MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - weber dcoe inlet manifold problems

We are using a weber 45 dcoe type 152 on a tuned 1900cc MGB engine. (Car is tuned and set up as rallye car.

The strange thing is that when using a long straight inlet manifold basically cylinder 1 and 4 run lean. (The ignition leads on 1 and 4 can be removed with almost no effect on idle).

For a change I used a short swan neck manifold (normally used on rubber bumper cars which lack space due to master brake cylinder) and then cylinders 2 and 3 run lean.

Has anyone had similar problems and how were they resolved?

Engine specs:
1902 cc B series engine
1:10 compression
fully ported head on large valves
718 sp kent cam
Lucas 43D4 ignition with pretronix ignitor II system
Marius

My car runs the same (1995cc/45DCOE/long manifold) and I am assured by people much, much more knowledgable than me that it's the expected result of the head/carb/cam combination and is normal. I was advised to check the plugs after a high speed run (without idling when you stop) to make sure the (true) running mix is OK, unless you have access to a rolling road and can do the job more thoroughly.
Steve Postins

Steve,

Thanks for the answer. Did you ever try to use a balance tube on the inlet manifold? (similar to what is used on the SU inlet manig=fold.

I am also interested to know what type of settings you are using for the weber.

We currently use:
38 venturi
180 main jet
f-16 emulsion tube
160 air jet
4.5 aux venturi
50 pump
aux pump closed

Thanks,

Marius
Marius

I thought about the balance tube but ruled it out as the car idles reasonably smoothly so it wouldn't be of much benefit, but would change the jetting, meaning more money at a rolling road! I have added a small diameter tube for vacuum take-off to help the fuel consumption, but I imagine it is too small to give any balance effect.

My set up is:

Choke 36
Emulsion tube F16
Main jet 160
Air jet 160
Pump 50
Idle 60F9
Aux venturi 4.5
Needle valve 225

So I'm running smaller chokes and jets. In view of this, the following may be of interest: "DCOE mixture is odd in the extreme - but generally, at high rpm you can achieve the same mixture with larger venturi and larger jets as you can with smaller venturi and jets. The difference lies in the mixture at lower RPM with smaller venturi and jets giving richer mixture."

Hope that helps,

Steve
Steve Postins

Your idle is not affected when pulling the plug
wires on #1 and #4? I would suspect that something is not 100% with your plug wire set.
This is just a guess. You might try swapping
in some other wires or inspect the engine
compartment, with the engine idling, during
a very dark night. No lights. Look for any faint,
tiny blue sparks dancing off of the wires or
distributor, etc.

My own engine wears a Warneford (curved branch)
manifold with a fabricated balance tube. The spark
plug colors on #2 and #3 tend to be very slightly
lighter than #1 and #4 (almost unnoticable).
I attribute this mostly to the fact that the #2 and #3
cylinders tend to run slightly hotter than the other
cylinders because of their siamesed exhaust
ports, and the fact that #1 and #4 get slightly better
coolant flow around their jackets. Some racers
put in a slightly colder rated plug in #2 and #3.
Daniel Wong

Steve,

Your settings are almost the same as what is quoted the MG B special tuning manual, only difference being that they quote a main jet of 170 and idle 60f8.

Mine actually is set up with 50f6 which is quite a bit leaner.

In the meantime I switched to the standard SU's and quess what, it runs on all four cylinders. The problem now is that these carbs are too small for the engine and I have a power loss. The SU's have a fairly large balance tube. In the coming weeks I will have a balance tube made for the manifold. I will post the results.


Daniel,

Regarding the ignition lead question. The whole system has been replaced as my initial thoughts were that this is a typical ignition problem. The only thing that still puzzles me is the pertronix ignitor II I am using. (Compared to the ignitor I this one has a varible dwell angle). When the engine is hot the rev counter at idle becomes a bit eratic.

The MGB special tuning manual also refers to the fact that 2 and 3 plugs run hot and these were changed for colder plugs. I am using 4 NGK 8es spark plug which are fairly cold.

Marius
Marius

Hi Marius,

I use the 7ES plugs which have done me well in traffic and on fast runs for a long time now.

I've also run the car with 1.5" SUs. I modified them for greater air flow following the Peter Burgess book, flowed and polished the manifold, and ground some needles very thin to get the mix rich enough at speed. The result was great low, mid and part throttle performance, but top end a tad behind the Weber. It was all extremely civilised, quiet even, but I went back to the DCOE because I missed the gruff, full on character it gives the car. The induction roar and high speed howl always makes me grin.

It was Peter Burgess, along with others, who mentioned the apparent redundancy of a couple cylinders at idle with the Weber and assured me it was OK. It's down to the set up really being intended for 3000rpm upwards and, depending on the head and cam, low rev running is going to be more or less compromised. Added to this, Webers really must to be set under load, hence rolling roads or plug watching. Anything done without the wheels moving is misleading. So if your car has the power you expect on the road, it's probably fine.

Daniel, any chance you could pull a plug lead or two on your car at idle to see what happens? I can remove 1&4 with little or no change in the note, but 2 or 3 kill it. If your car is different I'm jealous! What effect has the balance tube had on running and the mixture? Do you recommend it as a modification?
Steve Postins

Hello Marius,

I lost your e-mail adress and since we didn't compete in Holland this year, it is a long time that we had a chat.

My new engine is in the car since this season.

Yes I do have a balance tube between the manifold runners ( dia = 8 mm)

Idle is very smooth.I haven't done the test you described but since idling is fine, it is not that importent.
The engine has to deliver power under load, not at idle

I believe you that there is at idle a difference comparing with the SU's, mainly because the throat of the SU in 100% in line with the inlet channel in the head.
On the webber the mixture has to make a curve to reach the inlet channel. It is a fact that the atomizing of fuel is not that sofisticated on lower rpm's . Due to that is is possible that the larger/heaver fuel droplets have problems to "take the curve" in the manifold, so that the mixture on the outer side (cilinder 1 and 4) get leaned out...

It is dangeous to compare settings. The key to power is to get over the complete rpm range a fuel/air mixture around 0.92 lambda. Of course the head and cam have to be matched...

according to my memory I have the following settings

venturi 38
aux vent 4.5
main jets 210
air correctors 180
Idle 60F8
pump 45

regards,

André
André

Dear André,

I had thought of contacting you but also lost your e-mail adress. It is interesting to hear that your engine does idle properly as mine does not. Could you drop me an e-mail on my personal adress as shown in the heading?

Regarding your setting my guess is that air and main are the reverse of what you indicate.

Kind regards,


Marius
Marius

This thread was discussed between 22/09/2004 and 23/09/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.