Dellortos 48 drla leaning at the transition

[pnikas]

Hello everybody, I have a 2533 type 4 with Dellorto 48drla carbs
They have 40mm venturi
I put them on the engine and using a O2 sensor I am trying to tune it .
It was rich on the idle circuit, ok at high rpms and it gets lean at a about 3000rpm, specialy when pressing just a bit the throttle .
I tried reducing air size 200->190
    Increasing pump jet  55->60
    Increasing main jets. 180->190
 Used 9164.3 emulsion tubes instead of 9163.2(that helped a lot )
Still gets lean at around 3000 rpm

[Phil West]

Your problem is the idle air.  I had the same issue.  You need a smaller idle air jet.  As we found on the Dyno, it’s a critical tuning area.

In the end we used the tall idle air holder from the cb update kit and drilled it out very big.  Like 3mm.  Then we used Weber air jets that just fit nicely inside the tall holder.  This allowed us to change jets for the idle air.  After years of living with a lean out at transition I finally got it sorted on my drla’s.

Cheers
Phil

[pnikas]

Your problem is the idle air.  I had the same issue.  You need a smaller idle air jet.  As we found on the Dyno, it’s a critical tuning area.

In the end we used the tall idle air holder from the cb update kit and drilled it out very big.  Like 3mm.  Then we used Weber air jets that just fit nicely inside the tall holder.  This allowed us to change jets for the idle air.  After years of living with a lean out at transition I finally got it sorted on my drla’s.

Cheers
Phil

Thanks a lot for your answer Phil .
You might jut be on to something because I had the dumb idea to use the idle jet holders from my 40 drla carbs that had fresh o-rings on them , are the jet holders marked ?
Where from do they pick the air?.DHLA carbs have many options in idle jet holders .

[Phil West]

Fundamental weakness of the DRLA design - these idle air holders do not allow tuning i.e. they don't come in a whole range of sizes.  I found it was one of the most important areas for tuning.  For most of my motorway driving I am at the top of the idle circuit.  Main jets not so relevant.

[Phil West]

I'm sure there's many ways folk have got round the problem.  My solution was pretty basic.  Just fit weber airs into the top of the idle air holders in this kit.  You need to spread the base of the jets a bit to fit snugly.  Then you can swap airs at will.  Worked at treat.

Cheers
Phil

[volkskris]

Are these (or similar) idle jet holders available separately somewhere? I have 36's so no use for most of the parts that come with the update kit.

[Phil West]

Not that I am aware.  I also am not using the other bits.

[JIMP]

Hello

cb has the "7348 Jet Doctor Kit" so you can get only the tall holders needed for small price

Friendly

Dimitrios

[volkskris]

Hello

cb has the "7348 Jet Doctor Kit" so you can get only the tall holders needed for small price

Friendly

Dimitrios

Sweet thanks!

Anyone have any idea what size air jet I can start out with on a 1600 with 36 drla's?

[cel]

Fundamental weakness of the DRLA design - these idle air holders do not allow tuning i.e. they don't come in a whole range of sizes.  I found it was one of the most important areas for tuning.  For most of my motorway driving I am at the top of the idle circuit.  Main jets not so relevant.

I dont know but maybe the idle jet holders from the older (pre emmision) dhla carbs will fit? They are "emulsion type" and have air calibration holes.

https://www.dellortoshop.com/contents/en-us/p632_(12)-Idle-Jet-Holder-Emulsion-Tube-for-Dellorto-DHLA-carburetors-7850.html