Weber 38 Idle Woes

[2ndGenGuy]

I picked up a Weber 38 DGMS (manual choke) carb off of Craigslist for $190. I thought it was a good deal, I've been wanting the bigger carb to play with for a long time. And after trying to screw with rebushing the throttle shafts, I kinda got tired of messing and wanted to go back to a good ol Weber... I may never buy another used carb again.



Surprisingly there's not a ton of info about these online, and they seem to have a few differences from the other DGS models. First off, I guess the DGES/DGAS have different sized idle jets (physical size, not number size) depending on which barrel you're working on, which seems stupid since it's not progressive. My DGMS has the physical idle jets in both sides. Also, opposite of the accelerator pump is what appears to be a vacuum operated diaphragm. A vacuum line goes from the base of the carb to the diaphragm, and when I took that thing off, all the fuel drained out of the float bowl through two tiny holes where I'm guessing that thing pumps fuel somehow through a special circuit of some sort. Also, there are two fuel filler nipples it appears, and I have no idea why, or why when I apply fuel pressure to one, fuel squirts out the other. I have one capped for now.





Anyways, enough of that... the main issue I'm having is trying to get the damn thing to idle after revving the motor. The engine seems to surge as soon as it gets down to about 900rpm. It will surge up to about 1100, then down and almost stall, then it speeds up and almost stalls again. I thought for sure it was a vacuum leak, but I checked and I know for sure it's not.

I thought maybe it was running rich, so I sized the jets down, and it seemed to help, but does this seem like it's possibly a jetting issue? I have another smaller set of idle jets on order, but until then is there something else it could be?

I replaced the power valve assembly, which initially had cracks in the diaphragm, but it didn't really seem to help. I also checked and readjusted the float level which was way off. Fuel pressure is good, I have it regulated down to 3psi according to my pressure gauge.

Any other ideas what could be causing the surging at idle? I also get a really bad stumble if I drive it down the road and try to keep it at a steady RPM. Car seems to run decent while accelerating...

[2oodoor]

wow dude, you beat me to it, mine won't be here till Monday.. bought cygnus 's
Are your throttles closing sychronously? the gears have been known to get outtawhacky and worn so the plates do not close together, this causes gas to pull on one side on decel.
Still, get a AFM and start tuning ! OK now tell me, do tell, about the guts power you have with it over the progressive.?

[2oodoor]

Whats this about a diaphragm? does that bad rascal have dual accel pumps?

[Ichiban]

I betcha that diaphragm is an auxiliary accelerator pump, activated by a drop in vacuum, normally. My toyota has one.

[2oodoor]

I guess whenever you come back you can say if you honed out the adaptor to match the carb.I think any "stairs" left in below the venturi and throttle plates would cause puddling. It shouldnt have two diff idle jets for each barrel I dont think.
That was an OEM carb for something?
You can bump your ign timing up a good ways with this carb too.
see the other 38 troubles thread, my analogy about car wash dryers chasing water

[2ndGenGuy]

I suppose it might be an accelerator pump. It was my initial guess but I wasn't sure what benefit having two pumps would be, nor did I understand why you'd run it off of a vacuum like that. Maybe to keep the car idling after you come off the throttle is all that I could guess. Perhaps snapping both throttle plates shut cuts the fuel so quickly it would die...

I didn't port the manifold actually. I used the spacer for the 38 to avoid the butterflys not hitting the 32/36 adapter plate. I figured it might not perform it's best, but I could get it drivable first, then screw around with the port matching.

It IS entirely possible this was some sort of OEM carb. I don't really see the manual choke models for sale at all except in a few Toyota conversion kits.

What I mean by different jets, is not different jets like one is a 40 and the other is a 50. The "DGS" jet kit I bought came with a 50, 55, and 60 that looked like this for the "choke-side barrel", as it was labeled:


And a 50, 55, and 60 that looked like this for the other barrel:


Whereas my carb has jets that look like this for both barrels. Hence they're the same jets in both sides.


I think the reasoning for the newer DGES and DGAS carbs may have been to keep jet kits compatible with the DGV series carbs, so they can just sell the same jet kit for all of them and save some money. That's all I can figure anyways... I don't even know if the DGES/AS carbs have different idle jets for each barrel or not, I'm just guessing based on my jet kit.

Your analogy about the car wash was good though. It made total sense what you were trying to get across! I don't think this is my problem necessarily now... but I can see it helping with overall smoothness and drivability for sure... after I get this figured out.

How much did you buy Cygnus's for? It should be pretty close to tuned for your car even!

[Ichiban]

You sorta actually got it backwards. The way the aux accel pump on the toyota functions, is when the engine is cold, vacuum draws the diaphragm back, charging the fuel side of the pump and loading the spring. When you crack the throttle open and the manifold vacuum drops, the spring overcomes the now reduced vacuum force, and pushes fuel in through another accelerator pump jet, in effect supplementing the mechanical accelerator pump. Once the engine warms up, a thermovalve closes and the aux accel pump ceases to function. I believe it to be a roundabout way to reduce cold stumble. The whole Aisin carb setup is stupid, being a semi-synchronous vacuum activated 2bbl system, if you saw it function, you'd understand what I mean.

[2ndGenGuy]

I wonder if this one operates the same way. It would have to be some sort of internal thermovalve though. It would explain why you can only seem to find them mostly as conversion kits on Toyotas.

[2ndGenGuy]

So here is a video how it runs with 45 idle jets:

http://www.flickr.com/photos/chrisb27/2495815763/

As you can see, a touch rich. Also, I was looking down the barrels and noticed lots of puddling, and the accelerator pump sprays a LOT of fuel. I think it's spraying too much. If you just BARELY touch the throttle you can see big drops of fuel spraying out. The car also seems to drive just fine as long as I don't move the gas pedal. It accelerates smoothly at WOT, but low speed cruising causes bucking.

I'm getting my 40 idle jets tomorrow (I hope) and going to try swapping accelerator pump jets from another carb if those 40s don't help. I hope I don't have to order more parts. Waiting days between swapping little tiny parts is lame. Wish there was a shop in town that sold them...

[2oodoor]

You tried advancing the timing some, I am SURE it could use it now..
You can swap accel pump jets but they have to be from that carb I think. Can you adjust the pump stroke on that one?

[2oodoor]

there are any number of ways you can set these up. All of them can be acceptable and "right" Just depends on what you want out of it.
You can try turning everything back, throttle stop adj and get the base idle up slowly with that stop screw and ignition timing. then adjust everything else from there.
I am going to have fun setting this 38 up with an auto tx, ya think?

[A20A1]

Check for a vacuum leak on those adapter plates

How did the rebushing of the throttle shafts go? was it leaking from the shafts before?


On the diagram for your carb does it show that same type of jet or do you suspect the previous owner rigged it in there?

[2oodoor]

A20A1, do you know if it is common for the 32/36 to share the same jet fitament as the 38/38. Of couse the orifice sizes for them would be matching but different for tuning sake, I am speaking of the thread sizes and what have you. Mains, idle, tubes, the common items for DG and DF's. In clearer but hypothetical terms, if I had a set of F6 tubes, 137 mains, 50 idles from a progressive, would they fit the sychronus. Leaving Holley out of this question of course:

[2ndGenGuy]

Check for a vacuum leak on those adapter plates

How did the rebushing of the throttle shafts go? was it leaking from the shafts before?

I checked for leaks using some butane in a can. I can't find any leaks. This method worked for me to get my 32/36 running on my 81 Accord properly, so I'm positive there are no leaks around the throttle shafts or the adapter plates. Also, the leaky shafts I had before were on the Holley, not this one. This one seems good.

On the diagram for your carb does it show that same type of jet or do you suspect the previous owner rigged it in there?

That's the problem I'm having. I can't find a diagram for the 38DGMS anywhere. I called Carbs Unlimited, and they acted like they'd never even heard of it, and couldn't get me a proper rebuild kit for it specifically either. I don't think that the "big" jet would fit in either side, though now that you mention it, that's something I need to check.

I tossed the 40s in tonight, and it did fuck all. Car still runs like total shit, so I'm going to tear it apart (after I figure out why the Civic quit running ). I'll check for adjustment on the accelerator pump and probably will tear the carb down and soak the parts in carb cleaner...

[2ndGenGuy]

A20A1, do you know if it is common for the 32/36 to share the same jet fitament as the 38/38. Of couse the orifice sizes for them would be matching but different for tuning sake, I am speaking of the thread sizes and what have you. Mains, idle, tubes, the common items for DG and DF's. In clearer but hypothetical terms, if I had a set of F6 tubes, 137 mains, 50 idles from a progressive, would they fit the sychronus. Leaving Holley out of this question of course:

The jets look identical between both my 32/36 and my 38 (aside from the choke side barrel). I would say yes, and hope so assuming that's what I've been swapping out!!

[A20A1]

What about the throttle plate being open too far, could you get it to idle with less throttle?

[2oodoor]

^^^ So you are saying it is trying to idle in the transition circuit I suppose.
That would be the purpose of backing off the throttle stop adj. and move the timing up a little, then using a tach start adjusting the mixture screw.
Correct me if am wrong here, but cant these 38's be set up to run all the way up to slight part throttle on the idle circuit. That way you can get some economy out of them.

[2ndGenGuy]

If I drop the idle back any more, it won't idle at all. I'm convinced that there's a clogged passage or something. It's going to get the full soak and teardown in carb cleaner. I just won't have time for about 2-3 weeks, so the 2gee will have to sit for a little while...