Carb Idle Fine tune tip

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61porsche

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Just so the carb guys don't get left out with all the MS talk....

I got to thinking about the carbs, ( I know that leads to trouble) regarding idle, quality, and how to make it better without pulling out my set of jet reamers.

On the Zeniths, there's a small air bleed hole on top of the carb next to the jets. It's just a hole. It let's air go into the idle jet which is very small.

Using a piece of small diameter wire, half the size of the hole, inserted and see what the idle does- get's smoother? Needs a little more opening of the idle screw. Rougher- turn the idle screw in a bit.

It works and mine was pretty good before. Let's call it sync'ng the idle jets.

Now I just gotta hook up that wideband meter one day after I drop the header.
 
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must be this

200220111944.jpg
 
Jerry your tip on putting a finger over the hole to see how the idle reacts up or down was one of the best tips when I rebuilt my carbs and when fiddleing with the tuning. I love my carbs - they woosh with air when starting up and I love feeling the secondaries kick in when I mash the accelerator! :mrgreen:
 
Air correction sizes

DQ,

Can you read the numbers on the brass air jets and confirm pls. I think I see 100 on the primary and 120 on the secondary?

You're photo has me thinking. Check against the specs for the carb in the manual.
 
61 Porsche: OK, I give.

Please share-What's the manual call for in terms of the jets? The MS discussion has been super interesting, but for now I have Zeniths on a 3.5.
 
air correction jet

In DQ's picture, I would usually see 80 on the primary and 4or6n on the secondary. Auto equipped cars had 100 on the primary for 2800s. But that's about it.

The same size jets were used on the Mercedes Zeniths.


I tried the 100 from another set I had, didn't help me.

Tell me what you're symtoms are or what more you want from the carb.

choke, idle, off idle, or getty up?

Here's the link to jet sizes-http://www.e9-driven.com/Public/Library/BMW-E9-Manual/pages/en/13000000.html#refertoc

In your case- look at the carbed csl. You're secondary fuel jet needs a little help most likely.

Dripping fuel?

Stumble?

Idle quality?

There is an interaction between them all.
 
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61porsche said:
DQ,

Can you read the numbers on the brass air jets and confirm pls. I think I see 100 on the primary and 120 on the secondary?

You're photo has me thinking. Check against the specs for the carb in the manual.
Click to expand...

you are right, it is 100 and 120, but those are not my car mounted carbs, but a spare set
i will check the other this weekend

what should i expect to see ?


regards
 
dear 61 porsche

marvelous tip

did it this morning before departing for a 150 km trip

exactly two different situations, the front carb when obstructed the hole goes iddling up, so a little opening of the screw was done

the rear carb when obstructed dies, so a generous closing of the screw

fantastic results

but now iddle has gone to 1000 rpm´s, what to do to return to 900 ??

thank you
 
If it's running well you may consider leaving it at 1000 (mine is), it's a 40 year old car and 100 rpm out of spec isn't going to matter much and I'd rather have it idling smoothly. To really dial it in you'll need a carb air meter (or the tube stethescope technique) to see which carb if either is sucking more air. The manual will tell you that all final idle adjustments are done with the mixture screws, but in your case you may consider just very slightly backing off one or both throttle stop screws depending on what your air meter and the hole trick tells you.

deQuincey said:
marvelous tip

did it this morning before departing for a 150 km trip

exactly two different situations, the front carb when obstructed the hole goes iddling up, so a little opening of the screw was done

the rear carb when obstructed dies, so a generous closing of the screw

fantastic results

but now iddle has gone to 1000 rpm´s, what to do to return to 900 ??

thank you
Click to expand...
 
Turn boths screws equally now. Then recheck again slowly. Yep. it works.Your idle quality should be much improved. This allows for a slower idle than the 950 +-. It also helps smooth the transistion to off idle.
 
I performed this innovative test, after setting the idle via Blue Book procedure, and found that I must also be running idle rich; coverng the hole lowers the idle by 100 rpm with some stuttering. Based on this and others' comments, it seems that many of us are running richer than is necessary for smoothest idle. But if leaning out improves off-idle transition then I'm all for it.
 
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