Dellorto DRLA 40 modifications

[Don Cichocki]

I'm tuning a Porsche 356 with 1720 P&C, stock "C" cam (mild), 9:1 CR, and a centrifugal only distributer, 33 deg max advance. I had Weber IDF 40's on it and it ran reasonable. After hearing how superior Dellortos are I bought a set from Alfa 1750. I had him install 30 mm venturies and he gave me a jet combo of 55 idle, 122 main, 175 A/C, 35 pump, .2 emulsion tube. I went through the jetting procedure like you would for Weber: took out main stack, set LBI, backed out the pump to "o" squirt, drove around on just the 55 idle. I'm using an AEM wideband and with the stack out I'm in the 12.2-12.8 range, not bad. Put a 160 main and 160 A/C in to see the transition and got a good feel for pedal position, about 1/8 throttle, when it goes pig rich. Then backed off A/C until I was at a 180 A/C when I could see the wideband start to go lean, 14.5-15. Started dropping the main and I'm now at 132, and have the lean spot on transition but at WOT I'm rich, 11.2-11.6. I've tried a 190 A/C to richen up the transition to main and it worked buy leaning out the low range, didn't do much to the middle, and am still rich WOT, 11.6-12.1. After trying various combinations 55I, 132M, 190 A/C seems to be the closest all around to a 12.-13.2 A/F mixture, BUT at transition up a hill in 4th gear it goes into the "death Zone", 14.5-15.5, and stays there until I back off or the road levels out.
My question is: Has anyone tried to add a 6th progression hole above the existing 5 on this carb? Or are there any modifications I can make to the e-tube to help richen this lean spot. I do have a .1 and a .3 e-tube coming just to experiment.
I did read all 102 pages on the wideband post.
Thanks!

[kadub]

Have you tried larger idle jets? Should richen up the transition.
_________________
Karl

aka Turbobaja

[74 Thing]

You are still rich on the mains-should be in the 125 main area with 170, 180, 190 airs.

Leave the idles as they are at 55s.

[Don Cichocki]

I will be going down on the mains but I doubt that will help the lean transition. 55 idles feel right. The Dellorto's don't seem to respond to A/C jet changes like Webers. Instead of turning on the mains sooner they lean out everything if you go bigger. After 190 A/C if you go bigger the A/f gauge goes all over the place, seems like that's the limit.
That's why I'm asking if anyone has tried the modifications I mentioned above.
I had good results modifying the F11's on my Webers but those are entirely different than the Dellorto e-tubes.

[Alstrup]

Sounds like the float height is too high. Lower the floats to 5 mm from gasket.
Try #1 E tubes. Main air depends a lot on WHICH Dellorto model. There are three. A picture of the carb from the top says more than a hundred words.
Dellorto´s are generally more difficult to dial in on a 356, because that engine wants the idle system so rich. Then the midrange on the mains suddenly becomes the problem.
I also think you have some false measuring on the idle circuit. Try and block one exh. pipe while on the idle circuit and in the transition. I´m pretty sure that will alter your readings rather significant.

[Don Cichocki]

Alstrup,
Floats are correct at 5 mm from bottom of gasket. When I started having problems I checked that first thing. I have bungs welded to the #1 and #3 lower header pipes coming from the heads going to a stock 356 muffler. Out of curiosity I switched the O2 sensor from one side to the other and got similar readings.
That you said Dells are a bit difficult to dial in for midrange on a 356 is why I asked about adding a 6th progression hole like a DRLA 36, or possibly modify the e-tubes. I was a machinist for 30+ years so I feel confident in doing the work.
I'll try and upload some pictures but I'm an idiot when it comes to computers.

[Don Cichocki]

Bung location on both lower header pipes. I don't have a picture of the top of the carb but I'll get one soon.

[modok]

Check the size of the transfer ports
are the top two holes 1.4mm or 1.8mm?

[Don Cichocki]

Modoc,
Will have to remove a carb to do that, I might do it tomorrow.
I've seen some of your posts where you aren't afraid to modify something if it won't work, that's the state I'm in. These certainly are beautiful carbs but "one size doesn't fit all". I'm sure that with some perseverance and thought they can be made to give excellent performance.
I'll post the sizes when I get them along with some pictures.
Thanks to all who responded!
Regards,
Don

[kadub]

How does it drive? I'd be concerned about trusting readings under all rpm/load conditions with that exhaust and sensor location.
_________________
Karl

aka Turbobaja

[modok]

Don, I think it's going to be a pleasure working with a fellow machinist.

Understanding a carburetor as a series of calibrated holes and sizes, takes all the mysteries out of it. We're going to have this sorted so quick the peanut gallery might miss it if they aren't paying attention.

[Alstrup]

If you are confident with the jetting in general, try changing the pump jet throw, perhaps even go up to, 40 jets.

[Don Cichocki]

Pump jets on order.
At VERY slow throttle openings the accelerator pump should not come into play. I'm taking my A/F readings at constant pedal location across the board up the rev range.
I can get it to run fine by "seat of pants" but the A/F gauge is telling me I'm either way rich at spots or in the "death zone"
Going out this morning to remove a carb to measure the progression holes and take photos … be back soon.

[Don Cichocki]

carb top photo:

progression holes:

!st progression hole (on left) at idle setting:

Modoc,
Measured top holes on both carbs at 1.4mm (.055"). The next 3 lower holes were also 1.4mm. What surprised me was the bottom holes on the right carb were 1.0mm (.039") and the bottom holes on the left carb are .95mm (.037"). Was wondering if I should open up the smaller holes to 1.0mm just to be symmetrical?

[Alstrup]

[Don Cichocki]

Alstrup,
I can certainly lower the float to 6mm if necessary. I do run velocity stacks and a .25" spacer plate to clear the fuel inlet boss. Total height of spacer plate and stacks is 63mm (about 2-1/2"), to the top of the air filter element is 40mm (about 1-9/16").

[modok]

Alstrup didn't notice the boosters are UPSIDE DOWN
but you did request pics, so, your still my hero

[modok]

40drla from alfa, the ports are 100-140-140-140-140

40drla like CB sold 100-160-160-180-180

All of them are the same pattern, but the hole sizes are different.

Do not pay attention to where the holes are in relation to the cover, the holes are drilled in relation to the throttle plate, and the cover is just machined to match the casting.

[modok]

Your lean spot will be cured by installing the boosters the correct direction.

If you were running 32mm or 34mm venturis then i would recomend enlarging the transfer port, but, I think more testing is needed before doing that. Although, yeah I'd go ahead and fix the .95 with a 1mm drill.
oxidation can make them grow smaller.

[[email protected]]

Re-do everything with the aux venturis oriented properly. That was your headache.
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