Idle question

[Gorwanvfr]

Good evening to all.
Have just finished getting my 2+2 back on the road.
I have a question about setting the idle rpm. The car is an automatic (Borg Warner 8). The issue I’m facing is with the car cold and in neutral the idle is 500 - 600 rpm. In Drive with foot on the brake the idle drops a little maybe 50 rpm.
Once the car heats up (Normal running temps). The idle in neutral rises up to 1000 - 1000 rpm. In drive with my foot on the brake it drops to 500 - 550rpm.
I don’t understand what is driving the idle so high once the car heats up.
I can’t lower the rpm anymore as the centre carb idle screw is fully closed. The carbs are balanced with regards to airflow and lifting the pistons to check the mixture settings works as it should.

Any help or ideas most welcome.
Thanks Steve

[Jack the lad]

Hi Steve,
I would say that if I've read your post correctly, you do not have a problem. With the car on level ground it should remain static or with minor creep with drive engaged but the brakes off, the 500/550 idle seems to be fine. The reason the rpm rises when the engine/transmission warms up is that friction in the mechanism is reduced so in effect the engine is making better use of the fuel you are giving it. Cold oil in the engine and the transmission do make a big difference. You probably will need to do few few hundred miles before finally re-setting everything anyway. Hope this helps, best of luck, John.

[christopher storey]

If the centre idle screw is completely closed your carbs are a. wrongly set and b. are relying , at least in the case of the centre carburetter , on the throttle being cracked open slightly. You need to start from first principles, and first make sure that the throttles are all completely closed when at idle, and only open - together - when the accelerator is pressed . This may require some readjustment of the linkage to achieve. Then equalise the airflow using the idle screws - note that the passages controlled by the screws admit not only air , but mixture . Then check mixture strength on all 3, noting that each carburetter although it predominantly feeds 2 cylinders also has some influence on the others in a 4.2 because of the generous interconnecting passages in the manifold . Then go back and recheck idle airflows . This really is only a short resume - there are plenty of threads on here which will give you more detail

[vee12eman]

Hi Steve.

When the idle screw is at the lowest adjustment, but idle is still high, it may be because there is an air leak, allowing air to bypass the normal route. If air can do that, it’s similar to opening the throttle, which also allows more air in, don’t make the mistake many do and believe the throttle affects fuel flow, it’s the reaction to the throttle opening which increases the fuel - to match the throttle opening. Check all vacuum sources, carb and manifold gaskets and also the spindle sealing of the throttle and any other linkages. Gaskets can be replaced, vacuum leaks cured, but leaking spindles require carb replacement or restoration with new bushes and spindles.

Good luck.

Regards,

Simon

[Gorwanvfr]

Cheers Guys for the replies. I think I have some home work ahead of me. Something is telling me my main issue is the centre carb.
Will let you know what I find.
Thanks
Steve

[Gorwanvfr]

Small update
I pulled the middle carb off the car this evening.
Before (don’t mind the missing screw I had removed it before I remembered I had not taken a picture)

2C993EE1-2D06-48DA-83D4-1A924240F185.jpeg (201.21 KiB) Viewed 3512 times

After

F633F3A8-8E17-4635-9F5F-49F05383E5E6.jpeg (188.5 KiB) Viewed 3512 times

Now I have to open the air bypass screw to maintain idle .

Cheers
Steve