Carb adjustment.

[Robert Mose]

I am trying to adjust my idle mixture. Its a 34 DMTR. I warm up the car and set my idle at around 800 rpm. When I turn the mixture screw there is no effect at all. I screwed it in all the way and even took it out.
What am I missing?

 

[ianlawson]

Are you sure.............??

....When I turn the mixture screw there is no effect at all. I screwed it in all the way and even took it out.

......you have located the actual MIXTURE screw??
My 1300 X has a 32DMTR Weber carb, and the mixture screw was initally hidden in its drilling location with a waxy sealing gunk (to prevent "non-professional" tinkerers from altering the initial production idle mixture/emission levels).
It takes a bit of searching to actually locate the idle screw, after probing away this sealant plug, and then using a thin bladed screwdriver to adjust the idle mixture.
If you have actually completely removed the TRUE idle mixture screw, the engine will stall!!
I kinda suspect that you are futzing with something other than the actual mixture screw??
Have a close look at the carb parts diagram in the Haynes workshop manual, or even better, in this Forum's online parts catalogue to ensure you are dealing with the correct adjustment screw.

cheers, Ian - NZ

 

[Black-Tooth]

Well... that screw allows gas to be syphoned...

... around the CLOSED butterfly and regular RUNNING fuel circuit.

If it runs when fully screwed in... it is either NOT seating, the WRONG needle, or the idle is turned up too high so that the butterfly is slightly open and the regular RUNNING circuit is drawing fuel into the system.

First make sure the butterflies on BOTH venturis are completely shut. Disconnect the cable and try forcing them closed some more.

Let us know what ya find... more to come...

 

[EricH]

The 34 DMTR is quite different than the 32

I am trying to adjust my idle mixture. Its a 34 DMTR. I warm up the car and set my idle at around 800 rpm. When I turn the mixture screw there is no effect at all. I screwed it in all the way and even took it out.

The 34 DMTR (and DATR and DAT) is quite different than the 32 DMTR/DATR/DMTRA/DATRA here. The 34 idle mixture screw is a little teeny thing set into the base of the carb on the side opposite from the throttle linkage. Is that the screw you are working with?

Depending on the age of your 34 DMTR , there are two ways of setting the idle speed. The later ones set it the way you'd expect, with a stop screw that holds the primary throttle plate a bit open. On the earliest DMTR (both 32 and 34) that stop screw is fixed with a lock nut, and the idle is adjusted with the idle bypass screw which meters air through a passage from above the butterfly to below. The throttle is set to let essentially no air by. Check out http://www.artigue.com/fiat/carbs/32_DMTR.pdf for an explanation of how this works (although it's for an early-style 32 DMTR not 34, so the idle adjustment screws are in different places from on you carb).

If turning these screws all the way in doesn't stop the engine, then it's not idling on the idle circuit - you may have the throttle too high so that it's running off of one of the transition orifices.

I'd suspect that you have a vacuum leak and are running on the primary circuit... except that then you probably wouldn't be able to get the idle down as low as 800.

If you don't have a vacuum gauge hooked up to the manifold vacuum port, you want to have one.

Check out

 

[Robert Mose]

I have the carb with the tiny screw on the side of the carb base ( drivers side). I cant find any other adjustment screw except for the idle speed on the throttle. It has a screw with a spring and not the stop screw. The car is at the garage now getting an alignment and once over before I take it for the final mech inspection on Thurs. I will take some pics and post them when I get it back.

 

[EricH]

Throttle stop screw with a spring?

I have the carb with the tiny screw on the side of the carb base ( drivers side). I cant find any other adjustment screw except for the idle speed on the throttle. It has a screw with a spring and not the stop screw. The car is at the garage now getting an alignment and once over before I take it for the final mech inspection on Thurs. I will take some pics and post them when I get it back.

Throttle stop screw with a spring? That's the later style, and you adjust the idle speed by turning that screw to crack the throttle slightly open, just like you'd expect.

And yes, the little screw on the side is the idle mixture screw, and the only other adjustment. If you can turn that all the way in without killing the motor, something is wrong. The idle circuit may be totally blocked so the screw does nothing and you're running on the transition circuit, or as Tony suggests something is wrong with the needle at the end of the screw.