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Weber Carb Tuning Problems

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Weber Carb Tuning Problems
  This topic is about my 1972 MG MGB
steve123 Steve P
Conestogo, ON, Canada   CAN
1972 MG MGB
Hello Gents, your help with this Weber carb problem would be greatly appreciated. I have a ’73 B with a Weber 45DCOE. I’ve installed a holley fuel pressure regulator and set it at 2.5 psi per weber spec. Everything else is bone stock, except the rust which comes free in Canada.

The original problem was a very rough idle; the engine wouldn’t run at less than about 1500 RPM. Also, the “idle” was unresponsive; if you gave it gas and the quickly let off the gas, the engine would rev up, but not rev back down again as you would expect. I believe that this was a result of the speed screw being set at almost 2 full turns in, rather than the recommended 1/4 turn to 1/2 turn. The incorrect setting on the speed screw was resulting in the throttle plates opening up to much and exposing additional progression/transition holes in the throat of the carb. It was drowning in gasoline.

So as of yesterday, based on what I found on this site, I have:
: reset timing to 15 degrees
: adjusted float clearance to 14 mm
: reset idle speed screw to 1/2 turn in
: ensured that that throttle shaft is not over tight or binding
: verified that floats are black plastic-like material, not brass
: closed airbleed screws
: verified that the jets under the wing nut cap are “55F8" idle jets (2 jets) and “F2" main jets (2 jets).

Results are:
: with the idle speed screw between 1/4 and 1/2 turn in and the engine warm, the engine runs smoothly and responds quickly to the gas pedal, as long as the choke is set at about half open.
: I was able to get it to idle smoothly at just under 1000 RPM for the first time.
: adjusting the mixture screws all the way from fully closed to FIVE TURNS OUT has very little effect on rpm or smoothness.
: I can’t speak to mixture screw adjustments affecting engine POWER, since it’s winter here still and I can’t actually test drive the car.
: at these settings, even with the engine warm, if I close the choke, the engine loses rpm immediately and dies.
: the only way to keep the engine running without the choke partly open is to turn the idle speed screw in 1 or more turns. When I do this, the old symptoms return: unsteady idle, high rpm idle (between 1500 and 2000 rpm) and very slow response to gas pedal.

Any suggestions? If I need choke for a smooth idle, doesn't that mean I’m too lean? Does that mean re-jetting? How do you interpret the lack of response to mixture screw adjustment?

Thanks for your help.
Steve in Waterloo Ontario

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Wachtmans Avatar
Wachtmans Wouter Strodijk
OVERVEEN, Noord Holland, Netherlands   NLD
1974 MG MGB "The Bee"
1974 MG MGB "The Bee"
1974 MG MGB MkIII "The Bee"
Get rid of the Weber. Problems all over the place: for normal road use SU are the way to go.

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ken472 Avatar
ken472 Ken Petersen
Luverne, MN, USA   USA
SU a better chioce but I suspect the power valve is bad or the wrong one for your application

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lewk Avatar
lewk Silver Member Keith Lewis
Cambridge, ON, Canada   CAN
Many people on here use the Weber 45 DCOE and will chime in with helpful tips. Don't be discouraged. BTW the mixture screw out 5 turns is way too much, should be only about 21/2
Look here:

Or here:

Edited 3 time(s). Last edit at 2019-03-14 01:46 PM by lewk.

DCOE TUNING (2).pdf    384.4 KB

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ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
I have almost zero experience with a DCOE so take this with a grain of salt.

Maybe someone with expertise will chime in shortly, you seem to understand something about Webers based on your initial post.

The good news about Weber carbs; they are infinitely adjustable. The bad news it seems to take a good bit of expertise to know what needs to be adjusted or changed. float height, chokes, emulsion tubes, idle jets, main-jets, air correction jets, ...

Did you buy the carb specifically setup for a B? If so, I would go back to the supplier and see if you can get some technical support from them.

The cheap fuel pressure regulators are not that good at maintaining steady pressure. A pump that outputs in the proper pressure range might be a better solution.

Are you SURE your ignition is in perfect condition and set exactly correct? (The oft quoted saying 99% of carb problems are in the ignition)

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Norsk62 Avatar
Norsk62 Lyle Anderson
Renton, WA, USA   USA
1974 MG MGB "Braken"

You will probably get some backlash from the purist for choosing to run the Weber, and especially the DCOE. So be prepared. However I am willing to assist.

The problem you are describing is not necessarily related to the Weber. It also has all the symptoms of a vacuum leak.

If you are applying the choke while the engine is warm and the carb starts to behave normally. Doing so, lessons the air flow through the carb and richens the fuel flow.

You might want to check if you have a vacuum leak at the manifold or at the carb base plate. Then check the O-rings on the base of the carb. I have seen dry rotted cork gaskets on some Weber's in the past so check that as well.

Also, the tuning sheet from Red-Line Weber states the setting are basic settings to be used as a starting point in tuning the carb and you will need to tune it for your car's engine. The carb will react differently in a stock engine as compared to one that has been slightly modified or in race trim. I am running and Weber Outlaw 38 on my 74 B with a fast street set up, at 9.75:1 compression, and had to tune it for how my engine is set up. I chose this over an DCOE because it is more drivable from off idle to 4500 rpm and the car still pulls hard in 4th gear and over drive. If my car was set for the track, I would have gone with the DCOE or HS6 carbs.

In tuning the carb you need to take in consideration the overall condition of the engine. Compression, Cam Shaft, Ignition timing and timing curve, pistons, Intake manifold, header vs stock exhaust, valve train, even altitude, will impact how the card function at idle and through the power band.

Have fun and try not to get frustrated while tuning.

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Notelluc Avatar
Notelluc Kevin Culleton
so, if i read you recent update correctly you said running and throttle response good on a partial choke? and dies when choke is disengaged?

considering nothing else but that, i would say to rich on the mixture.

Every trip is a gift.

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Yes No Thank Avatar Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
No such thing as an F2 main jet, that would be the emulsion tube. The main jet is hex shaped and at the bottom end of the tube. The air corrector is round and at the top of the tube under the retainer screw cap. What size chokes? Vacuum advance on the dizzy? Also 45DCOE chokes do not affect air flow AFAIK. Just enrichment. So, what jets, what pump nozzles? What model 45DCOE?

Member Services:
Dave Headley, dba FAB-TEK offers full service race car parts and preparation for MGB & MGA race cars, SCCA and Vintage. Dave is a mechanical engineer and has raced MGBs since 1963.
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tvrgeek Avatar
tvrgeek Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
I might suggest looking at the "Skeletons" thread as the basics for DCOE and DGV are the same.

Be sure your ignition is 100% and then some. Trying to dial in a carb without perfect ignition is a fools errand.

If you search, the factory tuning manual is online somewhere.

List your idle, main, AC jets, and E-tubes as well as your venturi size compression, and cam. Those running them need to know all the baselines.

OK, SU's are easier as MG already spent the time and effort to dial them in, but the Weber is a fine bit of hardware, just a bit big for a stock MG.

Cogito ergo sum periculoso

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MT-B Avatar
MT-B Matt T
City, NY, USA   USA
The basics maybe the same in some very general sense but they have very different operational principles.
For instance when Lyle who ownes a DGV said that the choke restricts the airflow through the carb and can be used to diagnose a lean condition, in his case that is correct, his carb uses a choke plate to restrict airflow and thus riches the mixture. However a DCOE uses a totally different enrichment system (sometimes casually called a choke) which only adds fuel without reducing the airflow. The DCOE does use replaceable calibrated chokes to create the low pressure venturi effect which draws the main fuel charge up from the well, but these have nothing to do with the starting or enrichment circuit and not understanding how they differ can lead to a lot of confusion.

In reply to # 3908716 by tvrgeek I might suggest looking at the "Skeletons" thread as the basics for DCOE and DGV are the same.

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RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Like Noah's Ark, DCOE carburetors require two of everything. I ran one for a decade and it took me two years of trial and error before I got it running satisfactorily. Also, the tuning components aren't cheap. If you really want to use one it can be done, but it will cost you a lot of time and money to get right. RAY

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