Weber Carb Questions

T

Todds Coupe

Member
Messages
19
Reaction score
7
Location
Denver, CO
I have 32/36 Webers on my 71 2800 CS and am having difficulty getting them to adjust. I rebuilt them a while back and the car runs OK, however the Oxygen sensor numbers are very high (18 to 20) and I cannot get them down. The fuel mixture adjustment screw doesn't seem to do anything. All gaskets (carb to manifold and intake manifold) are new and there are no air leaks. Any ideas would be helpful. Also, there are a couple hoses/lines that I would like some direction on. I have the vacuum hose on the distributor connected to the underside of the air filter intake on the Crank case vent. And there is another small one, coming from the base of the second carb, that I have connected to the other port on the underside of the air filter housing. With this one capped or plugged, the engine runs smoother though? I have also heard that the shafts on these carbs can start leaking? Yes? Thanks everyone!
 
You can check for leaks by using carb cleaner with the spray straw to spray around the base and the shaft of the carbs, and around your intake manifolds. If the engine runs faster then you have found the leak.

Basics of making sure the carb linkage is the same, measuring with an air flow meter.

Once your linkage is adjusted (if needed) you can adjust for best idle and other adjustments.

Some sources for adjustment:

D3 shooter on YouTube has some really good videos on Weber carb operation and adjustments.

Top end performance has a lot of Weber adjustment info on their website.

I think your distributor vacuum line is not hooked up properly, and (from my reading of your post) it is likely that you have a vacuum leak.

vacuum advance needs ported vacuum, not manifold vacuum. You should look for a vacuum source above the throttle plates. You could use one carb or “t” off, but use the same source.

Plug or cap the manifold source.

I’m sure that others will have good or better advice.

Let us know how it goes.
 
Todds Coupe said:
I have 32/36 Webers on my 71 2800 CS and am having difficulty getting them to adjust. I rebuilt them a while back and the car runs OK, however the Oxygen sensor numbers are very high (18 to 20) and I cannot get them down. The fuel mixture adjustment screw doesn't seem to do anything. All gaskets (carb to manifold and intake manifold) are new and there are no air leaks. Any ideas would be helpful. Also, there are a couple hoses/lines that I would like some direction on. I have the vacuum hose on the distributor connected to the underside of the air filter intake on the Crank case vent. And there is another small one, coming from the base of the second carb, that I have connected to the other port on the underside of the air filter housing. With this one capped or plugged, the engine runs smoother though? I have also heard that the shafts on these carbs can start leaking? Yes? Thanks everyone!
Click to expand...
Your description of the problem could be more specific. A car that runs “OK,” seems somewhat inconsistent with lean A/F numbers. Is the problem more pronounced at idle, part throttle, cruising, during acceleration? Does the engine experience any misfiring? When? Any vacuum gauge readings? Condition of the ignition system? Is it stock? Timing properly adjusted? How about some images of the spark plugs and their firing tips?

Was there a specific symptom you were attempting to address when you “rebuilt” the carburetors? How long after the carburetor rebuilds did you start experiencing run-lean symptoms? How extensive were the rebuilds, e.g., gasket replacements or complete disassembly and replacement of all internals? What is the jetting situation in both carburetors, i.e., what jet sizes are installed, including mains, airs and idles?

Throttle body wear at throttle shafts is observable by carefully examining throttle and linkage operation. Unless your carburetors have experienced years of daily use, such wear is not impossible, but it is unlikely.

https://e9coupe.com/forum/threads/low-rough-idle-question.48335/





iu


iu


iu



iu


iu
 
High A/F ratio combined with no change when moving the mixture screw is a vacuum leak. Boonies has hit on the likely source of your problem. The connection from the base of your second carb to the air filter housing is creating a huge vacuum leak. Remove this hose and cap the connection on the carb. Do this, drive the car and record AFRs. And as nashvillecat notes, it is very helpful when evaluating carbs to collect AFR data at multiple times - idle, part throttle, cruise and full throttle. I collect data for each of these conditions regularly.

Do this and report back.
 
Todds Coupe said:
I have the vacuum hose on the distributor connected to the underside of the air filter intake on the Crank case vent. And there is another small one, coming from the base of the second carb, that I have connected to the other port on the underside of the air filter housing.
Click to expand...
I agree with Chris that the hose from the base of the carb to the air filter housing is causing a vacuum leak. Remove the hose and plug the port on the carb.

The hose from the vacuum advance to the air cleaner housing results in no distributor vacuum advance at any condition. I would disagree with Boonies and hook this hose up to manifold vacuum...not ported vacuum. Either will provide the same vacuum condition off idle, but ported vacuum will not proved vacuum at idle. Ported vacuum was used in the early years of emission control to generate more combustion heat at idle to improve idle emissions. Idle speed and quality will improve using full manifold vacuum for distributor advance. You will likely have to decrease the idle speed after switching from ported to manifold vacuum.
 
Last edited:
As to manifold vacuum, the brake boosters in our cars use manifold vacuum to enable braking assist. You can tap into the same connection that is used to run your brake booster to provide vacuum to your distributor. That's what I have done in my car (althought my distributor is not currently programmed to use this connection).

Here again, do that stuff after you plug the vacuum leak at the base of carb 2.
 
Ohmess said:
As to manifold vacuum, the brake boosters in our cars use manifold vacuum to enable braking assist. You can tap into the same connection that is used to run your brake booster to provide vacuum to your distributor. That's what I have done in my car (althought my distributor is not currently programmed to use this connection).

Here again, do that stuff after you plug the vacuum leak at the base of carb 2.
Click to expand...
I am a little confused. Should I connect the line from the distributor to the base of carb 2?
 
Dick Steinkamp said:
I agree with Chris that the hose from the base of the carb to the air filter housing is causing a vacuum leak. Remove the hose and plug the port on the carb.

The hose from the vacuum advance to the air cleaner housing results in no distributor vacuum advance at any condition. I would disagree with Boonies and hook this hose up to manifold vacuum...not ported vacuum. Either will provide the same vacuum condition off idle, but ported vacuum will not proved vacuum at idle. Ported vacuum was used in the early years of emission control to generate more combustion heat at idle to improve idle emissions. Idle speed and quality will improve using full manifold vacuum for distributor advance. You will likely have to decrease the idle speed after switching from ported to manifold vacuum.
Click to expand...
Dick, should I connect the vacuum line from the distributor to the base of carb 2?
 
Well now, this is very familiar. I had my reckoning with my own pair of 32/36 Webers a few years ago. Your comment that the mixture screws do not seem to do anything brought back a memory that finally lead me to a solution. May not be your problem, but should be investigated and eliminated as you have received a lot of very useful instructions above. In my case, I ended up removing the mixture screws and found one of the pair was seriously gummed up. I cleaned and chased that thread and put everything back together, and I was amazed when, after following the standard weber 32/36 initial set up instructions, car ran like new!

I must add that I had been optimizing every other component of the drivability equation i.e. timing, valve adjustment, elimination of vacuum leaks, well fitting ignition wires, new distributer cap, and so on. But that cleaning out of the mixture screw bore....that really made a difference.

Good luck and keep us in the loop!
 
Last edited:
Maybe we need to back up and get some history on your ownership of the car. Also maybe a little background on your mechanical ability and willingness to work on the car. It sounds from this thread, and your prior ones, that the car is new to you and you are discovering a few problems? Do you have experience with vintage BMWs? How about your comfort with tuning, repairing cars in general? I think some more info will help us better answer your questions.
 
Dick Steinkamp said:
Maybe we need to back up and get some history on your ownership of the car. Also maybe a little background on your mechanical ability and willingness to work on the car. It sounds from this thread, and your prior ones, that the car is new to you and you are discovering a few problems? Do you have experience with vintage BMWs? How about your comfort with tuning, repairing cars in general? I think some more info will help us better answer your questions.
Click to expand...
New reply being posted.
 
THANK YOU ALL for the feedback and insight. I will try to be more specific in my description.

I am a shade-tree mechanic, not a trained one by any stretch of the imagination. I am very mechanical and follow directions well. I've had my 2800 CS since 1981, at that time I did a complete motor rebuild and body restoration. Not off-frame but a pretty complete one. I rebuilt my 32\36's about 7 years ago because of hard starting. I took the carbs completely apart and cleaned everything very well. Before the carb rebuild, I had to pump the gas between 7 and 9 times on cold start, then had to ride the accelerator to keep it running (around 1500RPM) until it warmed up. Warm start was immediate. Idle has always been a little rough due to a higher performance cam. After the rebuild, I had to pump the gas only 2 to 4 times, but still need to ride the accelerator until it warms up. Once warm, it idles between 12 and 1400 RPM. My mechanic discovered many air leaks around the base of the carbs and the intake manifold. All of those gaskets have been replaced. There are no more air leaks. There may be a very slight leak around the shafts, but it is negligible and after the engine is warm is pretty much gone. I am in Denver and am not sure what the best jetting for altitude is? I do not know the numbers or sizes of the existing jets but will investigate further. If the air/fuel mixture screw has little or no effect, could it possibly be the jet before (above) the mixture screw? If yes, what size should that jet be? We have used a broad band O2 sensor. The mixture screw, after 2.5 or 3 turns has no effect on the O2 readings which are between 17 and 19. The mixture screw tips are clean. Air flow meter was at 5 (both barrels) for the rear carb and 4 for the front (on his meter). We couldn’t get any change or fluctuation on these values.

Now regarding the vacuum lines. I suspect that my mechanic MAY not have remembered or replaced the lines where they were originally. He is not an old coupe expert, but he is an expert mechanic. Should the vacuum line coming from the distributor be attached to the base of carb 2? Right now it is attached to the bottom of the air filter on the crankcase vent port, which, from previous feedback, I am told, supplies no vacuum. The line from the #2 carb is capped off. The vacuum port at the base of carb 1 is connected to the brake system. I have electronic points.

All feedback and questions are welcome!
 
There are 2 vac ports on each carb. The port below the level of the throttle plate is “manifold” vacuum which is highest at idle. This supplies the brake booster. So if that is running from #1 carb to the booster, cap the same port on #2 carb. “Ported” vacuum comes from above the throttle plate and is zero at idle and increases on acceleration. In a stock configuration, this supplies the distributor so connect that to the “ported” port on one carb and cap the other “ported” port on the other carb.

Make sure the bases of each carb are tightened down (4 nuts each) and inspect all vacuum lines for integrity. The booster can also leak so as a check you can temporarily remove the booster vacuum hose and cap that carb port to see if that changes your idling quality.

If you have any lingering emissions crap, remove it and cap all potential leak sources.

Ensure that the carbs are synched.

Ensure that your timing is correct.
 
Last edited:
nashvillecat said:
However, additional information
Click to expand...
Great - thank you so much for sharing!
I am deeply in love with the late Gruppe 5 / IMSA bodykit - the cow-catcher airdam is quite borderline but the sweep back wing is so much more attractive than the "usual" upright CSL wing...
It is a pitty that there are so many rare and expensive parts in/on the care but the details do not align completely; a Group 2 engine is great for itself but in a Group 5 bodykit? The dry sump system seems to be a modern one, probably it is also not built upon the original Group 2 pump? And I now definitely understand why a buddy told me not to use the current interlinks etc anodized in red or blue on a vintage car (they should meanwhile also be available in black for that purpose).
But nevertheless, a very impressive car! I may not sleep well today :)
 
I removed and cleaned the idle jets and the air/fuel mixture screws. The jets are "50's", I am in Denver, is this the correct one for altitude? I have connected the distributor vacuum line to the manifold port on the #2 carb. Is this the recommended position?
 
Todds Coupe said:
I removed and cleaned the idle jets and the air/fuel mixture screws. The jets are "50's", I am in Denver, is this the correct one for altitude? I have connected the distributor vacuum line to the manifold port on the #2 carb. Is this the recommended position?
Click to expand...
There are 2 vac ports on each carb. The port below the level of the throttle plate is “manifold” vacuum which is highest at idle. This supplies the brake booster. So if that is running from #1 carb to the booster, cap the same port on #2 carb. “Ported” vacuum comes from above the throttle plate and is zero at idle and increases on acceleration. In a stock configuration, this supplies the distributor so connect that to the “ported” port on one carb and cap the other “ported” port on the other carb.

Make sure the bases of each carb are tightened down (4 nuts each) and inspect all vacuum lines for integrity. The booster can also leak so as a check you can temporarily remove the booster vacuum hose and cap that carb port to see if that changes your idling quality.

If you have any lingering emissions crap, remove it and cap all potential leak sources.

Ensure that the carbs are synched.

Ensure that your timing is correct.
 
Todds Coupe said:
I removed and cleaned the idle jets and the air/fuel mixture screws. The jets are "50's", I am in Denver, is this the correct one for altitude? I have connected the distributor vacuum line to the manifold port on the #2 carb. Is this the recommended position?
Click to expand...
https://e9coupe.com/forum/threads/altitude-affect-on-webers.16047/

https://www.opelgt.com/threads/refining-high-altitude-tuning-and-jets-for-a-weber-32-36.59410/
https://www.yotashop.com/jet-kit-weber-32-36-carburetor-high-altitude-jet-kit-701-dgv4-a/
 
You must log in or register to reply here.
Back
Top