Vacuum leak at mixture adjustment screw - 83 911SC

[sigchuck]

Hi guys, I fabricated a smoke machine (which works great for the $30 in parts) to try to determine whether I have a vacuum leak. Unfortunately, it’s pretty apparent that there’s one at the location of the mixture adjustment screw. I can put an Allen wrench in and get positive engagement, and if I push down something (presumably the screw?) makes contact with the air sensor arm (forgive my terminology if incorrect).

Any ideas? Is the mixture screw stuck “down” in the adjustment position? I think I recall reading this is bad as it can damage something (not sure what though). The car seems to run alright, just have an overrich issue.

Thanks again for your help!

[boyt911sc]

Sigchuck,

When you perform a vacuum leak test in a CIS motor, you do it only to areas or sections that vacuum exists during operation. The fuel mixture set screw area is under atmospheric condition. Are you familiar how the test is done? Ask and you will receive.

Tony

[sigchuck]

Tony, apparently I am not, though I’m hoping that’s a good thing since it sounds like I may not have a problem!

I based my test off a video a user (forgot who) did with a cigar. It involved wrapping the top half of the airbox in a trash bad and inserting the smoke hose through the plugged hole in the passenger side of the lower half of the airbox.

Thanks for your quick help!

[75 911s]

That set screw is part of the air sensor arm. The arm is basically a teeter-totter with one end on the air intake to the throttle body and the other directed from the fuel distributor - and simply pressing down on the screw will open the air intake slightly. That's why adjusting the 3mm set screw is fussy because if you apply downward pressure, you move the arm a bit.

Tony is right of course, that set screw is on the atmospheric side. So smoke emanating from this area is not a problem in your test.

Over rich by how much? What does the tail pipe look like and what do the plugs look like? There is an amazing CIS thread - I'll see if I can find it and link it here.

[sigchuck]

The engine has around 2k miles on a new build. Valves were adjust around 1k Miles. Around 200 miles ago it developed an issue where on deceleration the idle speed would bog down and aloccasionally stall (same happened with a quick rev at idle). I’ve made some adjustments to the mixture screw (probably a quarter turn total over the past few weeks), but the issue is still present. I was checking for vacuum leaks before getting into checking CO.

Does a quarter turn on the adjustment screw seem like too much? Overall, it still runs great, doesn’t seem to have any flat spots, and pulls extremely well at higher rpms. Just the deceleration issue and sometimes hunting for idle.

[T77911S]

depends on which direction you turned it. CW is richer. richer could make your problem worse.

how was the mixture set after the rebuild
have the CP's been checked and what are they. have they been checked since the problem started
is the O2 sensor connected and how old is it
have you checked fuse 18
is the FV vibrating with key on

[Sunroof]

In my 1973.5T with CIS the previous owner added "blue locktite" to keep the mixture screw from moving (like its not tight enough already!). He flooded the hex nut cavity! I used dental pics to clean out as much of the stuff as possible to get that allen wrench adjuster in their to adjust it. But YES, if I press down too hard with the hex tool it impacts the sensor plate and the engine dies! What I do is remove the airbox, place my hand under the sensor plate to support it in the up position to make tiny adjustments. That way, I can adjust it one "click at a time", with the engine off of course. RIGHT-RICH, LEFT-LEAN.

Good luck
Bob