Idle stays high before dropping

[Danishbanker]

I am new in the 911 world and have a question for you guys.

I have a 1977 3,0 Carrera where the idle stays ad about 1800-1900 RPM for 4-5-6 sec before it drops down to about 900

Any idea what to do ??

Thanks
Claus

[pete3799]

Is this after a cold start or after the motor has warmed up?

[Danishbanker]

Originally Posted by pete3799

Is this after a cold start or after the motor has warmed up?

When starting could it is at abourt 1200-1300 and drops down to about 900...

So my issue is when the engine is varm and I rew it or clutch down when comming to a stopsign

[Hx911]

My '74 targa cis does the same. All is fine till the engine is at full temperature. Then it sort of 'hangs' at nearly 2000 and drips to 900 after maybe 10 or more seconds.
If I drag the revs down using the clutch at standstill, the revs stay low.
Have not found a hint on the internet yet ....

[tirwin]

I’m not too familiar with your model year. In general, when false air is introduced (i.e., vacuum leak), the extra air causes an increase in idle speed. And since it’s happening on deceleration, that makes me think decel valve.

Are you familiar with the decel valve? A simple test would be to disconnect the vacuum line, plug it and then go for a drive. The rubber hose could be dry and cracked with age. Try replacing it with a silicone one.

[Danishbanker]

Originally Posted by tirwin

Are you familiar with the decel valve? A simple test would be to disconnect the vacuum line, plug it and then go for a drive. The rubber hose could be dry and cracked with age. Try replacing it with a silicone one.

Yes I am - the vacumline, is that the thin one on the top ?

Plug the line og just leave the vacum stud open ?

[boyt911sc]

Originally Posted by tirwin

I’m not too familiar with your model year. In general, when false air is introduced (i.e., vacuum leak), the extra air causes an increase in idle speed. And since it’s happening on deceleration, that makes me think decel valve.

Are you familiar with the decel valve? A simple test would be to disconnect the vacuum line, plug it and then go for a drive. The rubber hose could be dry and cracked with age. Try replacing it with a silicone one.

Tim,

Your suggested test will not work for early CIS with the pear-shaped decel valves. Plus they use a vacuum assisted WUR’s. The later SC’s with the saucer-shaped decel valve is installed or connected differently and use non-vacuum WUR’s. In summary, they have to be tested differently.

Tony

[tirwin]

Thanks for the clarification, Tony.

Do you have a suggestion as to how Claus could test his problem?

[tirwin]

Originally Posted by Danishbanker

Yes I am - the vacumline, is that the thin one on the top ?

Plug the line og just leave the vacum stud open ?

Tony has broad experience with the differences in CIS over the years.

My suspicion is that you have false air somewhere. Unfortunately I don’t know enough about your year model of CIS to be of much help on specifics.

If you search in the forum for “smoke test” or “homemade smoke tester” you will find many creative ideas. Basically the idea is to use smoke and a little positive pressure to identify any vacuum leaks.

Another possibility is that your fuel mixture is off. CIS is highly dependent on fuel pressure and no vacuum leaks.

I suggested to start with vacuum because it should be easy enough. But if you really want to fix this problem, it is a really good idea to test methodically to avoid guessing.

[Danishbanker]

Thanks Tim - It sounds plausible with a vacum leak somewhere ;-)

Lets hope Tony will chim in again ;-)

[Paulporsche]

I have the pear shaped decel valve.

I haven't done this in years but I think you can take the top tube of the valve and turn it to vary the amount of "hang time". Try turning it clockwise, as viewed from above, and see if things change. It is similar in concept to squeezing the round type valve.

I currently have the top tube open with the line to it plugged.

[Danishbanker]

This is What I got

[Paulporsche]

That small tube off the left side (top) can be adjusted, I think, via the nut. You can leave that tube end open but you have to plug the hose that goes to it. I can't remember if the nut moves the tube in and out or if the nut loosens the tube so it can be pushed in or pulled out.

BTW all this is done once you have verified you have no vac leaks and are sure your points, distributor and other ignition parts are good, and your timing and mixture are correct.

[boyt911sc]

Originally Posted by Danishbanker

This is What I got

Claus,

Please take a picture of the decel valve showing the top and bottom sections. The early CIS have the tee-connection at the bottom and later moved to the top when the TTV (thermotime valve) was introduced.

Anyway, your decel valve could be the culprit but we do not know that as a fact. So, test and verify it. A good decel valve (normally closed below 18” Hg) could be bench tested easily using a hand operated vacuum pump.

Could you take a picture of the TTV (thermotime valve)? It is installed between the WUR vacuum line and the decel valve if you have one. Keep us posted.

Tony

[Danishbanker]

Tony, I got this when the PO had the engine out - does that help ?? ��*♂️


Arggghhh cant get the pic posted ����*♂️

[porschetub]

Just a suggestion but is your distributor ok ? if the rotor plate is sticking or your mechanical weights are gummed up you will have the slow way back to idle rpm of 900 like you have.
Can't remember set-up on these but if the distributor is stuck up it will effect the vacuum advance/retard functions to the point where they can't overcome the mechanical drag and return to idle be the issue.
Good luck.