CIS help - engine RPMs drop drastically when you let go of the throttle

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Hi, guys. I have a '77 930 that I bought in a basket. Had to do a full rebuild to get the motor running, and I still have one remaining CIS annoyance.

When you release the throttle, as in whenever you push the clutch in to shift, the RPMs drop to essentially 0 for about 1 second, and then recover. This makes shiftig much more difficult, although it's never stalled.

I followed Superman's advice on an earlier thread, and blocked the air valve - this is the diaphragm valve that lets air bypass the throttle body until the engine starts, and then vacuum snaps it closed. Sup said in his post that these things can open again on decel when they get old, and when the engine surges they close, and when the RPMs drop they open, and then the engine surges, and ...

Sounded exactly what happens to me sometimes, so I tried it. Absolutely no effect. Dang, it sounded so good!

Anybody have any other ideas?

[Targa77]

Have you had the exhaust system checked for restrictions? I have heard of other cars with similar symptoms and they had a restriction in the exhaust. If you are running a cat, that would be the first thing that I would check.

[stormcrow]

It could be your deceleration valve. When you push the clutch in is it when you are shifting? Try pushing the clutch in when the engine is at idle. If it doesn't change, then raise the rpm's and do the same as if you were shifting while driving. If the rpm's change then it's the deceleraton valve.

Haynes has a test in their manual for checking it out so I would look to doing that to see if it's what's causing you problem.

Steve

78 911 SC Targa

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I don't think I have an exhaust restriction - more likely the reverse. The PO gutted everything out of the muffler - now I set off car alarms in parking lots. However, I'm getting a new GHL muffler - I'll let you know if there's any change when I bolt it on.

I know my deceleration valve was bad. I put a ball bearing in the vacuum line controlling it because it would "hang" during upshifts. At 5000RPM on boost, if I shifted the engine RPMs would actually jump UP about 500 - 1000 before the valve would let go. At low speed it did nothing - I still had this "trying ot stall" behavior.

When idling, pushing in the clutch causes no RPM change. Sitting in neutral, if I goose it to 3500 and then let go the accelerator, the RPM crashes to essentially 0 - 50 RPM for about 1/2 - 1 second, then recovers to a 950 RPM idle. The mixture is set at 3% (spec).

If the muffler doesn't help, I'll replace the deceleration valve and see if a new one works better than mine.