DHLA Spec 5 on 907. Idle jets 50 -> 55 made for terrible idle. Why?

[BrianK]

I'm running DHLA 45D on my 907 - they are from an Excel HC. Being from an Excel, they were running a calibration that was ill-suited for the 907, so I had them rebuilt to Spec 5 calibration a couple years ago. 

Spec 5 uses size 50 idle jets. Word on the street - and my own driving - suggest that 50 is a touch too small. I've read that 55-58 is the way to go.  I just installed 55s all around and my idle went south.  It sounded (and almost assuredly was) running on 3 cylinders at idle & would pop on overrun.  When given a little more than idle throttle (say, 20%), it would run smoothly & had great power, but would fall back to a stumbling idle. To make sure it wasn't just a fowled plug, I drove the car a good bit, but no change to idle after that.

I pulled the idle jets and re-seated them, but no luck.

I then went back to 50s and she purrs like a kitten.

Any idea why 55s would idle so poorly?

Is it possible I got a bad jet? Any way to tell?

 

Worth noting: the new jets were a very snug fit in their holders - so snug that I couldn't fully seat them, so I got them 80% of the way in, then just installed them and allowed them to fully seat as I screwed them in. Could that have anything to do with it?

 

 

Edited September 18, 2022 by BrianK

[910Esprit]

Even if I change from 40 idle jet to 43, I heed to turn in the mixture screw, approx one turn, to maintain an acceptable idle mixture (measured on a gas analyser) - ie if you literally just swap the jets and do nothing else, its likely you are simply running very rich (Mine is a turbo but principle will be the same).

I don't think your technique for fitting the jets would be the issue, but I'd be cautious of overtightening.  I suppose you also need to consider if they could be counterfeit, if they are not consistent with the original tolerances

[jonwat]

On 18/09/2022 at 04:00, BrianK said:

  I just installed 55s all around and my idle went south. 

"Idle Jet selection

Idle jets cause a lot of confusion; although their name suggests that they govern the idle mixture, this is incorrect. It is true that the fuel consumed at idle is drawn through the idle jet, but the idle mixture is metered not by these jets, but by the idle volume screws mounted on top of each barrel."

Taken from "Selection and tuning of Dellorto DHLA carburettors"

You really need a gas analyser or Colortune in order to correctly set the idle mixture, anything else is guess work   

 

 

[BrianK]

Thanks for the tips, guys.

I naively thought that changing jets was all that was needed.  I already own a Colortune, so I've got some homework to do. 

Will report back after I get a chance to tune...