Itbs:- high idle

[Myles1970]

Hi folks....
I'm still tipping away at the project (Ae86 4age 20v blacktop), and I've finally got it running with a standalone ecu - speeduino.

The only issue at the moment is that it idles at 2500rpm. Some have suggested that it's a vacuum leak but as far as I can see all the vacuum lines are connected or plugged.
So, aside from looking for a leak, I've noticed that the idle drops when I push against the throttle cable stop (the cable isn't too tight). I wondered if this was normal?

I have removed the intake manifold to facilitate adjusting the throttle as per the manual, once I've done that I might remove all possible sources of vacuum leaks such as the brake servo and the pcv pipe etc. Failing that I suppose I'll be looking for a car balancer to adjust the air bypass valves

Any thoughts/advice would be much appreciated

 

[Nomad]

Hi, is it carbs or fuel injection?
I'm coming close to getting fueling working on my classic project that has been standing for many years, lessons I've learned ....

Disconnect and blank off any vacuum hoses to eliminate possible problems (mine has a lot of clever - fragile - small bore hoses and sensors)
Your throttle cable is slack (good), check for anything else holding the throttle off the stop (choke in my case, the throttle also has a "clever" deceleration device that is misbehaving)
Fit an in-line fuel filter so you can view the fuel supply and check for air bubbles. I had a classic that suffered with occasional fuel starvation (a flake of junk blocking the outlet from the tank), my latest project is pulling air bubbles from the tank (lines from the tank to to engine are ok so I'm now suspecting the pickup inside the tank)

Remove & clean all sensors on the inlet, look for broken wires around the sensors.
Check for cracked or mis-seated hoses or pipework on the inlets.
Run the car at tickover (2500 rpm on yours) and spray carb cleaner or similar around the various flanges & joints, if the revs change you have found a leak. Fix and repeat.

In your case my priority would be to find what is holding the throttle off the stop.
Disconnect the throttle cable then look and see what else could be holding the throttle open.

Hope it helps, keep us updated

Congrats on the stand-alone ECU, that's something I'd like to try at some point but have never got around to ...

 

[colnerov]

Check the butterfly disc in the shaft is not displaced and consequently not closing off the venturi properly, allowing too much air through.

 

[Myles1970]

@Nomad thanks lad, it's fuel injected, so in a sense it's simpler but at the same time it can be quite complicated lol. I was thinking the same thing, ie eliminate all possible sources of vacuum leaks and work from there. The thing is, pushing against the throttle stop lowers the idle - it does take a fair bit of pressure, but it seems to lose the 4 butterflies enough to lower it....
@colnerov there's 4 butterflies so I'm thinking they're good

 

[minimutly]

Not synched or slightly open throttles - one ore two pots will bring the idle speed right up - no load see...
Nothing runs without air.

 

[Myles1970]

... So it could be an open/leaking vacuum line or 1 or more butterflies not fully closed? It does take quite a bit of pressure on the throttle linkage to get the revs to drop..

 

[Misterg]

1 or more butterflies not fully closed?

The first one to close will hold the others open if they aren’t synchronised.

Have you set the static synchronisation between them? (Usually something like setting the throttle stop so they are all partially open, then adjust each butterfly individually so that a thin pin just fits under each one. This gets refined using vacuum gauges once the engine is running.)

 

[Melvyn Best]

What throttle bodies are you using? Bike tbs often use a second cable to close them , it would be easy to rig up a return spring in its place then as others have suggested synchronise them /check for air leaks

 

[Nomad]

As above, butterflies not in synch so one is hitting the stop and holding the others open.

If you have the manifold off the car hold it up to the light and look through the bodies - you might find one butterfly passes no light (or air when it’s on the car)


Edit: with the manifold off the car, undo the throttle stop full then look through the bores - it will be obvious if one butterfly is fully closed (no light) when the others are part open and letting some light (and air) past.

Do you have a set of vacuum gauges? I find them invaluable for balancing carbs on big bikes.
There was a set for sale here recently but I can’t find the ad.

 

[Myles1970]

What throttle bodies are you using?

Oem itbs, operating off a single accelerator cable

 

[Myles1970]

As above, butterflies not in synch so one is hitting the stop and holding the others open.

If you have the manifold off the car hold it up to the light and look through the bodies - you might find one butterfly passes no light (or air when it’s on the car)

Yes, will do...

Edit: with the manifold off the car, undo the throttle stop full then look through the bores - it will be obvious if one butterfly is fully closed (no light) when the others are part open and letting some light (and air) past.

Do you have a set of vacuum gauges?

No gauges yet, will invest