74's 2.7 cis backfire issues
 

Sorry newbie question time here - Just got my gal back from the shop.. they've adjusted the timing (new distrib cap leads rotor points etc), and I note that the slight hunting when warmed up has gone - to be replaced with stuttering and sometimes backfires when coming out of idle IE: Pulling away from lights when engine hot until the revs climb past 2-2.5k she's a stutter monster and sometimes she'll backfire if I stay in this low rev band for too long.

Done some searching here and I'm a little confused but I think perhaps I am running lean? Mainly because the slight hunt has gone and this stutter crap has started.

Outside of this lowish rev band she goes smoothly and idles when warm at about 750-700 ish (lil low and I might adjust it up a bit).

My concern is the backfire possibly damaging the air-box.

sounds like a timing issue. did they adjust the timing?

The title of your post implies you think it to be a CIS issue. Did your shop check the CIS? If so you should have been informed what they did regarding checking the fuel pressures and checking for air leaks. Without basic knowledge like this, both you and others reading this will only be guessing as to what might be causing your problems.

fuel pressures, cold and warm need to be checked first.
make sure the advance is working properly.
no air leaks
check mixture

is the shop a prosche shop?

As the others have said, get them to verify those fuel and control pressures. It's very common to have a failing WUR that, because of a "compensating" mixture setting, may give you a too rich mixture @ start but a good one after warming up. Or, if someone leaned the mixture to prevent the startup hunting, then yields a too lean mix once warm. Only a correctly operating WUR will allow you to have the correct mixture across the range of temps.

Also, check the plugs and wires, and get that idle speed up to spec.

In addition to the excellent advice above I can speculate that you possibly have a lean condition due to vacuum leaks. I say this b/c the old owner of my 74 described how frustrated he was with the Porsche dealer for failing to repair his stuttering problem. He described it exactly as you did. After weeks of the run around, he took it to specialist who found the leak.

Fuel pressures/mixtures can also make for lean conditions so take it to someone who knows CIS.

Write down the suggestions.......
 

wozzle,

Take note of the above suggestions and visit your mechanic. Your mechanic is responsible for making your engine run good to enjoy it. BTW, have you mentioned this problem before to your mechanic? Keep us posted.

Tony

Nope it's not. Well they know the newer 996 onwards but I think a 74 CIS is a bit outside the box as it were.
Will be dropping by a CIS guru in the next week, but in the interim I've enriched it a little and raised the idle (now stable at just over 900).
Still a bit stumbly and sometimes a baby backfire, but better than it was.

Thanks to all for the excellent advice and pointers.
Will update when the CIS Guru delivers his verdict. :)

Main problem was the distributor was out by 2 teeth.
And the mixture was slightly too lean.
And the idle stop screw was set too low.

2hours of workshop time with someone who knows 911'S & CIS well was 2hrs well spent.
I now have a new mechanic.

the idle stop screw is something the should not need to be messed with, unless someone has adjusted it thinking it is the idle adjustment