Sputters and dies below 2000 RPM

Gearhead63

Low Mileage
I looked up the specs for a 38DGAS, and it appears the standard idle jet is a 45.

If it's anything like 36DCNVA setup (which also has two idle jets) and that carby definitely runs best with 47 or 50 idles... with the stock 40 idle jet there is no real change from winding the mixture screws back and forth, but a simple change to a 47 or 50 idle jet in both holders and the idle mixture adjustment screws will function as expected and the carb will hold a good idle.

At under 2k you are not on the main circuit, but with the throttle blade tipped at 2k revs you will probably be exposing the first progression hole to the airflow and at "fast idle" you are probably running off this.

Single simultaneous 2 bbl carby on a sohc, I recommend having a pair of 45,47,50,52 and 55 idle jets handy for tuning. The carb spends a LOT of it's time in the idle circuit for a street driven car so getting this metering right makes it much easier to set the carby up properly.

replace the idles with a pair of 50's and try again, leave everything else alone and make sure the float level is correct. The change may sound small, a 0.05mm change in orifice size, but viewed as a % increase of cross sectional area of the orifice (which is what matters) it equates to about 22.5% increase in the fuel volume available

and BTW that is a big carby for a small engine...I know you say your planning a 1.9 build and that's your justification, but it can't turn your sows ear into a silk purse no matter how hard you try...

38DGAS is just oriented all wrong for the application. including that it will put one of the mixture screws close to the motor right above ports 2 and 3... with a DCNF/DCNVA not only is the throttle shaft correct for application, but the idle jets are positioned equidistant from the engine, and positioned so the distribution of fuel is much more even at idle

SteveC
Thanks, Steve. I have pulled the carb and am looking to order some jets to experiment with. I did order some smaller mains too but will wait to modify that until I get the idle corrected. And, yes, the orientation of the carb makes adjusting the front mixture screw a little tricky but it is doable. I have heard concerns expressed here before about the transverse application of this carb on the car, but I know that DCOEs and DCNFs have both longitudinal and transverse applications as well.
 

fiatfactory

Steve Cecchele
well orientation depends on a few things...

Eduardo in his tuning bible definitely says for "best results" the carby shaft axis and the crank shaft axis should be aligned.

But a DCNF only really works well two out of the four possiblties, that would be float bowl to the front / shaft transverse to travel - like in an X19 or a 308 dino... and float pin to the front / shaft aligned with travel axis - as you would in an 850 / 600., and the 246 V6 dino

It does however also get used (well a DCN which is very close) on a V12 ferrari, and the DCNF on the Fiat Dino V6, and the throttle shafts are not aligned with the crank axis, but the float bowl is definitely forwards. (but it makes sense whe you see the flow parth)

but they perform poorly in the other two possible orientations, if you put the float bowl to the rear on an X19, or put the float pin to the rear with an 850... you get issues with the fuel level being affected by hard braking and going up steep hills.

I dont know what the float pin axis and float orientation is for a DGAS, at least the throttle shafts are aligned with the crank axis, but I'm not sure which orientation you actually have your carb? or 100% sure what one looks like when you take the top off.

I had kind of assumed it was a bit like a DGV, but looking at it again it may not be.

SteveC
 
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Jefco

Daily Driver
a bit like a DGV
Yes;
DGS orients like a DGV or DFV, float bowl forward; the F series is for a left-side application (left side primary) and the G series for right sides.
I have run a 32ADFA on my Volvo 140 (pushrod engine, right side manifold) with no ill effects, though the primary was on the inside.
Currently running a 38DGES.
Thanks for the jetting advice!
 
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fiatfactory

Steve Cecchele
primary to the inside / outside happens a lot with the same carby in different applications.

dmtr / datr on 128 / x19 puts the primary and idle jet away from the engine, but the same carby on a lancia beta has it positioned ahead of the engine, so to keep the float bowl orientation (the more important thing) correct, a beta has the primary and the idle jet positioned close to the engine.

In pedestrian engines, not a problem... no tuning issues.

But, if you bump the compression / change the cam timing and make other general tuning changes and retain a dmtr/datr style carb on a beta, I've always found issues with running on / dieseling to be a problem. The factory probably noticed the same thing as they were the first Fiat group cars I recall being fitted with idle fuel stop solenoids....

The exact same engine in a 124 or a 131, with an ADF/ADM type carb has no issues, and the only real difference I have found is the idlel jet fuel hole orientation difference. I think it let's cylinders 4 and 1 run quite lean on idle, and when you bring the engine closer to the limit of the fuel being used it starts to present a problem.

so orientation depends on a lot of things, but float bowl position seems to be king... but again it depends on the actual carby

DCOE's get used in all four directions with no issues for example.... but they have the fuel bowl down very low compared to the jets. carby throats and the needle valve position is central.. a side draft carby also benefits from a low "component" centre of gravity... so the float bowl / needle valve position works.

SteveC
 
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Gearhead63

Low Mileage
I replaced the 155 mains with 145s and the 55 primaries with 45 and replaced the 170 air corrector with a 200. Starts up and idles around 1500-2000 but when the choke starts opening, the idle starts to drop and then it won't idle. Have to keep the throttle ope and keep it about 2000 to keep it running. Runs fine at speed. Could this be lean primaries?
 

Kendall

Daily Driver
Sounds lean if I had to guess. The fact that it dies when the choke opens might back that up. You certainly leaned it out significantly changing all that. I believe that they say one step in a main jet is equivalent to 3 steps of the air correctors (so a difference of .05 equals .15). So with changing all that it's like you moved 5 or 6 "steps". I might change back the main jets and air correctors and see how it runs, then adjust it from there.
 

fiatfactory

Steve Cecchele
At under 2k you are not on the main circuit, but with the throttle blade tipped at 2k revs you will probably be exposing the first progression hole to the airflow and at "fast idle" you are probably running off this.

SteveC
changing the main and air correctors will have NO effect (i.e zero, none, nada , bupkiss, zilch) on the idle running, you are not on the main fuel circuit yet, ALL the fuel at idle comes via the idle fuel jets. Some (well most webers) carburettors also have an idle air corrector and in most (OE type factory fitted) webers this is a pressed in brass bush with a metered orifice.

It's the idle fuel jet, and the idle air corrector bushing that will be controlling the emulsified idle fuel flow, in combination with your mixtutre screw adjustment ... so as I said before (and I hate having to repeat myself) just forget about the others until you have the idle dialed in.

You've made a typical carburettor novice mistake, by making more than one change at a time....

SteveC
 
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