SC having problems retuning to idle without dieing!!

[air-cool-me]

SC having problems retuning to idle without dieing!!



I have a 77 with a 79sc motor in it.

Today when I started it up it took a second more to start then usual.

Then as im driving along to Wendy’s I put in the clutch at 3k and as the RPM's drop something funny happens.

It died!!!

So in the parking lot in neutral I snap off the throttle and it dies.

The RPM’s drop rapidly like there supposed too then they “bounce” off 1k RPM’s back up to maybe 1300 then it drops to 300rpm’s.

At this point it either dies… or stumbles and returns to idle at 850rpm…


If I set the RPM’s down slowly… bring it down to idle. It idle’s fine.

This has never happened before.

It seems to get a little better(it makes it back to idle from 300RPMS more often) as it get warmer…

HELP!!!!

Again: if the rpms drop rapidly(like rev’n it) it bounces off 1k up to 1.3k then down to almost dieing then dies or returns to idle…..

[MysticLlama]

That sounds like what mine was doing with the O2 sensor unplugged and it running a bit lean.

Can you check your O2 sensor to make sure that it didn't come loose or something, or do you run without it already?

[Oldporsche]

Don't know if the O2 sensor appeared before '80.


David Duffield

[Paulporsche]

Try plugging the vac line to the top of the decel valve.

Check for vacuum leaks.

Check mixture. I think you might be too rich, not too lean.

[VaSteve]

I would agree too rich. Mine was doing the same thing. You may try adjusting the idle as well.

[thabaer]

Vakkum leek

[rickeolis]

From your description, I would say a vacuum leak.
Check on the six tubes going into the airbox (underneath it) for broken seals, etc.
This is common.

[air-cool-me]

ahhh crap... vacume leak..... just what i dont want.


just for more information...

it ran fine at 0 feet and now im at 5000feet...

its always burbled a bit upon decel. a rich mix might be in combo with the vacume leak...

[Paulporsche]

Try the decel valve thing. Try leaning your mixture 1/16 turn.

Let us know if either helps.

[Al S]

From experience, I have to agree with the "too rich" crowd.

[Mysterytrain]

My 78 was doing the same thing..total rebuild with all new vac lines and intake boots. It was hanging between shifts and high rpm's when the car was hot. I pluged the vacuum line going to the decel valve and sealed the top of the valve and it cured the hanging rev's and the high rpm's..but I would stall out when ever I took my foot off the gas until it warmed up..replaced the decel valve and that problem is solved.

[Paulporsche]

Always try the cheap, easy, quick stuff first.

[Hladun]

1. First, check/set the CO with a proper gauge. If emissions are not a problem set at ~3%. Don't play with this adjustment without a CO gauge.
2. Do item 1. On rapid throttle release, the decel valve should hold the rpm's at about 1,300 to 1,500 for a second or two before rpm drops to 1,000. The decl valve is adjustable. Loosening the locknut and turning the top fitting out (CCW) will make the valve hold a higher rpm, but don't open it too much or you won't reach idle.