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MG MGB Technical - Weber 38/38 Idle/part throttle problems

'K...readers of this board know I've been REALLY struggling to get this car right. A 78 B with a Weber 38/38 carb...desmogged/Eurospec dizzy/wires/ etc etc all new...new valve job/good compression and only 39K on the motor. I now have throttle performance...but cannot get a consistent idle and the bugger STILL has a flat spot from overrun to part throttle. It's a bitch. I cannot seem to tune it out....ignition all works OK/everything is new and I've been through it all again and again....even redid intake manifold gaskets and carb to manifold gaskets looking for vacuum leaks...some inprovement but still cannot eliminate the hesitation. Soooooooooo????? Jets in the carb? Anyone.....I am at my wits end having rebuilt myt own motors for over 25 years I cannot sort this little sod from my own country....anyone out there had similar problems with weber set ups????????????
Thanks
Pete
P J KELLY

Well if you live in the NYC area you can come to my house and we'll test your mixtures with an a/f meter that I have. I don't know if it'll cure your problem but at least it'll be a start knowing what your mixture is.

A flat spot can be caused by the accelerator pump. I had that problem w/ a holley carb. Did you try spraying some carb cleaner around the gaskets to see if you have a vacuum leak? I also have a vacuum gauge.
Mike MaGee

Thanks...I live in Westchester. I also have a AF meter but even though I can get pretty low CO I still cannot tune the flat spot out. I have eliminated 1 vac leak I had at the carb base...made it run better but still cannot get it to idle OK what ever I do.....Carb is brand new and Accel. pump works properly...carb cleaner around the gaskets makes no difference at this point.
P J KELLY

Not an expert here, but both cases seem to be air velocity problems. Maybe the idle vacum is insuficient to keep good air velocity over the idle jet? On overrun to partial throttle, you go from almost no air velocity to a need for fast flow over the main jet. If I recall, the 38/38 weber opens both barrels simultaneously. On drag cars this is overcome by increasing the amount of fuel "shot" by the accelerator pump. There is often an adjustment to be set (much like valve lash) that determines how soon the throttle linkage acts upon the pump.

What manifold are you using? I have never liked the design of the downdraft conversion manifold; it dumps into a (largish) plenum and forces the air into a 90 degree turn. Until it is good and warm, I believe it separates fuel from the mixture. It also forces us to use a rather small air filter. As far as I know, you are pioneering the 38/38 conversion. You may have stumbled upon the reason it is not sold comercially.

Pete
Pete

P. J. What no.# is stamped on the acc. valve (nozzle)?
victor gardino

Is the accelerator pump adjustable? A quick google search indicates that there are different acelerator jets available. Stupid me gave a Weber manual away many years ago and now I have a Weber too but no other answers for you.
Mike MaGee

One more thing - a rich idle mixture can cause a poor idle that hunts.
Mike MaGee

The Weber 38/38 has 2 idle mixture screws just to make things harder...but still I cannot get even close to a consistent idle...all out performance ain't bad...it's just the idle and transition to part throttle that sucks...it constantly hunts?????
P J KELLY

Thanks for your replies...there is no # on the ACC. pump housing, but both ports do squirt fuel over the throttle plates as soon as the throttle is opened even slightly. Dunno what to do....maybe pioneer that new Moss Fuel Injection kit????
P J KELLY

Hi Pete, I use a DCOE on my B, but maybe I can help - you'll have to catch me on any comparisons I make that aren't appropriate for the downdraft carbs.

First, I think you are dealing with two unrelated problems. Idle hunting is probably separate from the bog off idle, you should sort one before tackling the other. I'd start with the idle problem, since I'd probably be able to fix it without buying/installing new parts. If the bog is still there after, I would come back to it.

Does the 38/38 open both throttle plates at the same time, or is there a primary/secondary? I'm guessing the former, since you say there are two idle screws.

For the idle:

Disconnect and plug any vacuum advance ports (at the carb).

Make sure that the throttle plate(s) are tight against the stops. If you have a balance meter and your carb opens both plates simulataneously, you should check the balance and adjust via the air bleed screws (if there is any), with one screw closed and the other just open enough.

Count the number of turns to set the idle mixture screws, then reopen them both the same amount. Turning both the same amount, adjust them to get the fastest idle (start at around 900-1000rpm). Readjust the idle speed if necessary too keep it less than 1100 and off the primary and progression circuits, and make sure the throttle plates are still tight against the stops afterward.

Once sorted, tweak the idle screws individually to try to further improve the idle.

BTW, it's possible that the idle screws are so far off you need to open the throttle to uncover the secondary ports (I'm assuming the 38/38 has separate idle (a misnomer BTW) and primary jetting) to run. If so, and the idle jets are mis-sized, it will hunt and idle poorly. Not sure how to tell other than looking to make sure the progression holes are covered by the throttle plate when they are in the idle position.

If the car still doesn't idle well, try leaving it running for 5 or 10 minutes. Pull all the plugs and check them, looking for carbon or whitish deposits, to figure out whether you're rich or lean, or perhaps have a unbalanced flow.

For the bog, it's helpful if you have a vacuum meter. Hook it up if so, otherwise go by ear. Very slowly, crack open the throttle. You should hear or see (on the meter) the stumble with the throttle as it starts to open before you hear the progression onto the primary fuel circuit. Now you have a choice - fiddle with idle jets or adjust the floats to change when the primaries come in. This is experience, so I'd say just try and see. Adjusting the floats brings the primary fuel circuit in earlier or later (higher is earlier) which can fix any transition problems from the idle circuit.

The trick with the floats is to record where you start, increase or descrease the float height to see what happens, and make sure you don't go too high or low. Too high allows fuel to drip into the manifold, not a good idea - you'll see it and hear the engine load up, esp after a closed throttle deceleration.

To decide what to do with the idle jets, open the throttle till you get a minimum reading on the vacuum gauge or the worst sounding engine. Then open (or close) the idle mixture screws. If the idle gets better when you open them, you need richer idle jets (higher number), and vice versa.

Are there any other symptoms? Popping on decelleration, smoke, etc?

HTH, or at least gets you further. Good luck!
Mike Polan

Great responses I really appreciate the help and your opinions
Pete
P J KELLY

This thread was discussed between 02/01/2004 and 06/01/2004

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