> OK, that seems reasonable. If doing an engine rebuild, would
> it be best to
> use the the same camshaft, an early camshaft, or something completely
> different? As an aside, my '73 B with it's tired engine pulls
> way better. I
> assumed it to be the combination of camshaft & dual SU's.
The cam and tappets on a B engine are a weak point. Choice of cam
is always going to be a tradeoff. More performance will entail higher wear.
The stock grind is a balance between performance and wear, and is very
usable. Most folks building engines go to a little more aggresive cam
because it is expected, and makes the car feel more sporty at higher revs.
>
> I'm not having any particular engine problems, but have a pretty leaky
> engine, so all the seals/gaskets need to be redone. And if
> it's that far
> apart, it seems foolish not to refresh the whole motor.
mmm. I could argue this point. I'd suggest fixing the major leaks,
and driving the car. This from someone who is pining away because he
doesn't have a drivable B.
>
> The advance on my '80 B is plumbed to vacuum connector at the
> base of the
> Weber DGV. Is this correct, or should it be connected to the manifold
> proper? Perhaps a tee fitting where the vacuum line runs from
> the manifold
> to the brake servo?
The correct vacuum port for that distributor should have full
manifold vacuum. I believe the port you are referring to, is off the
butterfly and will not give the correct supply.
>
> I thought the 25D was used on earlier cars, with later models
> getting the
> 45D, 45DE, or 45DM.
25D, 45D etc. are designations of distributor design. The 25D was
supplied on the early MGB. The design was superceded by the updated 45D
design and replacement parts for the 25D discontinued. This is the same way
the DM2 as used on the MGA was discontinued and replaced by the 25D.
The 45 series distributors have larger diameter caps to reduce
arcing, the points designs have self wiping points and a better lift cam
profile. All in all the 45D distributor is superior to the 25D.
Arcing in the cap, either between electrodes or of the rotor is a
major problem with high energy electronic ignition conversions of the 25D,
particularly in racing applications. The larger 45D cap reduces these
problems.
Would replacing the 45DM with a 25D or 45D be a sensible
> thing to do? Or
> does it only make sense when changed in conjunction with a suitable
> camshaft?
My recommendation is to keep to a stock specification. The 63-67
federal specification of 8.8:1 compression, twin carbs, cam and distributor
settings was optimized for performance and economy. Anything after that was
juggled around to keep the EPA happy.
Once you go from a stock specification you are on your own. If you
don't have access to a dyno, chances are any modification you make will have
some drawback.
That said. (disclaimer) Most of the fun with these cars is playing with
them. There are lots of things that can be done to make them work better,
you just have to know what you are doing and approach any modification
systematically.
> Now this is an area that I have no knowledge of. I installed
> the Weber and
> set it for "lean best idle" per the instructions. I know
> there is further
> tuning that could be done vis a vis re-jetting (or is it
> re-needling???) but
> this is beyond my limited knowledge at this time. What's the
> difference
> between an accelerator pump type carb and an SU type carb
When you stomp on the go pedal, nothing happens for a moment. An
accelerator pump type carb squirts fuel into the intake manifold, and
hopefully some of it makes the engine go faster generating vacuum to draw
fuel from the main jet. This is kind of a hit or miss situation with raw
fuel going all over the place. Many times the engine will "bog" under full
throttle till things get sorted out.
The Weber DGV and many typical V8 carbs have only the pump circuit,
the main circuit and an idle circuit. This makes tuning simple, but can
lead to mixture problems at off idle. The nice thing is that they work ok
out of the box, but they can be improved through careful selection of jets
and the pump return circuit orifice.
A constant depression carburetor like the SU or Zenith has a
blocking piston that only opens as far as the engine vacuum will allow it,
so is smoother and more efficient under transition. The engine will not see
the fully open butterfly until it can handle the increased volume of
mixture.
Weber DCOE and other expensive carbies have a number of tunable
progression circuits that are sequentially exposed. Each one has to be
tuned correctly to be effective, making this carb akin to driving a Formula
1 car on the street. Once set up correctly for the engine and load a DCOE
is an incredible work of art. The problem is most people do not have the
time/knowledge/tools/tuning parts to properly adjust them. They are also
not very effective for multi cylinder filling. (read MGB/Midget siamese port
engines)
oops, ran on at the keyboard again.
hope this helps
Kelvin.
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