Mixture screw setting - new info

zimmy · #1 (**permalink**) 10-05-2010, 11:49 PM

New information to ponder. Everyone running 48 IDA carbs has always heard that the idle mixture screws need to be 3/4 of a turn out or your idle jetting is wrong. When I run at 3/4 of a turn I have extreme backfires at decel and not very good accel. Recently I have discovered that my car runs very well with the screws turned out to 1 1/4 to 1 1/2 turns and the backfires have gone away.

Here is my current thinking - I am running the new Spanish manufactured carbs. I had access to a set of old Italian made carbs and I compared the idle mixture screws. Turns out that the new idle mixture screws are a larger diameter than the older ones. This would mean that there is less space in the idle mixture channels with the new screws, so it makes sense that you would need them turned out further so that the volume of mixture would be the same as that with the older screws. I'm not sure this is totally correct, but it is my story and I am sticking with it!

coinop101 · #2 (**permalink**) 10-06-2010, 03:23 PM

Well I found the best resuilts by using a remote tach . Once idle is set I start on the driver front carb and turn the screw in and out untill I reach max rpm . The needle is quite senitive to the movent of the screw . Then I move on to the next carb and start all over . My throught is each carb effects each cylinder and by maxing the rpm I'm dumping the correct amount of fuel .
What do you think

Gaz64 · #3 (**permalink**) 10-07-2010, 02:58 AM

It all depends on the needle taper and the idle port diameter as too how much idle passage "area" is presented to the engine at idle.

Changing either one of these changes the baseline setting.

Cougar · #4 (**permalink**) 10-07-2010, 07:03 AM

I've always had good luck on any adjustable idle by turning the needle in until the motor begins to falter and noting the position, and then backing it out until it falters and noting that position too. I then turn the needle to be a split beteen the two positions. This may or may not be the exact same as the highest idling setting, but it does seem to run smoothly and seems to give the best coming off idle response.

Gaz64 · #5 (**permalink**) 10-08-2010, 11:12 PM

Quote:

Originally Posted by Cougar

I've always had good luck on any adjustable idle by turning the needle in until the motor begins to falter and noting the position, and then backing it out until it falters and noting that position too. I then turn the needle to be a split beteen the two positions. This may or may not be the exact same as the highest idling setting, but it does seem to run smoothly and seems to give the best coming off idle response.

Yes, you're correct in your procedure, but doesn't apply to the subject of this thread.

Zimmy has stated that there can be differences in the idle mixture screws between Italian and Spanish carbs.

The whole idle circuit could be a different calibration from factory, but the taper of the needle, its diameter and/or the port diameter affect the "correct" idle mixture adjustment point out from seated.

Eljaro · #6 (**permalink**) 10-12-2010, 10:19 AM

I did quite some trial and error with the idle jets and screw, and no matter what, the screw had to be 1 1/4 to 1 1/2 out for the setup to run right. I just discarded the 3/4 turn out setting for the idle as something impossible to achieve and am happy ever since.
That 3/4 setting was probably reached by someone about 40 years ago as the ideal. Well guys, if it is the carbs which are manufactured differently ( probably) or because of the global warming (less so), the setting is best at 1 1/4 out or even more.
Good to know that there is a modified mechanical dimension which may explain this and justify the new idle screw setting.