Sputters and dies below 2000 RPM

Gearhead63

Low Mileage
I replaced my original carb (couldn't get it to work) with a 38/38 DGAS and cannot get the car to idle. It's a little better when starting cold but especially when it's warmed up it will sputter and die if you don't hold the RPMs to above 2000. Once on the road it runs pretty well. I have backed the mixture screws in and out which doesn't seem to make a difference. Also checked for vacuum leaks, adjusted timing etc. I can usually figure carb stuff out but this has me frustrated. Not sure where to go with this..
 

Jefco

Daily Driver
Agree with Don, these come with jets for 6-cylinder vehicles well over 2L. (I have one on my Volvo).
 

Gearhead63

Low Mileage
Thinking that but when I turned the mixture screws in all the way, it didn't seem to make a difference. Plus if it runs better on the choke, might that mean it's running lean?
 
You mentioned that you could not get the other carb to work. Is it possible that the problem is not all carb related? Have you checked ignition and valve timing and that you are getting a good spark?
 
The idle mixture screws will only give a certain range of adjustment. If the idle jets are too large, you might not be able to dial them in.
 

Gearhead63

Low Mileage
I am going to verify the valve timing as I replaced the belt while ago. Went with the larger carb as I am building a 1.9 for it. Thanks for the tips. Will post updates..
 

twincam69

True Classic
Was this a new out of the box carb or rebuilt? You may want to blow out the idle circuit with compressed air just to make sure there isn’t some blockage causing an idle issue.
 

Gearhead63

Low Mileage
Brand new. I did check the valve timing and it is spot on (thought I had double-checked when I installed it) Will try smaller jets.
 

Jefco

Daily Driver
Sorry I can't be more help (tuning), but I haven't done mine yet...
I plan to move it to the 124 2L I'm just about done building, going back to a DGV on the Volvo.
What kind of manifold are you using? A DGS isn't made to run with the throttle shafts across the car, the float pivots the wrong way.
 

carl

True Classic
If I get this right, your car wouldn't idle with the previous carb and now it won't idle with the 38? As noted above, this may not be a carb problem. Normally if you close the idle mixture screw all the way you close all fuel to the idle jet and that should kill the motor regardless of idle jet size. If the mixture screw is having no effect then this suggest the idle circuit is not working or you have the idle speed screw turned in too much and you are trying to idle on the main circuit, totally bypassing the idle circuit.
 

Gearhead63

Low Mileage
I couldn't get it to run at all with the old carb. With the 38, it starts but with a somewhat rough idle but will rev and has good power. Then when as it warms up, it starts to sputter and die unless I open the throttle to keep the engine about 2000 RPM. After it dies, I have to open the throttle up all the way to get it to start. I am thinking it might be too rich and is flooding/fouling the plugs as it warms up. I ordered some smaller jets and will experiment.
 

EricH

Eric Hamilton
Moderator
Then when as it warms up, it starts to sputter and die unless I open the throttle to keep the engine about 2000 RPM.
Do you have any vacuum lines attached to a thermovalve? A temperature dependent vacuum leak will act as you describe.
 

Kendall

Daily Driver
I assume on that carb it has some kind of progression holes. I had a similar issue where the mixture screw did nothing for my idle because the engine was idling off the progression holes. Take the carb off without changing anything and look from the bottom end to see if any progression holes are showing.
 

Gearhead63

Low Mileage
I assume on that carb it has some kind of progression holes. I had a similar issue where the mixture screw did nothing for my idle because the engine was idling off the progression holes. Take the carb off without changing anything and look from the bottom end to see if any progression holes are showing.
Thanks- Is this something that needs to be covered by the base gasket?
 

Kendall

Daily Driver
No, sorry I could have been more descriptive. The throttle plate at idle should basically be exactly in line with the first progression hole. If the throttle plate is higher and part of the hole or the whole hole is showing you're idling off the progression circuit, which is incorrect and not controlled by the mixture screw. I think that's the situation you're in but I don't know for sure. If the throttle plate was below the progression holes turning the mixture screw all the way in would kill the engine, as others have said.

My only other though is that I believe that carb is synchronous, so there may be two mixture screws you need to adjust. Maybe your idle is only running from one barrel instead of two?
 

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Gearhead63

Low Mileage
No, sorry I could have been more descriptive. The throttle plate at idle should basically be exactly in line with the first progression hole. If the throttle plate is higher and part of the hole or the whole hole is showing you're idling off the progression circuit, which is incorrect and not controlled by the mixture screw. I think that's the situation you're in but I don't know for sure. If the throttle plate was below the progression holes turning the mixture screw all the way in would kill the engine, as others have said.

My only other though is that I believe that carb is synchronous, so there may be two mixture screws you need to adjust. Maybe your idle is only running from one barrel instead of two?
It is a synchronous carb and I have been adjusting both screws. I will check where the throttle plates are with respect to the progression holes
 

fiatfactory

Steve Cecchele
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
 
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