Classic idle

Hi guys

I recently advanced the timing slightly on my 3.5efi which resulted in improved performance (with a small P) but the downside is a corresponding increase in the idle when in N.
According to the bible the adjustment screw on the plenum casing should be used to adjust this, but the slotted screw will not turn despite using the appropriate size screwdriver blade.

Is this likely to be due to simply lack of use or am I on the wrong track?
If the former, will application of a releasing agent do any harm considering it's an alloy casting?

Flapper or Hotwire?

Ratae said:

Flapper or Hotwire?

Click to expand...

Thank you.

Flapper

Sorry for not getting back to you before.

Presumably the gap between the butterfly & the bore of the plenum chamber is correct - max 0.05mm (0.0002 in) measured at the bottom - & the area is clean?

If the gap is too big then effectively you've a partially open throttle

If it's OK then, as you say, the idle speed is adjusted using the screw on the plenum.
If yours doesn't move then I'd think it's either seized or someone's put some threadlock or something on it.

Appreciate for your reply.

The idle revs increased as a result of the slight advancement of the timing, which is what you would expect so I doubt there's any problem with the gap. I'd had the dizzy out & having previously marked the position I turned it a just couple of mm following replacement. I have been running on 97-99 RON ever since the intro. of E10 & figured the car would benefit from a touch of advance, which it did without pinking.

I'll try some gentle persuasion on the screw, if no result I can live with the 1000rpm idle (at running temp) as the revs. drop to 600 when Drive is selected, which is only 100 above the original.

Mine's on LPG & that does work better well advanced BUT I don't have any fancy gizmos to alter it so I've set it as far advanced as I can get on unleaded without pinking.
Manual says TDC +/- 1 deg & I'm a little over 6 BTDC.
Around 600 rpm in 'D' is where I'm at but I'm around a little over 800 in 'N'.
Best done by road test & this from RPi does the job.
Its also worth noting that the best ignition timing in any given engine, is to achieve the most advance it can tolerate without pinking. ( Audible pre-ignition).
This is achieved by setting your V8 timing to about 4 deg. BTDC (assuming you've been through the distributor check list completely). Then tighten the distributor so that you can (with effort) still turn it by hand. ideally put a tip-ex mark or small scratch on the distributor body and engine block to record this spot.
Next road test the car and simulate high load by quickly shifting into a high gear or if Auto allow to change quickly up to 3rd or 4th Ideally you need to find a small hill or incline, now if you apply full throttle the engine should respond without pinking, find a safe place to pull over, open the bonnet and turn the distributor through a couple of degrees only, anticlockwise, this will add slightly more advance timing and if you do the same test, and repeat it until pinking is noticed you will be very close to your absolute best timing criteria
So now all you need to do is turn the distributor clockwise by the same amount by an amount to counteract the last adjust, retest for the absence of pinking and that the job done, you will have just achieved the best maximum timing position for your car

Having done this an ideal engine would run perfect in all areas and pull as you would expect from all load and rpm areas. you would also expect the final timing position to be somewhere close to 6-8 deg. btdc, (exceptions assumed)

Appreciate the reply.

The procedure you describe is essentially as was followed by old school mechanics on the basis that it set the optimum timing for that particular engine that had covered a good few miles, rather than the factory settings for a brand new engine.
I could put the timing light on it, just out of curiosity, but I'm happy as it runs well as-is.

My Classic is not a daily driver (less than 1000m PA) so the next time I'm in the mood I'll dribble a little releasing agent around the adjusting screw & try some more gentle leverage with the screwdriver.

Hopefully sorted .. a squirt of releasing fluid, leave to penetrate for a couple of hours & I managed to move the screw, albeit still quite stiff. Got the free idle revs. down to between 850/900 which drops to 500 when Drive selected. Think I'll leave it at that.

Appreciate all of your replies.