MGB Comparing the 45DCOE and the SU HS4
Many years ago, when my SUs were worn out, in a search for more power
for my ‘B I decided to try fitting a 45DCOE weber.
I’ve just recently completed a minor rebuild of my SUs (oversize
throttle spindles, new jets and floats) and have refitted them in place
of the Weber. This gave me the opportunity to compare the SUs and Weber
back to back, and I wanted to capture my thoughts before they faded into
the past.
The car is a 1970 MGB roadster, well worn, street use only. Driven daily to work unless
there’s snow and/or fresh salt on the road. 1800cc engine, 030 overbore,
standard grind cam, Crane electronic ignition. Hardened valve seats. The
head and seats were mildly cleaned up by me (not ported). 185×70 radial
tires on 14" aftermarket wheels. No overdrive. Stock manifold and
exhaust. I’ve tried an Ansa exhaust and header, made the B sound like a
truck with no noticeable performance improvement.
The recent increase in fuel costs motivated the switch, and is the
primary difference between the SUs and Weber. With the Weber I was
getting 19 to 21 MPG (imperial - 4.54L/gal), depending on the time of
year and my driving (highway vs the short drive to work). This is
typically 190 miles on a full tank, followed by an approx 42-44 liter
fill. With the SUs I get 22 to 26 MPG in the same conditions.
Weber 45DOCE 152
- main venturi 33mm
- auxiliary venturi 4.5
- main jets 135
- emulsion tube F15
- air correction 160
- idle 65F9
- pump 55
- Float 9mm
- Imitation K&N filter
- Modified air horns (shortened to fit inside filter)
- Longer canon intake manifold
- Old style (mushroom) PCV valve plumbed into the intake manifold
With the exception of the idle jets, air horns, PCV valve and float height, this was the
out of the box settings.
Driving
With the vacuum advance tube connected to the weber manifold (the
distributor VA can is the type designed for the SU carb take off), the idle
is a little lumpy - probably too much advance at idle. Without the tube the idle is rock steady, set to
around 900rpm.
There’s a definite difference in the intake sound, it’s more noticeable
and more aggressive vs the tinnier sound from the SU cans.
The vacuum advance seems to yield about 5% better fuel consumption, but
this is a very subtle effect for most of my driving which is not long
distance. Part throttle acceleration is definitely better. Without the
advance, more static advance is required for the same effect, but this
resulted in pinging and/or run-on. With the vacuum advance and the
milder static advance setting (10º BTDC) the car never runs on.
Throttle response is very crisp, the RPMs return almost instantly to
900rpm with my foot off the accelerator and there’s a very good engine
braking effect.
Power delivery is smooth and strong from 1800rpm up to 6000rpm, with no
signs of drop off.
There is a slight stumble when accelerating from idle with part
throttle, and a definite bog with a WOT acceleration from the lower
RPMs. Part throttle yields the best acceleration until the RPMs increase
to 3500 or so (this keeps the carb on the progression jets until the
intake air is moving fast enough to pick up fuel from the primaries).
Torque seems to be higher than with the SUs, it’s easier to
"chirp" the tires when starting off, and there’s certainly
more of a kick from 70-80mph when I open the throttle. There’s also a
nice, low, throaty roar from the intake when I do this. No problem
getting above 100MPH.
Starting cold is easy, one or two pumps of the accelerator and the car
starts first crank after sitting overnight, even in winter. The
enrichment circuit is difficult to use, and unless just right the car
bucks, hesitates and sneezes until it warms up completely. There is no
facility for a higher idle speed during warmup. It is a racing carb
after all…
Starting from hot, after a short time, can sometimes be a little
difficult. If I forget it’s hot and pump the accelerator, it will take a
few cranks to clean out the raw fuel that’s been dumped into the
cylinders. As soon as the car catches, cracking open the throttle will
immediately clear it out and all is well. While it struggles (compared
to a cold start), the car has never failed to start even when hot. After
a longer heat soak (say 1 hour), there’s no problem at all with
starting.
Setup
It’s a lot of work. I also found that the book I had wasn’t entirely
correct in it’s theory and set-up, and most of the ‘net help I found
applied to the DCOE-9. Out of the box, the 50F9 idle jet was totally
inadequate. The car would surge and sneeze at part throttle and low
RPMs, esp when cold, and pop and spit on deceleration at all times.
This was finally traced to two problems (after spending a lot of time
trying to chase a non-existent vacuum and exhaust leak). The first was
that the float height of 12.5mm stock does not work for the B. The cross
over from progression to primary jets resulted in a wide rpm lean
condition. I verified this by hooking up a vacuum gauge, and slowly
opening the throttles with the idle screw. The gauge will drop at
certain RPMs and the engine would stumble. Turning the idle mixture
screws can be used to figure out if it was lean or rich (it was lean).
I tried various idle jets to attempt to remove the stumble, the richest
being 65F9. The easiest way was to use the same vacuum gauge technique.
The 65F9s worked the best, but I needed to reduce the float height to
9mm, which removed the lean stumble and the deceleration pop at the
same time. However I was never comfortable that this lower float height
ensured raw fuel wasn’t dripping into the intake manifold (see hot
starting). I know that 6mm was definitely too low (though it worked even
better, eliminating the popping even with the 50F9 idle jets). For best
running, the idle mixture screws were set to the rich side, rather than
the recommended "lean is best" setting. It’s possible 65F8s
would be even better, but I don’t have them (they’re cheap, but shipping
and duty adds up…). It’s also possible a smaller secondary venturi
would work better, but they’re more expensive.
I believe the earlier (non-spanish made?) DCOEs had different
characteristics. For example, the 1.5 max turns for the idle mixture on
the DCOE-9’s is too little for the DCOE-152s, which need something more
like 3 turns (I believe the taper on the mixture screws was steeper on
the 9s).
I also experimented with a crossover pipe in the manifold into which I
plumbed the PCV valve. This seemed to smooth out the idle somewhat
further, but that effect was subtle and very subjective.
It was important to soft mount the carb, if I had it too tight it would
pop on underrun, and I suspect run lean on WOT and high speed cruising.
I found the thackery washers and soft washers inadequate (both
disintegrated after a short time). Instead I used short lengths of
heater hose, which worked well and lasted for many, many years.
BTW, I tried different main venturi (36mm) with of course different
mains and air correction jets. I tried F16 emulsion tubes, running with
and without air horns, different air cleaners. None of these had any
discernable effect on the progression to main cross over mixture. The
36s did run well at higher RPMs, but 33mm seems closer to the needs of a
stock engine, giving higher air speed and torque at lower RPMs (IIRC -
it’s been a while).
The linkage that came with the weber was hopeless. Good ones are
expensive so I made my own by using some brass nuts drilled to hold a
return spring between the manifold cable mount and the cable attachment
on the carb. It works fine, but I never did like the looks of it.
Vacuum advance was taken from the weber ports used to check the balance between the throats.
This is the screw in plugs just downstream of the throttle plates on the top.
I used brass barbed tube fittings tapped to fit the ports, joined with a plastic Y just above the carb and sent to the distributor.
The stock throttle cable can be used, adjusting the pedal travel with the bolt beside the pedal box.
I drilled and tapped the manifold boss that takes the end of the cable sleeve so that I could lock it in place with a small screw.
The choke cable doesn’t reach the Weber enrichment lever, which is on the front side of the carb.
I used a universal replacement that fits just fine and doesn’t look too out of place on the dash.
Conclusion
It was more expensive than the SU rebuild esp after factoring in the
additional jets and other bits. A good challenge and lots of fun to
fiddle with - a great learning experience. I know more now than I ever
thought I would - too bad carbs are obsolete :).
SU HS4
Driving
Very smooth acceleration from any speed, including right from idle.
No bucking or surge when negotiating speed bumps or stop and go traffic
in any gear.
Slower return to idle, very little initial engine braking (relative to the
weber). The car will run-on if I turn off the ignition before waiting a
few seconds for the idle to return to normal.
Quieter, tinny, hollow sound from the intake. Not bothersome, but not as
nice as the weber. Makes the MG engine very recognizable, I can always
tell the sound of one running on SUs from quite far away :).
Not as much torque as the weber in the 2000-3000 rpm range, it’s harder
to chirp the tires. Runs out of breath above 5500 RPM, there seems to be
little point holding the upshift much past 5000. Doesn’t have the same
kick going WOT from 70-80MPH, and I suspect it would struggle to get
over 100MPH (relative to the weber). Doesn’t have the same pull through
a corner as the weber.
Starting is easy. A little choke in this fall weather gives a slight
enrichment and just the right throttle, the engine usually starts on the
first or second puff of the first crank after sitting overnight. When
restarting later in the day to return from work no choke is needed, it
starts right up with just a touch of throttle. No issues at all with a hot start.
Anecdotal, and the memory is starting to fade already, but I’m pretty sure from a standing start
I couldn’t get my foot all the way down with the weber before it was time to upshift.
With the SUs I have more time at WOT in first. It’s possible this is because I can engage the clutch
and open the throttle at a lower RPM with the SUs (see earlier about the Weber bogging at low RPMs, WOT),
but the weber did feel faster off the mark.
Setup
Easy, even the linkage is a joy. Completely stock, including the air filters. Poppet valves still in
place on the throttle plates. Emissions equipment removed, except for
the PCV tubes (metered bleed into carb throats). Vacuum advance (carb take-off) in
place. Balanced with a unisys, tuned against idle speed and using the
lifting pins, with a final check for good and matching plug colours.
Holds the tune very well (so far at least).
I’ve got a carbon monoxide detector installed in my garage. Oddly
enough, when fiddling with the DCOE idle, it seldom went off. When
tuning the SUs or leaving the car to idle even for a short time, the
detector almost always goes off (the garage door is open of course).
Conclusion
The weber is more fun than the SUs. With more torque and
better engine braking it’s easier to "steer with the throttle"
around corners, and it gets better acceleration at speed. The
SUs make the ‘B a milder, easier and more economical car. If fuel prices every
return to previous levels (ha!) I’ll reconsider using the weber. For now
I’ll stick to the SUs, at least until they need another rebuild.
