Weber 38/38 DGS and various Distributors

Sickboy49

Sickboy49

Well-known member
I am creating this post as a case study on setting up a Weber 38/38 DGS (Manual Choke) carburetor with two different distributors. I hope this helps anyone going down the same path that I am. I also have a question or two at the bottom of this.

Distributors:
Stock Load-O-Matic (Vacuum Advance only)
Mallory Dual Point ZC313A (Vacuum/Centrifugal)

Engine:
Rebuilt 1961 223 with stock valves and cam
Bored .040 over

Intake/Exhaust
Clifford Engineering Intake for either a single two barrel/four barrel carb
Exhaust is a custom built header setup derived from a Clifford Engineering truck header

Weber 38/38 (Initial Jetting and settings)
Idles Jets: 55
Idle Mixture Screws: 1.75 turns out
Idle Speed Screw: 1.25 turns in
*Carb Note* I did a lot of reading on the setup of Weber carburetors. The on thing I can definitely say is that the 38/38 does pull vacuum at idle, which contradicts the documentation that I read. Vacuum line to the distributor is coming from the port closest to the choke.

*Note* The Weber Intake/Exhaust and Carb were bought used. I actually bought the whole motor which was from a 1963 truck. It was supposedly rebuilt but by the looks of it, it was leaking oil from everywhere. Also by looking at the plugs it was running very rich. I also purchased a Weber re-jetting kit just in case and I am glad I did. Also I realize this whole process is probably simpler with an electronic ignition conversion but I am trying my best to keep all electronics out of this car.

Case 1: Load-O-Matic

Most documentation says this setup (Weber/Load-o-matic) will not work. After changing Idle jetting the carb was still gasping when matting the throttle. I know this is due to the load-o-matic going to fully advanced at startup. I tinkered with heavier springs but could only get the timing down to 28 degrees advanced at idle. I also found that someone had try to re-jet the air correctors and main jets incorrectly. It was also determined that the idle speed screw only exposed the main idle circuit at 1.25 turns. All Weber documentation says it should be 1/2 turn after contact with the plate. Below is the best setup I could attain, but starts hard with such advanced timing:

Idle Jets: 55
Air correctors: 180
Main Jets: 155
Mixture(s): 2 turns out
Idle Speed: 1.25 turns in

Case 2: Mallory Dual Point

After discovering that I installed the distributor 180 degrees off (thank you guys for the help!) with the vacuum line disconnected the motor fired right up and was the smoothest it has ever been. At idle it was sitting at 14 degrees advanced. After plugging in the vacuum line, it behaved just like the load-o-matic going to full advance (30 degrees +). After tinkering with the carb settings and timing a little I got it down to 24 degrees at idle, but it still starts hard and runs rough at idle. However when you mat the gas she pulls for the moon.

Idle Jets: 60
Air correctors: 180
Main Jets: 160
Mixture(s): 2 turns out
Idle Speed: 1.25 turns in

Question/Observations:
I am thinking about disconnecting the vacuum line to the Mallory as all that the vacuum does remove the breaking affect of a plunger resting against the advance plate. I want to see how much advance at 2500 rpm I get without the vacuum. It is either that or heavier springs on the centrifugal advance arms. Any thoughts?
 
Does the Weber carb not have a ported vacuum port for the distributor? Ported vacuum solves all of the above. You have no distributor vacuum at idle, and manifold vacuum advance above idle.
14* base timing is enough. (Too much IMO). With vacuum advance at idle, it's way advanced and will idle poorly.
 
Supposedly the port that I am using is ported vacuum on the Weber. There is another port on the throttle side but if I remember correctly that is for positive pressure. I might just try it to see what happens. There is almost to much information out there and to many variations in this line of Weber's. I remember reading a post by a member that said there are 39 different variations to the same carb family.
 
If the port has vacuum at base idle, it's not ported. Ported vacuum port is usually higher on the carb than mani vac. Ported= no vacuum until the throttle is cracked open a few *, then it comes in quickly to the same vacuum as manifold.
 
Thank you Frank. I am going to try the other port out tonight. I will let you know the results.
 
Disconnected the vacuum line and plugged the port on the carb, and then spent the evening trying to fine tune everything before I try plugging the vacuum into the other port on the carb. I have gotten the timing down to about 10-12 degrees and am currently messing with the idle speed/mixtures/and advance in order to get the carb settings within Weber's specs. I also found a small vacuum leak on the port in the manifold. I properly sealed that up, so I think that was causing some of my issues. I think I still may be in the secondary idle circuit so I will have to mess with the mixture's and advance some more. I'll have more time this weekend to fine tune it.
 
I plugged the vacuum advance into the correct port on the carb and found it was pulling vacuum. So that means I have the idle speed set to high (even though it is at the lowest I can get it to keep the engine running) and the blades are opening and the motor is running lean at idle. I ordered #70 and #80 size idle jets. I'll see which on works the best. I should have them by Wednesday. I will keep you all posted.
 
Sickboy49 said:
I plugged the vacuum advance into the correct port on the carb and found it was pulling vacuum. So that means I have the idle speed set to high (even though it is at the lowest I can get it to keep the engine running) and the blades are opening and the motor is running lean at idle. I ordered #70 and #80 size idle jets. I'll see which on works the best. I should have them by Wednesday. I will keep you all posted.
Click to expand...
Yeah- having vacuum advance at idle is contributing to the idle issue. . Surprised that carb doesn't have a ported vacuum port.
 
I think the port I am using is ported.... but since I have the butterfly's open enough to keep her running, I think it is getting vacuum to that port. Heck, they both could be ported, but the one I am I using now is slightly higher then the one I was using closer to the base. I can definitely see why people go duel carbs since I have to get so much fuel into the motor using a single.
 
So to give you an update. I received the #70 and #80 idle jets. I installed the #80's first but it was running to rich. Then I installed the #70's and it was running better but I think I am still running to rich since I have the largest main jets with the smallest air correctors. So my thought is to start over again with the smallest main jets paired with the smallest air correctors and go from there, because the motor never had a problem after you got it out of the idle circuits. I will do that this weekend and let you know the results.
 
Well gentlemen the saga continues. I went through all the pairs of main jets, and am currently running #100 idle jet, #160 Mains, #180 A.C.. Car is obviously running waaaaay rich, but I cannot take the choke off without it dying. That is why I am running it so rich because my thinking was it not getting enough gas. Also I have the idle speed screw set so the butterflies are just below the secondary idle circuit ports. I am sure I could get the engine to run, but I would be in the secondary's which from everything I read is a no-no. Tuning a carb should not be this difficult. I also went over the entire intake manifold and carb checking for vacuum leaks again, and found nothing. I am about to chuck the damn thing out the window and get a 2 or 4 barrel Holley. The only other thing to check is the float level. One of my buddies who is a mechanic suggested that. I am going to check that out the next time I get to work on her.
 
Sickboy49 said:
Well gentlemen the saga continues. I went through all the pairs of main jets, and am currently running #100 idle jet, #160 Mains, #180 A.C.. Car is obviously running waaaaay rich, but I cannot take the choke off without it dying. That is why I am running it so rich because my thinking was it not getting enough gas. Also I have the idle speed screw set so the butterflies are just below the secondary idle circuit ports. I am sure I could get the engine to run, but I would be in the secondary's which from everything I read is a no-no. Tuning a carb should not be this difficult. I also went over the entire intake manifold and carb checking for vacuum leaks again, and found nothing. I am about to chuck the damn thing out the window and get a 2 or 4 barrel Holley. The only other thing to check is the float level. One of my buddies who is a mechanic suggested that. I am going to check that out the next time I get to work on her.
Click to expand...
Man that's so frustrating! With no vacuum leaks, the mystery remains.
Hang in there!
 
After I check the float level I am ordering a rebuild kit (since I did get this used) and rip it apart and let it sit in parts cleaner for a week and then blow it out. Maybe there is some crud in it somewhere that is blocking a passage?
 
Well I found some info online about the float levels, and checked them last night. They are waaaaay off. I am going to adjust them Friday and then put the smaller mains and idle jets back in to start from the beginning and see what happens. I will post the float settings if they work out.
 
I have not had a lot of time to work on the carb until last night. I pulled it apart and removed everything I could jets (idle, mains, air correctors, emulsion tubes), and the mixture screws. I shot it up with carb cleaner and let it sit in pan until tomorrow, where I am going to shoot it up again. When I pulled the fuel filter that is in the carb, it and the holder were pretty grimed up with dirt. After the second round of carb cleaner I am going to blow it out with a compressor and re-assemble.
 
Sickboy49 said:
I have not had a lot of time to work on the carb until last night. I pulled it apart and removed everything I could jets (idle, mains, air correctors, emulsion tubes), and the mixture screws. I shot it up with carb cleaner and let it sit in pan until tomorrow, where I am going to shoot it up again. When I pulled the fuel filter that is in the carb, it and the holder were pretty grimed up with dirt. After the second round of carb cleaner I am going to blow it out with a compressor and re-assemble.
Click to expand...
glad you're back- been meaning to post- a person in-the-know told me those carbs should have no more than 3 psi of fuel pressure. This could be the source of the whole issue, fuel pressure.
 
I have had Weber fuel pressure regulator on it since I bought it. Those are set to 3 psi out of the box. Fuel pressure was one thing I wanted to take out of the setup scenario since I read a lot about the carbs not behaving well with excess pressure.
 
Got an update for you. So I let the carb sit in carb cleaner and then blew it out with an air compressor. After reassembly and mounting it on the motor the same condition exists. Here is the jetting I installed:

ACs: 185
Main Jets: 150
Idle Jets: 60

I also verified again that the idle speed screw has the butterflies only open to were the secondary idle circuit is covered up. It's runs with the choke on, but as soon as you take it off it dies. One thing I found that may be a problem. When changing the idle jet out on the choke side of the carb the little rubber grommet on the jet holder never makes contact with the head of the holder and the casing of the carb. Would this create an air leak? The holder on the opposite side of the carb clearly make contact because you can feel it when you are tightening down the holder. The choke side feels like it just bottoms out.
 
If you have a clogged (or just restricted) passage, you might try clearing it mechanically with a piece of fine wire. It's been my experience that sometimes the crust that develops when ethanol infused gasoline turns bad can't be blown out with compressed air.

Lou Manglass
 
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