Where is my mixture adjustment screw?

[stuartrubin]

All, this is a dumb question, but I cannot find the mixture adjustment screw on my 1975 Spider. I believe it is an 32 ADFA (of course, I can't find markings on the carb to confirm). In the photo, I think it should be somewhere in the vicinity below and left of the blue-plugged port.

I can't seem to correlate what I have with Brad's excellent guide.

Any guidance? Thanks.

[aj81spider]

Yours looks like it's missing the screw. See this topic for a picture:

http://fiatspider.com/f08/viewtopic.php?f=3&t=6966

[stuartrubin]

AJ, thanks for the response. So, maybe I'm not that crazy that I couldn't find it. After I posted my question earlier, I went back and found this screw. (See the photo.)

I assumed, perhaps incorrectly, that THIS (See the photo) was the mixture adjust screw and started playing with it. Now the idle is completely bonkers (like 2000 RPM) and I can't get it down. I'm backfiring (literally a burst of fire came out the tailpipe according to me wife!), and generally running wrong (but running).

Looking at
(http://www.carburetion.com/Weber/adjust.htm) it appears that this is ALSO the "Idle Mixture Screw"...

So, maybe I'm confusing the "mixture adjust screw" (which is missing from me carb?) and the "idle mixture adjust screw"?!?!?!

[aj81spider]

I'm no carb expert, but here is a link to an ADFA writeup that might be helpful.

http://www.artigue.com/wp-content/uploa ... er_adf.pdf

[So Cal Mark]

the screw with the red arrow is the idle speed screw, not the mixture screw

[stuartrubin]

AJ, I actually printed a hard-copy of Artigue's ADFA reference as well as his engine guide!

Mark, thank you for pointing out what is now completely obvious, but was somehow eluding me in the thick of my challenge this afternoon. So, I assume that tomorrow I can just dial back the "mystery" screw (i.e. idle speed) and I can get my idle back to a reasonable 850 RPM.

BUT, do you concur with AJ that my mixture screw is just missing? Is this something that I can just buy and screw in? From the pictures of "good" carbs, it appears that there is a shaft coming out and I assume the screw is inside of that. Can this be repaired?

Thanks again.

Stuart

[So Cal Mark]

most carbs have the mixture screw in the location of the bottom brass insert. On your carb I would say the mixture screw is inside the boss that angles back toward the brake booster

[bradartigue]

What ADF is that? It has the mixture pre-set with a plug...very early 80's of whoever OEMed that carburetor. The mixture port is located directly in the center of the bottom part of the carburetor. In your picture it is the smaller port in the center line (top to bottom), in between the two angled ports and below the larger brass-capped port.

The kicker is what is behind your cap? If you extract it it may:

- A cap hiding a drilled port set to a specific size
- A cap holding a plug that is a specific size.
- The cap itself is calibrated to a specific size, the hole is threaded, and the port is drilled (best case)

It is not a terrible effort to drill and thread those ports, but you need a drill press and a tiny bit, and a mixture screw. That carburetor does not have the raised housing for the inset type screws on most ADFs so you'll need an external screw from a DHSA or DMSA or something like that.

[stuartrubin]

Brad, let me see if I understand what you're saying:

1. It's possible that instead of having a mixture setting screw, it has been fixed to a specific pre-set mix and cannot be adjusted. You imply that this is an 80's OEM design, but not original to my 1977 Spider. Why would they fix the mix?

2. Something is under that mystery cap! I'll pop it off tomorrow and see, but looking at data on other 32 ADFA's (if that's really what I have...), I can't see what it could be doing. What would a drilled port, plug, or threaded hole be used for? Some other adjustment?

Thanks again. You're a gentleman and a scholar!

[stuartrubin]

Brad et al, I took the blue cap off of the "mystery port" and snapped some photos. Does this give anyone a clue about what it is? Thanks!

[So Cal Mark]

vacuum port, looks like it would connect to the crankcase breather on the air cleaner

[stuartrubin]

Thanks, Mark. This is interesting considering a while back I posted a question about a mysterious tube which was not connected. See http://www.fiatspider.com/f08/viewtopic.php?f=5&t=30819 . Coincidence?

I placed the vacuum beather tube up to (literally right up to it) the air filter with the bracket sort of sandwiched between the top and bottom cover of the air filter. You can see it in the photos in the other post.

So, I guess this mysterious port is superfluous for my application?

And, I still don't know what the deal is with the mixture setting.

[So Cal Mark]

that carb port should connect with the small port on the breather plate that attaches to the air filter

[Scotsman]

Here is a pic of the ADFA32 carb on my 77 Spider. The screw in center is the idle mix screw. The upper LH port comes from the inlet manifold, the RH port goes to the fast idle deceleration system. These were hooked up at my last overhaul, but I see from your post that this may not be correct.

[stuartrubin]

Scotsman, definitely do NOT go by anything I have said, plus it is clear that my carb is not like the others! For example, mine doesn't even have the mix screw (or at least not where yours is).

Mark, if I understand you correctly, there should be a vacuum hose connecting the "mystery port" and the smaller port on the bracket which also connects the crankcase breather hose. Please see the picture and tell me if it makes sense. The blue line will be a new hose.

Also, what are the consequences of having this port plugged (the blue cap)? What changes should I expect once connected?

Thanks again for your expertise in helping with a complete novice!