Weber gurus, unite! (carbs, not grills)

After getting my 74 2002 back on the road after several years in pieces, everything is going pretty well, with one exception: there's a very unpleasant surging/bucking that occurs only with the sort of very light throttle used to maintain speed. Everything else is good: idle is steady at 1000rpm, decel is even, acceleration is smooth and linear all the way to the redline, and power is good. It doesn't stumble out of gear, but once there's a load on the driveline, the problem appears. If I was driving on track, it would be liveable, but on public roads it's really annoying.

Carbs are original Italian Weber 40DCOEs, properly sync'd and flowing evenly across all four throats. Linkage is good with no slack. Setup is as follows:

34mm venturis

50F8 idle jets

125 main jets

40 pump jets

185 air correctors

F9 emulsion tubes

2.00 needle valves

Engine is BMW M10, 300deg Motorsport cam, ~9.0:1 compression, long tube header, tii distributor (mechanical advance). Engine smells a bit rich, but it always has because Webers, and this behavior is new.

What do I need to look at here? It sure feels like a carb problem, but I have considered that it might be in the driveline; I looked it over and saw nothing obvious that might be at fault.

Sticky ignition advance mechanism first after a lay-up.

Is this the same setup you had prior to the car being off the road?
 

I have zero knowledge of car or weber carbs, but you are describing how my motorcycle runs when it sits and I have to clean the jets. Are the carbs clean/rebuilt?

Not a Weber guru by any means, but I have been living with Webers on my Elan for the last 35 years. 

Make sure that timing correct and is not jumping around at the RPM the surging/bucking is happening at. A lot of carb problems are actually ignition.

You didn't say what RPM this occurs at, but chances are the carbs are running on the idle circuits under light load and low to mid RPM. Make sure the idle jets/circuits are clean. Check float levels and check for stuck float valves. Check fuel pressure if possible, under 2 psi is good.

Do not try to re-jet the carbs to work around the issue. If they were jetted ok in the past, they should be ok now. I have been down that road and have hundreds of dollars of jets I will probably never use since I went back to stock jetting. 

Enjoy! All part of the fun of owning these things. 

I've pulled all the jets out and made sure they were clean. I saw no residue in the carbs, but I did not reconfirm float levels. This is the same basic setup I've had for years; I did make a few adjustments a year or two before the car came off the road, but I never had this problem. Fuel delivery is via a Carter 4070 low pressure/high volume pump, again the same thing I've used forever.

It occurs across the lower half of the rev range, consistent with the idle/transition circuit. I didn't think about checking for a sticky advance mechanism, but I will. The distributor was rebuilt a few years back, so it shouldn't be too dirty, but it could certainly need a bit of cleaning and lubrication.

After checking the ignition I would check the floats. Too many holes covered on the emulsion tubes from high fuel level can give you the problem you are getting.

02Pilot said:

Engine smells a bit rich, but it always has because Webers, and this behavior is new.

If it smells a bit rich perhaps it is. What is the fuel pump pressure and is it fairly steady?

Bucking and surging at light throttle is a classic symptom of a lean condition.

How many turns out are the - hold on, do those have idle mixture adjustment screws the let more fuel in, like a DGV (and most passenger car carbs), or do they have air bypass screws?

Whichever system they use, check to make sure adjustment is within recommended range with them set for best idle. For example, on a DGV, if you have to turn the idle mixture out more than maybe two turns to get enough fuel to idle, then you are probably compensating for a too-small idle jet, meaning you won't have enough fuel in the idle circuit for a clean transition to the main.

Your fuel smell could be absolutely anything. Could be a dried-out filler hose. Certainly I would not take that as evidence of a rich mixture, especially not during what sounds like a lean misfire.

Dang. I was headed to bed, but a few minutes with the Weber book wouldn't hurt...

For reference, the left side of this image shows the idle circuit, and the middle the progression circuit:

Unfortunately, my carbs have non-removable plugs over the progression holes rather than the screws shown here, so I can't easily check them. I did scope the throats and the throttle plates seem to be in about the right place.

Timing advance would be logical, something is hanging up from sitting.  Tuning the carb would be a lot easier with a wide band O2 sensor.  Consider welding a bung in and getting one.

Just to make sure all the bases are covered - vacuum leak? Spray carb cleaner around throttle shafts while running to check if they are worn. If using o-ring soft mounts, check if o-rings are in correct position.

I am also on team "90% of all carb problems are actually ignition" but....

How many turns out from the bottom are your idle screws? 1.5 to 2 turns is the general rule for DCOEs. If you are less or more than that then the idle jet needs to be resized. This is important because the idle jet is actually in effect up to 2500 to 3000 rpm.

02Pilot said:

For reference, the left side of this image shows the idle circuit, and the middle the progression circuit:

Unfortunately, my carbs have non-removable plugs over the progression holes rather than the screws shown here, so I can't easily check them. I did scope the throats and the throttle plates seem to be in about the right place.

That image matches what my Weber book indicated last night. Throttle plates being more or less in the right position at idle means the progression holes should be <ahem> progressively exposed to vacuum as the throttles are opened and the butterflies sweep past them. So far, so good.

Before we go any further, can we confirm that the engine and carbs and everything else are in the same configuration as they were when you last drove it? If the combination hasn't changed, then it may be best not to introduce any new variables until other factors are ruled out.

The carbs may have gotten gunky from sitting, and a restriction in the idle circuit could cause that surging. It's too bad your carbs don't allow direct visual inspection of the progression holes.  What you could do is remove the idle mixture screws (count the turns out from fully closed first) and shine a penlight into them and peek down the throats to confirm that light is visible through all the idle and progression holes. It probably wouldn't hurt to shoot a little carburetor cleaner through the idle circuits from any angle you can reach (idle mixture screw hole, idle jet well, etc), let them marinate for a few minutes, and then follow with compressed air to blast out any remaining boogers.

Update: good news and bad news (but mostly good). I pulled the covers off to check the float levels, which were OK, however I discovered a partially sunk float on the rear carb. Not enough to cause the bowl to overflow, but clearly enough to throw things off. Fortunately, I had a new set of plastic floats sitting on the shelf, so in they went. Got them adjusted (12.5mm to 25mm, measured at the tip of the float) and put it all back together. Bowls fill, all good.

Checked the advance mechanism on the distributor and it's 100% free. The distributor is still spotless from when I rebuilt it a couple years ago. I checked the points just to be thorough and found them clean but a little uneven, so I pulled them out and sanded them flat, then reinstalled and regapped.

Car fired up and the test drive showed major improvement, with the stumble all but gone, and anything that remains is probably just down to the nature of carburetors (it's really, really minor when it does happen, and that's not often). It's back to what I consider normal for this car; I've had it for close to 30 years, all with this configuration, so I think I'm qualified to make the judgment.

Thanks for all the suggestions.

02Pilot said:

It's back to what I consider normal for this car; I've had it for close to 30 years, all with this configuration...

Just curious - have you ever tried slightly fatter idle jets to see if you can eliminate that occasional stumble? It may be worth a try for the minimal effort required. 

Fuel composition may also have changed since the car was set up. If it was jetted just right for straight gasoline and is now burning E10 then it's probably a little leaner than it was 30 years ago.

Good to hear!

Where do I collect my GRM internet points for the float suggestion? I have some differential questions I want to spend them on.

In reply to DarkMonohue :

I've never tried other idle jets, but I've also never heard of anyone running these on a street 2002 with anything other than 45s or 50s. Never thought about the ethanol factor, though it may explain why the top end improved by fattening the mains from 125 to 135.

In reply to 02Pilot :

Light throttle/low RPM can be tricky because a stumble can simply be weak ignition.  At the low cylinder pressures involved, it can be difficult to light the mixture, especially if you have vacuum advance, which tries to light the fire before compression is pretty far away from max.

It's amazing how many "carburetor problems" can be fixed by new plugs and wires   Making it richer can mask it a bit because a richer mixture may be easier to light.

In reply to Pete. (l33t FS) :

Plugs and wires are almost new and perfect, as are the cap and rotor (all OE stuff). I used to have a Pertronix in it, but it crapped out and I just went back to points. Coil is a Bosch red that's newish.

For as minimal as the remaining stumble is, I'm totally willing to live with it. Most people wouldn't even notice it, at least people who have driven cars with carbs.

Drove about 60 miles yesterday, and the worst of it seems resolved, but as noted there is still an occasional mild interruption in power. Going through everything again, the only additional thing I've found is that the idle screws are only out about 7/8ths of a turn, which would seem to suggest that smaller idle jets might be an avenue to explore. Oddly, most charts and calculators suggest 55F8s, but that seems counter-intuitive to what I'm seeing. I don't know that I have any additional jets to test here, but getting some 45F8s, and maybe some 55F8s, will be my next step.

Not sure how you adjust idle mixture in those, but with my 38 I had that problem, and it sort of solved itself when I realized the instructions I saw had different starting points for the idle screw.

Using the smaller value (1/2 turn in, as it turns out) and adjusting my idle mixture as normal yielded perfect results.  Before that I changed a couple jet sizes trying to find that light throttle stumble.

I had similar stumble problems again recently when I lost the tab that holds the choke plate to the electric choke, so it might be you're getting a bit too much fuel at part throttle.  With a new choke plate assembly and proper adjustment, perfect running is back.

Might be something in transition from idle to main, even if your jetting is "correct".

02Pilot - Sent you a PM

In reply to ГУЛАГ мальчик УР следующий :

There's no choke on DCOEs, so it's definitely not that. I tuned the idle by adjusting for highest idle speed, which seems to be ~7/8ths of a turn (just simple screws like most carbs). If it's too much fuel, which it could be, it defies all the basic Weber tuning guides, by which I'm already on the lean side of what's recommended. I agree the transition is the area of interest; unfortunately, there are about four different things that can affect it.

In reply to so_slow:

Replied - thanks.

In reply to 02Pilot :

Right, no choke on DCOEs, just a data point that even with the "right jetting" the adjustment aspects seem a bit finicky on Webers.  

The fine tuning of them seems a bit mysterious even when everything is "correct".

If you can get through to Bud at Weber Redline he is worth a call. 

Genuine genius with the things. His intuition is always right. I have put his suggest jets in a dozen cars and rarely needed to change them during the dyno runs. He can tell you if the idle jet is off just from reading the list of jets to him.

And DCOE's DO in fact use a choke. It is the lever off the back of the carb. People just delete them and run their cars too rich to compensate.