1981 911sc backrap/idle issue

lanceogrady · 10-15-2021, 03:14 PM

Firstly, I know this has been covered in many posts, but I'm still quite new to the 911sc community and can't tell what is and isn't relevent for my specific car.

I've made up for that with some great '80's pics that a New Zealand based automotive photographer (https://www.instagram.com/bradlongworth/) took recently.

So, background:
I had some idle issues, which were causing a massively fluctuating idle when stationary. My mechanic discovered the airbox (which is fitted with pop valve) was not actually firmly affixed. It had been broken at some point and the bolts had stripped thread off so wasn't sitting nice and flush and secure. We managed to epoxy up the holes and refit new bolts to sort the unmetered air issue. The mechanical advance had also seized, but he managed to get that moving again with some careful lubrication and movement. Happy days, car is running smoothly.

I then went to get it's warrant of fitness (required every 6 months in NZ), and it failed due to the gasket seals on exhaust being too thin/leaking, and the bolts were loose. New gaskets made and fitted, bolts tightened:

Passed it's WOF, back on the road. Now, I get backrap when accelerating then quickly taking foot off accelerator. Idle also feels a bit chunky too, bit rough. It's sitting at just above the marker down from 1000RPM.

I've checked all vacuum hoses etc as did a bit of reading it could be those, can't see anything not plugged in. This was the only hose I could find that was a bit average in terms of condition, but If I'm honest, I have no idea what it's for.

Any ideas or easy places to check/things to try are appreciated. It's driveable, but after owning for 6 months and not making this noise, it's giving me some anxiety.

Here's some more stylised shots:

Paulporsche · 10-15-2021, 05:31 PM

That hose is for the decel valve. It keeps revs up between shifts. Some people prefer that the revs fall more quickly so they disconnect the top hose and plug it. The top opening can be left open. I would check that small hose to see if it is leaking and replace it, or at least cut off the unraveling outer covering. Typically a failed decel valve keeps the revs up all the time, not just between shifts.

lanceogrady · 10-15-2021, 06:32 PM

I also just had the break master cylinder replaced, and seems there's a vacuum leak between booster and master that could have caused the issue. I can hear a hissing noise, so will get that seen to. Shame about that, the weather is great, perfect day for a drive.

Reiver · 10-19-2021, 08:58 PM

What AFR (air fuel ratio) did your mechanic set it at? Often idle and a bit of backfire is simply a too rich/too lean issue.
I assume you have a ROW 930-10 engine....and if so your enrichment under throttle is controlled by your WUR (warm up regulator) thru vacuum provided to the WUR. An out of spec WUR can also cause fuel delivery issues...
Check the AFR first (13.5-14.1 I believe by the manual that should go into the 11's at wot) and get it into spec then set your idle a tad lower...900/950 is nice on the Euro's.