from a 30 minute drive and no more idle surge of any kind. I don’t know if it was the vacuum leak or the floppy unsecured throttle rod (or a combination of both) but the car is a pleasure to drive.

I pulled plug 2 and 5 and if wondering if someone may provide feedback. I’m at 2 1/2 turns mixture on both carbs. 

Howard

 

Edited April 1, 2018 by 882993md

[882993md]

On 3/16/2018 at 7:47 PM, Mark Maras said:

Will the rpms drop if you pull up on the pedal? I'd start raising the mixture screws in 1/2 turn increments to see if the dark plugs will clear up.

I did bump the pedal and the idle increased by a couple hundred rpm when sitting at a light.  I reached down and pulled on the pedal and it moved towards me (maybe 1/16th of an inch?) and the idle returned to normal.

I see some slop in that plastic spherical bearing, but I can’t see anyway of replacing it. 

Howard

Edited April 1, 2018 by 882993md

[Patcon]

They look a little rich to me...

[Mark Maras]

There are a few things that would cause dark plugs at 2 1/2 turns.

 Nozzle?s are sticking in the down (choke on) position. You can manually push up the nozzles with the choke off to see if they're sticking. They should be all the way up with the choke off.

 Choke cable?s mis-adjusted and not allowing the nozzles?s to go all the up. Readjust choke cable clamps at carbs.

 Float?s adjusted too high. Easy way to tell is remove the tops and pistons. With the choke off, there should be no fuel above the small hole in the nozzle that the needle fits into. The fuel level should be below the hole.

 I've also seen nozzles stick down because of the little flat bar linkage that attaches to the nozzle on the bottom. If both mating surface on the ends of the linkage aren't exactly parallel to the other mating surfaces, the linkage will bind. It's easy to fix with a slight twist after careful scrutiny.  

Edited April 1, 2018 by Mark Maras

[esmit208]

Upon first reading I suspected a vacuum issue. The cruise control in an 82 I had would not remain constant on uphill grades and it turns out a vacuum line behind the radio was the culprit. Dry... brittle etc. I suggest go back to basline settings , if you already haven't and work back up from there. While chasing the wrong issue I'm positive you adjusted yourself out of tune. Good Luck!