34DMTR idle/mixture settings


True Classic
Today I replaced my 32DATRA with a 34DMTR and am having trouble setting idle and mixture...

SteveC advises to adjust idle speed via the idle air bypass screw and not to touch the throttle stop screw.

After getting the car started, I could not get it to idle at all. Only way I could do so was to screw in the forbidden throttle stop screw. Now it idles, but jiggering both the idle air bypass and/or mixture screw through their entire range appear to have no effect at all.

On the road, I can feel some potential for increased performance, but acceleration is really jerky, head-wobbling stuff, particularly when under load - like up hill.

(Car ran OK up until carb swap)

Any clues?




Tony Natoli
Well... I hesitate to answer here...

... as its difficult to determine without 2 or 3 other senses observing what's going on.

OBVIOUSLY, its the carb, if there are no other vacuum leaks.

DO INDEED use the throttle stop idle adjustment to get the car to idle a bit... but as Steve C said, the PRIMARY idle adjustment is with the air/fuel (a/f) mixture adjustment screw.

If the a/f screw is not making a difference, it COULD BE because the main throttle stop has the engine idling so fast that the "idle circuit" for which that adjustment controls, has been overriden by throttle itself being too far open, and it has been effectively BYPASSED.

I suggest doing the following first.

1. Make sure the CHOKE idle adjustment is not interferring and that the choke itself is WIDE open, and the engine is at correct operating temp.

2. Turn the a/f screw clockwise all the way in until it LIGHTLY hits its stop. Then back it out 1 1/2 turns.

3. Start engine and idle it down as low as possible, using the throttle stop, to around 700 rpm.

4. Now carefully turn the a/f screw IN (leaner) until the engine starts to die, and then back it out a bit. Note its position. Now turn it counter-clockwise (richer) and note if the engine speeds up even more... and also note its position when it no longer makes any difference.

5. Some folks like to adjust the a/f screw to a mid-point between these two LEAN and RICH points... I personally like to adjust it to the LEANEST and SMOOTHEST point. Then re-adjust the throttle stop so that it is at 750 - 950 rpm, whatever your preference is. (Clear the carb by juicing the engine between adjustments and insure it comes back down to idle and is steady after each time you change a setting.)

6. You can go back and forth and do this several times until you find the point you like.

NOW, if this method fails to work from the beginning... I suspect that the carb has some internal issues.

ALSO... use an unlit propane torch or WD40 and spray around the gaskets and vacuum lines to insure there are no vacuum leaks. If there are, the engine will speed up.


Kevin Cozzo

True Classic
swap out your jets

couldn't get mine to run well with the jets that came with the carb, took the old jets out of my 32 and put them in the 34, ran good enough to drive. eventually did trial and error with many different jets and emulsion tubes till I got it (somewhat) right. Somewhere out here there is a chart with a listing of jets that different people used when doing this swap (uh Jim D, master of the archives) Of course no two cars are the same , certain variables in your engine and modifications will have a bearing on proper jetting


Waiting for Godot...
Lots of stuff on carbs out there in the archives

I probably won't be much help because I am never sure how it all relates to any specific issue though. I know next to nothing about carb jetting.. OK.. probably nothing at all. :)

I am pretty sure the 34 DMTR an 34 DATRA are basically the same carb with different choke setups, so I searched for both.

Here is a thread from PaulV with s nice carb diagram. You can print a few out and track your changes as you experiment:

A thread from Dean Rees with a table link:

Long thread from anhthonyG

Some PBS info from Mira, Scroll down and you can find a carb discussion:

Then I hit Google to see what popped up. More of the same chart:

and then this, which looks important, but doesn't seem to be focused on the carb in question. There is some info on the DMTR buried in it though:


True Classic
Tony, Kevin, Jim,

Thanks for your replies.

I'll continue fiddling next weekend and let you know the outcome.




Tony Natoli
And fiddling and fiddling...

Imagine what the engineers went through to get it right in the first place.

Next to air/fuel mixes... you can try FIDDLING with spark plug heat ranges.

Ya gotta kinda expect this sort of thing when you "go outside the lines" of the stock settings. It took me a long while to get mine to what I call PERFECT... but I betcha I could start FIDDLING with it again if I wanted to...