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 - DCOE hesitates on acceleration

The MG from Hell seems to be doing well for its second year in a row now. The only real problem is a hesitation when accelerating hard. The car has a Weber sidedraft, and other than this fault, it starts, idles and runs great. Is there a way to either increase of modify the "shot" from the acceletaor pump on these carbs?

Pete
Pete

Pete
I'm no expert with DCOE's but if there's an external linkage you may be able to get an earlier shot even if you can't increase the volume.
Dave
Dave Wellings

In a word: Yes.

========

1) - look at the "Pump Jets" (also sometimes called: "Accelerator Pump Jets").

On DCOE's, they work as a high-speed power
device and inject a stream of raw fuel directly into
the throttle bores during sudden jumps into
warp speeds.

They're located undeneath the brass screw plugs
(with rubber "O" rings") on top of each throttle bore.

Be sure that the rubber "O" rings that are installed underneath each brass plug are in absolute 100%
condition. When you stomp on the gas pedal, this
area becomes momentarily pressurized with fuel.
Any leak here will goof with the metering of fuel
being sent to the Pump Jets and can cause a high
speed stumble or hesitation. Look for any signs of
dried fuel or "varnish" in this area. Sometimes,
this leak is a very, very tiny stream, and on a warm
engine it can often evaporate soon before you can
notice it unless you are looking at it very intently
in strong daylight (use googles for this!!!).

Also, be sure that the tiny aluminum washers
underneath each pump jet are actually there
(many times these are lost and not noticed)
and are not worn, split, or squashed flat.

For stock or hot-street MGB's = 50

For very hot street MGB's = 55

For more radical, track MGB's = 60

=========

2) - look for the "Fuel Chamber Inlet Valve with
Exhaust Orifice". This is also sometimes called
the: "Accelerator Pump Inlet Valve".

This item (one per carb) is located at the bottom
of the main fuel chamber. It controls the amount
of fuel that is available to the Pump Jets.

The "Exhaust Orifice" is a tiny bleed hole that is
drilled on the side of this valve - and it is this that
does the "regulating" work. When you stomp your
foot on the gas pedal, a tiny ball valve inside the Inlet Valve closes and some of the fuel is pushed
directly to the Pump Jets - while simultaneously -
some of the fuel is also allowed to bleed back into
the fuel chamber through this bleed hole.

Generally, on all MGB's (stock, street, track) = 50

You can increase the volume of fuel available to
the Pump Jets by gradually swapping to smaller
bleed hole numbers (ie: 40, 30, etc.).

A lesser used item is the "Closed" type of Fuel
Chamber Inlet Valve. This one has no small hole
drilled on it's side, and is unmarked (no size I.D.
stamping). This fella makes all of the fuel in the
fuel pump well available to the Pump Jets for a maximum volume of fuel shot. Of course, when
you use this one...your car may seem happy...but
you could also be throwing money away in wasted
fuel each and every time you goose the gas pedal.

======

3) - Lastly, you can also change the total gross
volume of the shot of fuel going to the Pump Jets
by experienting with various lengths of Pump
Rod(s) and Pump Spring(s).

The Pump Rod regulates the amount of fuel in the
pump well. The Pump Spring governs the force
applied to the Accelerator Pump Piston - which is
responsible for pushing the fuel through all the
passageways in the accelerator circuit.

"If It Ain't Broke...Don't Fix It".

In 99.999% of cases, the "stock" Pump Rod and
Pump Spring that was installed in the carb at the
Weber factory is more than adequate to work in
our case - just be sure that all of it is working (no
binding, no worn or broken parts, etc.).
Daniel Wong

Daniel:

WOW, thanks a lot. That should keep me busy for a while. I do know that it is going lean for a while because if I stopmp it and keep it down, it wants to rattle until the rpm goes up and the torque load comes down a bit. Plugs look good when I pull them.

The carb is in good shape with no apparent leaks. I've been told that webers are never that good on transition.I have a hard time believeng that since they are so adjustable.

Pete
Pete

"Low speed gasp" is well known Weber problem , all you can do is ensure your acclerator pump is working well . This is where the SUs score as a street carb.
S Best

Besides tweeking with the Pump Jets and
Fuel Chamber Inlet Valve...

You might try hooking up a vacuum gauge to
your intake manifold while road testing.

Whenever there's gonna be a DCOE put on an
MGB for the road, I tend to favor a smaller size
choke as well as a cam with more lift and only
a conservative amount of added valve overlap.
This helps keep the manifold vacuum up and
results in a more "crisp" throttle response.

45DCOE's have rather large throttle plates and
when you aggressively stomp on the gas there's
often a big, stall-inducing drop in manifold vacuum.
Sometimes, this can = 0 Hg, and it takes a healthy
shot of fuel from the Pump Jets in order to force the
engine to recover from this (on DCOE's, this shot
of fuel is not reliant on manifold vacuum).

On my own car, whenever I stomp on the gas,
the vacuum needle will bounce to 0 (zero) for
only a millisecond, and then quickly rise to
4 - 8 Hg as I transition to Warp Speed.
After that, I can back off the throttle a bit
and hold the needle at around 15 Hg
while leveling off at a steady cruise.

Sometimes you can minimize the part-throttle
stall by advancing the ignition timing a bit. This
will also help you gain a few Inches of Hg in manifold vacuum and lessen the part-throttle stall.
Of course, there are limits to this, as this can
also induce pinging and knocking, etc. if the
ignition timing is advanced too far.
Daniel Wong

This thread was discussed between 19/06/2004 and 20/06/2004

MG MGB Technical index

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