Should the secondary throttle be open AT ALL during idle?

[stuartrubin]

Guys, here's an easy question that will be a big help to me. On a 34 ADF carb, during idle, should the secondary throttle be open at all? Does the secondary "participate" in idling or just the primary?

Thank you for once again indulging my dumb questions...

Stuart

[AndyVAS]

Only the primary should be open at idle.

[stuartrubin]

Andy, thanks for the reply. That does pose more questions:

• If it's supposed to be closed all the time, then why is there a secondary idle stop set screw at all? (I know it's so the throttle doesn't slam into the bore or something, but it seems like there would be another way.)
• I also stumbled into this "The secondary throttle gap on a 32 ADFA is 0.04 to 0.05mm (0.0015 to 0.0019) or the thickness of tracing paper." from http://www.fiatspider.com/f15/viewtopic.php?t=10748. Granted 0.05 mm isn't much...Could this be a difference between the 32 and 34 ADF?

My real problem is that I can't get a good, low RPM idle. I was hoping that maybe this was the problem since I know it's already screwed up a little; the secondary set screw is set too far back and not touching the throttle at all and it's stripped so I can't adjust it.

Thoughts on that, anyone?

Thanks!

[blazingspider]

Regarding your real problem, what have you done in order to try and resolve the issue? Have you removed the throttle cable to ensure it's not holding the primary throttle open further than the stop on the idle speed screw? Have you backed the idle speed screw all the way out to see if the idle speed drops as it should? Have you tried adjusting the carb per the redline weber procedure? Have you checked the choke and it's settings? Have you checked for vacuum leaks?

[AndyVAS]

You're right, it is open a hair. The hair that it is open is to prevent the blade sticking in the throat and not opening properly. Didn't even think of that. One of those things you set without thinking during a rebuild. It shouldn't be open enough to effect idle.

Has the car had an idle issue since you got it or is this new? If new, what work was done most recently?

[stuartrubin]

Andy, I got the car a few years ago which had some kind of "Frankenstein" Weber 32 ADF carb with all kinds of anomalies. I replaced it with a Chinese 34 ADF. I've never been able to set it up right, specifically, I can't get a smooth, low-speed idle.

Another hint may be that the car generally smells like "old car". I can't say if the smell is exhaust, overly rich mix, lean mix, etc., but it's there.

I think I can rule-out the set-screw on the secondary idle, assuming that opened "a hair" is the same as not opened at all from an air point-of -view. I'll take my chances that the throttle will neither stick nor get bent up.

Blazingspider, I've tried to do the initial setup procedure, but without much success. As far as I can tell there are no air leaks. I've sprayed carb cleaner and see no difference, etc.

Could there be a problem with the jets being wrong? How would I even check those?

Thanks, guys!

Stuart

[AndyVAS]

Most of the time the jets and/or emulsion tubes need adjustment on a new carb.. Since you likely don't have boxes of them laying around, call a local VW shop. Somewhere that has been around since dirt was new. Those guys will have thousands and thousands of little carb. bits. Ask them to tune the carb. Chances are they can have it running like it should in an hour.

[stuartrubin]

Just for the heck of it, I swapped out the idle jets from the original carb (the Frankenstein Weber of unknown provenance) into the new Chinese carb. I did not see a difference. Turning the idle mix screw has no affect on the idle.

The Weber sizes were 60 for the primary and 90 for the secondary. The Chinese ones have no markings. 60 sounds high for the primary idle, as least based on what I've read in the forum. Could that be a cause of my problems?

Also, the exhaust smell is unbearable. My eyes were watering and I felt nauseous, even with the car on the driveway. (Another hint?)

Thoughts? Thanks, guys!

FYI, the Weber jets:


And the Chinese jets:

[aj81spider]

According to the chart I have on jetting for Fiat carburetors the primary should be 50 and the secondary 60:

[stuartrubin]

AJ, this is a good chart. Thanks for posting it.

The more I thought about it, though, the wrong size jet doesn't fully explain things. If I screw the idle mix in all the way, I still idle. As was mentioned in another post, maybe I'm "idling" through the main circuit, not the idle. Could be that my throttle is set wrong (in a way I can't see), or there is a leak from the accelerator pump (which I can't quite understand short of a punctured diaphragm).

I may pull the carb off again and re-check the throttle, etc.

More comments are welcome!

Thanks!

[baltobernie]

I'm not familiar with the 34 ADF, but the foolproof method is to connect a vacuum gauge to "ported vacuum". This is a nipple located on the engine side of the butterflies. It is only active when the main circuit is active i.e. the butterflies are open. This is where the vacuum advance line going to the distributor is attached.

When you are tuning the idle circuit, the vacuum gauge MUST read zero. If it shows any value at all, you have activity on the main circuit, and it will be impossible to tune the low-speed ("idle") circuit. One manifestation of this condition is the ineffectiveness of turning the mixture screw (because the main circuit is doing some of the work). Your throttle linkage must be loose enough to allow the butterflies to completely close. If you're seeing vacuum here at idle, snap the throttle open and closed a few times to see if there is friction or obstruction.

I spent hundreds of dollars in jets (and FedEx ) before learning this.

As you may have heard here or in various Weber tuning books, the "idle" circuit is responsible for a huge percentage of driving conditions, not just idle. You've got to get this right before moving onwards.

[stuartrubin]

baltobernie, that makes a lot of sense. I took a quick look but did not see a port on the engine side of the carb, but there is one on the driver's side. Could you mean the nipple with the rubber cap on it? Picture are below.

So, if there IS a vacuum on the port, what could be causing it?

Thanks!

Driver's Side


Engine Side

[baltobernie]

Yes, the yellow cap. If the butterflies are closed and you read vacuum, perhaps there's an air leak somewhere. Check blazingspider's post above.