Idle Speed

[Robbie 2B]

I have a 2B with the bog standard 2 ltr Sierra set up, Weber carb and ECU.

 

When the engine is warm and at tickover, foot off the accelerator, the revs start to hunt between 1,000 and 1,700 in a cyclic pattern. The cycle is about 3 seconds for the revs to rise and then fall. Blipping the throttle does not help, the hunting just starts again once the revs died.

 

I have given it a few runs at 6,000 revs but again with no effect.

 

Has anyone come across this and if so how do you rectify the problem?

 

Many thanks

 

Robbie:angry:

[Mitch]

is there a hole(or pipe off) in the vacume hose(servo to carb) i had this once on a dohc Granada i used to have.

 

just a thought

 

Mitch

[Longboarder]

Hunting like that usualy means you're running a bit rich. Check and clean the carb, check float does float and that its set so the fuel level is correct and check the needle valve in the float chamber is in good nick and the fuel chamber vent is clear. If you have an air filter make sure its clean and not obstructed.

 

Nigel

[BigLee]

I've had hunting at idle problems.

 

As longboarder says usually mixture related. I've had similay symptoms from inlet leaks, either inlet manifold gasket or missing/holed pipe.

 

 

Cheers

 

Lee

[Guest grahamdale5]

I had this on an old sierra years ago. F plate if memory serves. It was caused by poor conductivity between the throttle stop screw and adjuster plate. If this is the same as yours, and I could probably tell from a phooto, try using emery cloth between the surfaces. This worked for me.

 

The ECU uses this contact surface as an input to tell the throttle position, very crude and doesn't like not being used for a while.

 

Let me know

[Guest Cantbearsed]

I'd start by looking for air leaks on the inlet side. Sounds like what most carbs do when they find a source of air from the induction side...

 

and then go for the throttle stop plate.

 

Hopefully a cheap fix, whatever it is.....

[Guest WHEELNUT]

sounds very much like a stepper motor problem as grahamdale5 says. the little plunger will be in and out like fiddlers elbow if the seat it sits on is dirty. However be careful how you clean it as roughening the surface too much may make the problem worse , carb cleaner may be better but do it when engine cold as it doesn't half go up quick if it meets something hot!! I once inadvertently sprayed some near achoke bi-metallic spring heater , luckily a mate happened to have a bucket of water handy!!

[Robbie 2B]

Thanks to all who replied.

 

It was the stepper motor contacts. Had the day off work and cleaned the outside of the carb. Did a run down and around Crawley, lots of starts and stops and hey presto no hunting.

 

Thanks again for your help.

 

Robbie