Idle jets and idle air jets

[Mervyn Hyde]

My 356B T6 has the Zenith 32 carbs. I have cleaned the carbs carefully and put good quality new kits in them. They run well. However, the air mixture screws are very sensitive and minor adjustments can mean a significant difference in idle stability. I have the standard 120 idle air jet (inside) and the 50 idle jet (outside). Do these deteriorate over time and would a smaller jet allow more adjustment at the mixture screw?

[Mike Horton]

Merv, what fuel do you get? Here in TX, I can only get the P.C. 10% ethanol, and in my '68 912 (as you have a '68 911, you'll know this), with leaner jetting from the factory, for the then new U.S. EPA emissions rules, up here on the TX South Plains, at 3300' elevation, I had to change the idle fuel jets from 55s, up to the solid shaft 57.5s, and the mains up to 127.5s, to get my oil temps back to normal ranges, in these 100+*F TX summer heat, and...I got my fuel mileage back, up to 32.5 MPG, on all highway driving, city driving, obviously, much poorer,

Read all of David Jones' tech articles on the changes needed for the modern fuels, especially, the ethanols...

[Ron LaDow]

Do these deteriorate over time

No. But they may have 'evolved' courtesy of someone with a drill. Confirm the sizes.

[Mervyn Hyde]

Thanks Mike and Ron. Our fuel here in Australia is still pretty decent. Ethanol is not compulsory as yet, although the sugar growers political lobby is trying to make it so. Our octane is 98 (your 95 RON equivalent I think). Also we are a sea level. I am thinking I need, as Ron suggests, to check the current idle jet and also the idle air jets for accuracy. The last owner of the car had it for almost 50 years and I don't think he fiddled with these jets, but there is only one way to check.


Need to find 50 and 120 thou jets to compare to

[Bruce Smith]

Merv -

Where are you starting the idle mixture screws? A good start is 1 1/2 turns out. Your problem sounds odd if it is happening with all four cylinders. Check to make sure the needle tips aren't damaged. Re. the idle jets, check to be sure they aren't plugged. Also, check the ports in your throttle bases. Again, odd with issues for all four.

- Bruce

www.cfi-auto.com

[David Jones]

Merv, Just to clarify. There are three common ratings for octane. RON, same as German ROZ and MON which is a lower number. Over here the combination of the two give us the pump rating we see here. This link is a good explanation. 99 octane sounds good but is the same as 93 here.
https://en.wikipedia.org/wiki/Octane_rating

[Mervyn Hyde]

It is interesting Bruce. The air mixture screws are all new, sharp, straight and no ridges. When adjusted seemingly correctly they are all less than 1 turn out. That seems wrong to me. I checked the .5 idle jet apertures with a .5mm nylon guitar strand and it fits through snugly. "...ports in the throttle bases"? You mean the progression ports in the base of the ventures? I have checked the three on each side (two above the butterflies) and all are clear and flowing properly.

All air mixture screw are about 1 turn out.

I can add that the car idles well now with no 'hunting' up or down and there is no popping. I am just worried that I might be too lean.

[Mike Horton]

Vic, Ron, Bruce, it's been a long time for me, but were the "C" Zeniths calibrated richer in the idle range? The rest of the engine specs matter...

[Bruce Smith]

Merv - One turn out with air cleaners off? Any different with them on? If you run it lean, it may get too hot. 1 1/4 turns with the mixture screws may be safer. - Bruce

[Mervyn Hyde]

The car has been tuned by using the best lean method as per usual. The air mix screws are very sensitive. Sensitive in the meaning that when screwing in the air/fuel screw until engine stumbles and then backing it out to the fastest possible idle before it stumbles again is between an 1/16 to a 1/8 of a turn. It then falls off to a stumble on the way out. So the sweet spot is very small. All the screws now sit about a turn out. The screws are new. Perhaps little rich but I will check after a long hot run. Temps on the centre of the rear exhaust pipes about half way along the rocker cover area, are pretty equal however. One issue I did note on #3 was that the spring on the mixture screw is very strong, even stronger than the others and they are all pretty strong. I wonder if they had been changed out at some stage?

I was trying to pick up a couple of idle jets (50) to test a theory that there may be a problem with adjustment of these old jets. However NLA in this neck of the woods. I will try to find a couple with in the Porsche community.

[Mervyn Hyde]

Does anyone have a spare 50 idle jet or two that I can buy? Also have others had any problems with the mixture screw tension springs being too strong to allow easy adjustment?

[Jim Alton]

Does anyone have a spare 50 idle jet or two that I can buy? Also have others had any problems with the mixture screw tension springs being too strong to allow easy adjustment?

You might try 356 Carburetor Rescue as a source of spare jets. They show up at the Lit Meet with a lot of parts.

[Dennis ODonnell]

You haven't mentioned much about your engine itself but one variable is the idle screw. There are long needles and short needles that will fit, only the long ones are correct. The originals were long and allowed for a gradual adjustment. I had some in a rebuild kit that were less acute; a 1/4 turn with them was the equivalent of a whole turn on the correct originals. Obviously blunt is not better. I was curious but tossed them and cleaned up the originals.

Your originals might have been replaced with the incorrect pieces because they had been overtightened, distorting the needle end. That has the effect of making the cone more blunt. You might replace yours with known correct parts.

[Mervyn Hyde]

Could be Dennis. Edit: I have now checked all of the 'needles' and they are all straight, equal and all the same length as the ones removed. I did note that the new ones in the kits were a little less sharp on the points.

I guess the spring tension is needed on them to maintain their relative position. It is obvious however that the tension on #3 is greater.

[Dennis ODonnell]

" I did note that the new ones in the kits were a little less sharp on the points."
Doesn't take much.

A tight spring will have no effect, one that's too soft will.