High idle after a good run at highway speeds (110 kph)

[EdHolly]

Any theories out there about why an engine runs an extra few hundred revs at idle after a good run on the highway etc. Even my twincams in the various Brabhams I have driven will add up to a thousand revs with the foot off the throttle after a race - 1500rpm on the dummy grid 2500 rpm on the run back to the pits.

Yesterday went down the Princes Highway to Bowral (NSW Aus) around an hour at 3500 rpm - at the end instead of idling at 1000 rpm, was around 1500. It'll happliy idle now at 1000 again the next day - so why the differences ?

Ed

[oldchieft]

It would sure to be some reduction in the parasitic loads.

Could be the running clearances are better when there has been a good heat soak.

Also the oil might be thinner and less going back to the sump via the relief valve and the oil thermostat will be open for more flow through the cooler.

This will give less pumping loss.

Battery will be fully charged, so less alternator load.

Jon the Chief

[rgh0]

HI Ed

It is generally due to the position of the throttle butterfly versus the progression holes and the first progression hole being exposed at idle. Open up the idle screws and back off the idle stop on the throttle and you will close up the butterfly at idle a little and it should go away. You can see the butterfly position versus the progression holes if you remove the plug in the body above the progression holes. Check all 4 butterflies to ensure you don't have twisted throttle shafts also.

Good to see your enjoying the car

cheers
Rohan

[richardcox_lotus]

The other thing you might want to check is the condition of your carb mounting o-rings. As these heat up and expand, they may not be sealing as they should.

Regards
Richard

[Galwaylotus]

You should go to http://groups.yahoo.com/neo/groups/sidedraft_central/. Keith Franck has done a massive amount of research and development on sidedraft carbs particularly Weber DCOEs,