how to adjust the idle

[DEKE]

Hi All
How do you adjust the idle on a 350i? when starting from cold hunts a wee bit around 600 to 800 rpm should it not be a slightly higher than that also when hot sits around the same and some times will cut out, don't think it did this before but have just fitted new plugs would that be the problem?
Cheers Deke

[adam quantrill]

I just used to fiddle with the stop screw that sets the throttle opening when the accelerator is slack.

You might get people saying "no that's bad practice" but it does the trick quick and nasty.

There is a bunch of stuff you have to do to do it properly. Depends how much time you have.

[KKson]

I just adjust the idle screw on the plenum. Seems to work okay. Ticks over about 900 rpm usually.

[mrzigazaga]

DEKE said:

Hi All
How do you adjust the idle on a 350i? when starting from cold hunts a wee bit around 600 to 800 rpm should it not be a slightly higher than that also when hot sits around the same and some times will cut out, don't think it did this before but have just fitted new plugs would that be the problem?
Cheers Deke

Hi mate..600-800rpm is way too low..The bigger motors are okay at that but you really want to be anything from 950-1000rpm..If the idle screw in the plenum has no resistance when you turn it then put a small rubber grommet on top of it when you have finished adjusting it..Make sure it pushes down on it as they can vibrate loose again..Ziga

[DEKE]

Thanks to all will give it a go
Cheers Deke

[honestjohntoo]

DEKE said:

Hi All
How do you adjust the idle on a 350i? when starting from cold hunts a wee bit around 600 to 800 rpm should it not be a slightly higher than that also when hot sits around the same and some times will cut out, don't think it did this before but have just fitted new plugs would that be the problem?
Cheers Deke

New plugs are unlikely to be the problem, more likely need to check the idle air and breather galleries are not choked with hard carbon deposits, getting worse as time passes. They never get better by themselves!!!!!

Either one, not being clear, will upset the idle speed and also make it difficult to adjust.

To a lesser degree the CO idle mixture control on the AFM can be used to influence idle speed.

This image shows the two controls.



The plenum breather gallery connects to the crankcase ventilation pipe and both galleries are best cleaned with the plenum removed especially the idle speed adjust - virtually impossible otherwise.

Typically the idle speed control will be 5 to 6 turns out from fully home. The CO control on the AFM should be approx 2.5 turns out from fully home.

Anything significantly different from the above suggestions (for a std 350i) indicates a problem or maladjustment elsewhere.

'Hunting from Cold' is an age related feature of the Std Efi system but if the owner is not prepared to tolerate it or it becomes too violent, it can be overcome in a couple of ways.

One is a 'workaround' additional component, the other is to get a collection of 'age related' issues sorted out.

Probable contributing age-related causes for Efi Hunting when cold.

• AFM Carbon Monoxide (CO) adjustment is maladjusted or the bypass gallery is cruddy reducing the available air at idle, inducing a degree of enrichment.

• Crankcase ventilation system partially blocked thereby reducing the fixed (not metered and not rogue) amount of foul air coming into the plenum from the crankcase, again inducing partial enrichment..

• The little hole (0.040" dia) in the left hand rocker cover breather blocked, having similar effect.

• An age-weakened flap spring in the Air Flow Meter (AFM) inducing unwanted enrichment at idle.

• A stiff and ageing Fuel Pressure Regulator (PFR) will raise the fuel pressure beyond 36 psi, enough to enrich the mixture across the whole range, including idle, again when not needed.

• Its probably not air leaks into the plenum, but taken together with other things, who knows?

• Any combination of the above!

Essentially, the problem is caused by slight enrichment of the mixture at idle and I have written a treaty on the subjects causes and how it is resolved:

http://www.vintagemodelairplane.com/pages/Snippets...

If looking for perfection - you'll love it! Otherwise - boring!