DCNVA Low Throttle Tuning

AKimball92

AKimball92

True Classic
I have been having some issues running my engine with the 36 DCNVA carburetor. The engine is a very overwarmed 1300 with all the goodies in my signature. The car accelerates and drives fine expect when at light throttle, city cruising which is what I normally do around here. The car leans out to the point of wanting to die. I have an air fuel meter so 99% certain its turning lean.

I can drive fine and get anywhere I need to by very slight twitches of the accel pedal up or down. If held constant it just creeps up to 20+ (super lean) within about 5 seconds. Applying gas at over 20+ to correct back to 12-14 A:F it will buck once, probably from nearly dying.

My thoughts are obviously somewhere between the
  • air jet, emulsion tube, fuel jet (175, F36, 150 respectively)
    • if full accel is good (near/full WOT) would the air-fuel jet combo could be right and the emulsion tube hole combinations be off? more holes/larger holes at the bottom? or low vacuum needs smaller top holes for less air to pull through?
  • Somewhere on the vacuum system shown on page 18 of the PDF: Weber Tuning Manual
    • Does the DCNVA (DCNF) have this circuit? there is a vacuum line #12 to the "Dispositivo sgolfamento" (deflection device?) #23 via this website:
      DCVNA Exploded View
  • Float? possibly too low resulting in more air at mild vacuum (in combination of the air, emul, fuel jets). This carb is rather difficult to pull apart to check float. There are two linkages on this that are not on the DCNF I believe that block make this a 3-4 hands process.
  • Would the center hole through the base cause this and phenolic spacer? The hole is there, uncertain how well its aligned but would prefer not to interrupt the seal if I can avoid it.
Its been a while but thanks for the help everyone!
 
light throttle cruise you are pulling fuel thru the idle jet and possibly in the first part of the progression circuit, you are not on the main circuit so air corrector/emulsion tube/main jet changes will have no effect.

the central hole is to pull vacuum for the choke pull off diaphragm, that is all it does, the only way that can leak air is if the diaphragm is holed and allowing air in... easy check to see if it works.

Tweak you idle jet sizes up progessively until the issue is resolved, check your float level is correct

150 on the mains look a bit rich to me too... I normally settle around 135/140 as I've said to you before.

SteveC
 
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fiatfactory said:
light throttle cruise you are pulling fuel thru the idle jet and possibly in the first part of the progression circuit, you are not on the main circuit so air corrector/emulsion tube/main jet changes will have no effect.

the central hole is to pull vacumm for the choke pull off diaphragm, that is all it does, the only way that can leak air is is the diaphragm is holed and allowing air in... easy check to see if it works.

Tweak you idle jet sizes up progessively until the issue is resolved, check your float level is correct

SteveC
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Thanks for the response. I wouldn't have thought it would be the idle circuit. Once warmed up, the car idles perfectly fine at ~13.5. RPM sounds "low" but I don't have a good way of actually measuring RPMs. It does require light throttle help until warmed up, otherwise it dies.

My idle jets are: 47s. Would adjusting mixture screws correct this as well and not just the jet? I did do the mixture screw, throttle screw combo per the standard tuning steps to get to that point with the idle jets used.

Edit: Now that I think about it closer. I had issues with idle that the jets couldn't be fully screwed in or idle too would rise to completely lean at approximately the same rate. a slight quarter turn or anything off fully seated and it was fine. All worked well after that and I just assumed you didnt fully seat those.
 
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at light throttle you are not on the main circuit at all... that happens aound 3k revs usually, if you look down the carby and rev the engine up slowly, you will see when the main circut is activated, that's when fuel begins to issue from the central diffuser (the bar that runs across the troat up towards the top) until that point all fuel gets pulled thru the idle jet, and spills into the engine via the a) idle mixture screw hole, b) the progression holes

"Edit: Now that I think about it closer. I had issues with idle that the jets couldn't be fully screwed in or idle too would rise to completely lean at approximately the same rate. a slight quarter turn or anything off fully seated and it was fine. All worked well after that and I just assumed you didnt fully seat those."

this doesn't make a lot of sense?

the idle jets not fully seated will be excessively rich as you have no metered hole with the jet off it's tapered seat... what I think you might mean is the idle mixture SCREW, which is not a jet, but a variable tapered pin in an orifice to control fuel flow thru at idle thru the port which the fuel spills into the engine, this point is below the throttle butterfly.

The progression circuit fuel flow all comes via the idle JET as well and the holes which the fuel spills into the engine are gradually exposed by throttle blade movement. these holes ae fixed in size and position but the amount of fuel flow available to them is controlled by the idle JET (not the SCREW)

SteveC
 
fiatfactory said:
at light throttle you are not on the main circuit at all... that happens aound 3k revs usually, if you look down the carby and rev the engine up slowly, you will see when the main circut is activated, that's when fuel begins to issue from the central diffuser (the bar that runs across the troat up towards the top) until that point all fuel gets pulled thru the idle jet, and spills into the engine via the a) idle mixture screw hole, b) the progression holes

"Edit: Now that I think about it closer. I had issues with idle that the jets couldn't be fully screwed in or idle too would rise to completely lean at approximately the same rate. a slight quarter turn or anything off fully seated and it was fine. All worked well after that and I just assumed you didnt fully seat those."

this doesn't make a lot of sense?

the idle jets not fully seated will be excessively rich as you have no metered hole with the jet off it's tapered seat... what I think you might mean is the idle mixture SCREW, which is not a jet, but a variable tapered pin in an orifice to control fuel flow thru at idle thru the port which the fuel spills into the engine, this point is below the throttle butterfly.

The progression circuit fuel flow all comes via the idle JET as well and the holes which the fuel spills into the engine are gradually exposed by throttle blade movement. these holes ae fixed in size and position but the amount of fuel flow available to them is controlled by the idle JET (not the SCREW)

SteveC
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Unfortunately I am confident in idle not being steady when the JET was turned in all the way. I cannot remember if this was with both of my jet sizes or just one. It was on both sides though and would quickly clean up and smooth out idle with the slightest 1/8-1/4 turn loose. I will need to look at their sizes but that was last year when I played with this last.
Wednesday I will go driving again and take note at what throttle and speeds the AFR goes lean. My road nearby is maybe 35 mph. I might have been cruising 35-40 mph through most of this.
 
if the idle smooths out when you crack the JET off it's seat, that straight away tells me the jet size is too small, go up to the next size (it needs to be the same size on both sides) and FULLY seated with an O ring on the jet holder

The jet will only function as designed when fully seated and with an O ring sealing any possible air ingress

SteveC
 
fiatfactory said:
if the idle smooths out when you crack the JET off it's seat, that straight away tells me the jet size is too small, go up to the next size (it needs to be the same size on both sides) and FULLY seated with an O ring on the jet holder

The jet will only function as designed when fully seated and with an O ring sealing any possible air ingress

SteveC
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These idle jets are my largest ones yet. Given shipping might be more than the jets, any other items you would recommend I add to the cart? I might do a couple base seals.

I think you are right, SteveC (like always). All the way seated the car idles at ~800 rpm (per the gauge) where it’s most comfortable, AFR is upper 14s touching into low 15s. I aimed for that lean/stoich for economy. However any additional throttle to try to hold ~1800 RPM and the ratio shoots to 18 and rising. It required blipping to keep it low.

I backed out the jets an 1/8th turn, idle is now 12s but it can retain steady light throttle.
Would the next size up idle jet be enough to fix that or should I double up and go up another size as well? Would it then idle more/too rich to retain light throttle where it’s needed? Any way to keep idle in the 14s and light throttle there as well?
 
if you have 47's in there now, get a pair of 50, 52 and 55 idle jets...

a new top gasket as a spare is always a good thing to have, same with base gaskets.

a set of 140 and 145 mains might be handy too as I think your overfueling the main circuit in your attempts to make the light throttle cruise richer, but as I explained this will have zero/zilch/nada/zip/bupkiss effect on this part of the carburettors function, as that's the idle fuel jets job.

You spend so much time in the idle and progression circuit on a street driven car, so it's the most important fundamental setting to get right.

The idle "trim" is what you use the mixture screw for... fuel flows via the idle jet into the circuit, so the maximum amount of fuel flow thru the circuit is controlled by the idle jet orifice size. You get to "fine tune" the fuel flow capacity by the idle mixture screw, as you once again introduce a restriction to fuel flow (but now the fuel is also emulsified with air) with the variable tapered valve that is the mixture screw, but this ONLY affects the fuel flow which is spilling into the engine via the hole under the butterfly, the idle jet is also responsible for metering fuel flow to the progression circuit.

SteveC
 
It's been nearly a month...

Did you get the idle jets changed?

Did it make a difference?

Did it fix the lean out issue?

SteveC
 
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