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Technical Information (MGB 19)

Carburetor Choices

Last Modified - 03/05/01

Alternate carburetor choices for the MGB are very limited. I have taken the opportunity to discuss several popular carburetors. Please reference end of article for abbreviations used.*

WEBER 32/36 DGV (all MGB's 1963-1980)
While a very reliable carb/manifold set up, a disadvantage with this carburetor is the fact that the primary venturi accounts for two thirds of the throttle open position (no CFM* figures are given for this position) with the secondary venturi becoming active in the final one third of throttle movement. This carburetor requires excessive initial ignition timing advance to avoid stumble on take off (if the ignition timing was retarded, we would have to open the primary throttle plate to obtain the necessary idle; this would expose the progression bleed holes which in turn would require that the idle mixture be leaned to compensate for this richer mixture condition. The end result being very little progressive richening upon further throttle opening, especially at snap throttle and so the stumble). Carburetor can be modified to allow the primary & secondary throttle plates to open simultaneously plus modifications to the progression circuit and accelerator pump. Unfortunately, the company that used to provide this service no longer offers this modification. Maybe a search on the web will turn up a company that can help you.

WEBER 45DCOE (MGB 1963-74 1/2)
The period carburetor for your car, suitable for large bore applications, use with long or short style inlet manifolds. There are several manufacturers and different styles of manifolds on the market such as Cannon 801 (5 1/2" long), Warneford (6" long) & TWM, to name a few. I prefer the Cannon 801 casting. This casting has provisions for fitment of a balance tube pipe, which should be, utilized for distributor vacuum advance purposes on engines using 10:1 GCR or less. It is important that the combined inlet manifold, carburetor air cleaner assembly should not exceed 13 3/4" in depth, this is the maximum allowable for inner fender clearance. If difficulty is encountered during fitment, examine the condition of the engine and transmission mounts replace if necessary, be sure that the L/H spacer is fitted between the engine mount and frame bracket, however, it may be necessary to install a thicker spacer plate. Recommended initial jetting: main 150, air 165, emulsion tube F7, slow run 50F8, pump 65, venturi 36mm.

Because of inadequate clearance, i.e. brake servo interference, a short style inlet manifold (approx. 3 3/4") will have to be used, along with a restricted size air filter assembly. Requires early exhaust manifold or exhaust header assembly.

MIKUNI 44PHH (All MGB's 1963-1980)
Although not your period carburetor, this remains our favorite choice. With better progression circuitry and generally an all round improvement to the Weber 45DCOE. Kit P/N Z44-640 contains short style 3 3/4" inlet manifold (flange thickness 15/32") with provisions for balance tube. Recommended initial jetting: main 150, air 200, emulsion tube (jet block assembly) OA, slow run 57.5, pump 40, venturi 34mm. 1975 on require early exhaust manifold or exhaust header assembly.

Unfortunately, Mikuni America has confirmed that they no longer import carburetors into the USA, however, you may come across a good used carburetor/manifold assembly, in which case, I highly recommend you purchase it.

Due to the varying thickness' of inlet manifolds, i.e. manufacturer to manufacturer, it may be necessary to fit step washers to secure exhaust header and intake manifold. As with the intake manifold, exhaust headers may also vary in thickness from manufacturer to manufacturer.

Carburetor manufacturers normally quote CFM* figures at WOT* and can be quoted in inches HG* or inches H2o*. To convert HG* to inches of H2o*, multiply by 13.6. To convert inches of H2o* to HG*, multiply by .0735515. The presumption is that these CFM* figures at WOT* is all the information we need, therefore, PTO* position information is not given.

255 CFM*
400 CFM* (32mm choke), 420 CFM* (34mm choke), 440 CFM* (36mm choke)
309 CFM* @ 12" HG*, 393 CFM* @ 18" HG*, 422 CFM* @ 20" HG* (all 30mm choke)

*Remember that we must divide the CFM* flow rate figures by 2, for each venturi, in practicality, supplies only two cylinders.

Theoretical Air Consumption (TAC) at maximum BHP The requirements for this TAC* depend upon max BHP* at an indicated RPM*. When using camshafts of 214 deg. (OEM) to 225 deg. duration at .050" lifter rise (we consider these camshaft duration figures to be the maximum for street and street performance), this max. BHP* will occur around approximately 5500 RPM*.

The formula for this is as follows:
Engine Displacement (cu/in) x RPM

MGB 1800cc = 32CFM per 1000 RPM or 175CFM at 5500 RPM
MGB 1924cc = 34CFM per 1000 RPM or 187CFM at 5500 RPM

Actual Air Consumption
Items that directly effect this are:
1 - Intake & exhaust valve timing events
2 - Flow characteristics such as cyl/head combustion chamber design, intake port design, intake manifold, carburetor and air cleaner combined CFM* flow rate
3 - Intake manifold vacuum (HG*)
4 - Barometric pressure and temperature
5 - Exhaust restrictions, such as cly/head exhaust port design; exhaust components including exhaust header, pipe diameter and exhaust system.

The K&N air filter will flow 6.03 CFM* at 1.5" water pressure drop (.11" HG*) of air per of effective filter area; this realistic figure is what K&N uses for their calculations. The formula to determine the minimum size filter for a given application is:

A = CID x RPM A = effective filtering area
           20,839 CID = cubic inch displacement
  RPM = RPM of engine @ max. BHP

At maximum brake horsepower (5500 RPM):

Engine Size 1800cc A = 110 x 5500 = 29
                                            20839       = 29 x 6.03 = 175 CFM

Engine Size 1924cc A = 117.5 x 5500 = 31
                                            20839          = 31 x 6.03 = 187 CFM

K&N have a catalog that contains very useful information (highly recommended), and can be obtained by sending $3.00 to: K&N Engineering Inc., PO Box 1329, Riverside, Ca. 92502.


All the following K&N and ITG air filter assemblies provide more than enough CFM airflow, therefore these figures are not quoted..

Weber 32/36 DGV (all MGB's 1963-1980)
K&N P/N 56-1070 5.5" x 9" x 1.75" with replacement element P/N E3441 5.25" x 8.87" x 1.75". Unfortunately no air horn fitment is possible due to the short height of the filter assembly. Fitment of an air horn would be a definite improvement to the air flow characteristics. .

ITG P/N 3-JC-20 back plate with element P/N JC20-25. Here again the air filter height prevents the fitment of an air horn. Just for the record, ITG P/N 3001-4050 air horn is available (height 3 1/2") which obviously would require modifications to the hood, which would not be practical. Adaptation to the crankcase ventilation system is also required.

Weber Moss P/N 222-365 - supplied to Moss Motors by Pierce Manifolds, contains K&N element P/N 3321, no air horn fitment due to filter height, however provisions are made for crankcase ventilation adaptation.

Weber 45DCOE (MGB 1963-1974 1/2) Cannon 801 or Warneford Manifold
K&N P/N 56-1310 (4.5" x 7" x 1.75") with P/N E3321 element (4.5" x 7" x 1.75") or P/N 56-1330 (5.5" x 9" x 1.75") with P/N E3441 element (5.25" x 8.87" x 1.75"). Use shorty air horn TWM P/N 2719-4512 (12mm).

ITG P/N 1-JC-20 back plate with element P/N JC20-40, air horns available to fit are: Weber P/N 52840.032 (32mm exponential), TWM P/N 2719-4520 (20mm exponential), TWM P/N 2719-4512 (12mm shorty). Full radius air horns will not fit inside filter housings, due to their larger outer diameter.

Weber 45DCOE (MGB 1975 on)
Unfortunately, due to brake servo interference, we are restricted to what we can use as regards air filters. At the time of writing, we have only been able to fit Ramflo Series 400-foam filter available under Moss P/N 222-245. Obviously, air horns cannot be used.

Mikuni 44PHH (1963-1974 1/2) Short 3 3/4" Manifold
K&N P/N 56-1350 (4.5" x 7" x 3.25") with P/N E3341 element (4.5" x 7" x 3.25"). Use air horn TWM P/N 2719-4512 (12mm shorty).or K&N P/N 56-1360 (5.5" x 9" x 3.75") with P/N E3461 (5.25" x 8.87" x 3.25"). Use air horn TWM P/N 2719-4512 (12mm shorty) or TWM P/N 2900-4835 (full radius).

ITG P/N 1-JC-20 back plate with element P/N JC20/40, air horn available to fit is: TWM P/N 2719-4512 (12mm shorty)
or ITG P/N 1-JC-20 back plate with element P/N JC30/65. Use air horns TWM P/N 2900-4835 (full radius) or TWM P/N 2900-4850 (full radius).

Same conditions apply as to Weber 45DCOE (1975 on) paragraph, i.e. Ramflo Series 400 air cleaner, Mikuni P/N Z70/175.


In the past, we have generally stated that the HS4 and HIF4 type carburetors were inadequate for MGB large bore applications, including British Automotive's own 1924cc (83mm) conversion. However, experience gained through many hours of testing with different engine component combinations, such as, camshafts, cylinder heads, air cleaners, exhaust headers and exhaust systems etc. etc. has lead us to the opposite conclusion. So, if you are thinking about investing in a pair of expensive HS6 carburetors, my advice, unless you are going racing, is to save your money.

SU Carburetor Flow Rates
The following CFM* flow rates were extracted from "Tuning BL's A Series Engine" by David Vizard. Although this book deals with the "A Series" engine, much of the information within its contents can be applied to the "B Series" engine; this book is highly recommended and well worth the investment.

All carburetors tested at 1.5" (HG) equivalent to 20.4" (H2o). Readings are per carburetor:

Carb CFM Flow Rate
HS4 142
*HIF4 No data given
HS6 210
HIF6 240

* By virtue of their design differences (HIF versus HS), the assumption is that the HIF4 carburetor would have a higher CFM* flow rate over that of the HS4 carburetor. Comparisons between the HIF6 and HS6 carburetor CFM* flow rates would lead us to believe this. Apart from the increased CFM* flow rate, another advantage is the integral float bowl design which reduces fuel surge within the chamber.

1 - The bi-metal metering jet adjustment fixture inside the float bowl also acts as a temperature compensating device which, supposedly dependent upon temperature, richens or leans out the fuel mixture the appropriate amount. Unfortunately, high under hood temperatures, especially at idle and also after the vehicle has been driven and parked a short while, before being driven again, we have a very difficult time obtaining a stabilized engine RPM.
2 - The spring loaded bias metering needle, common on this type of carburetor, actually makes contact with the jet tube assembly and over a period of time wears the jet tube to a point where it will have to be replaced along with the metering needle.

Starting in 1969 through 1971 (AUD326, AUD405 and AUD465), the HS4 also incorporated a bias metering needle into its design. The jet tube and metering needle are subject to wear in the same manner as the previously mentioned HIF4 carburetor. These carburetors also have an improved positive crankcase ventilation design (see Tech Article MGB9 "MGB Engine Oil Use"). 1968 HS4 carbs (AUD265) had a "boss" incorporated in the housing design; these carbs can be drilled to accept a 5/16" O.D. x 1/4"I.D. x 1 1/4" long thin wall tube (epoxy into position). This is an highly recommended modification as outlined also in Tech Article MGB9.

Fixed Metering Needle v. Biased Metering Needle
Fixed metering needles were used on the HS4 carburetor through 1968 and, when centered within the metering needle jet tube, will have a long life. On the other hand, bias metering needles (HS4 1969-1971, HIF4 1972-1974 1/2) and metering needle jet tubes will wear and will have to be replaced every several years or even sooner.

To change bias metering needles to fixed needles we could substitute the carburetor piston with the early type, making sure that the piston slides freely in the piston dashpot. We are now able to supply you with an adapter for this change over. The part number is NEED/ADPTR

Further increases in CFM* flow rates for HS4 and HIF4 carburetors can be made by following the modifications outlined in the previously mentioned publication.

Using the HS4 carburetor CFM* flow rate as our example:

TAC* @ 5500 RPM ( 1800cc ) = 175 CFM divided by 2 = 87.5 CFM

@ 6000 RPM ( " ) = 191 CFM " " " = 95.5 CFM

@ 6500 RPM ( " ) = 207 CFM " " " = 103.5 CFM

@ 5500 RPM ( 1924cc ) = 187 CFM divided by 2 = 93.5 CFM

@ 6000 RPM ( " ) = 204 CFM " " " = 102.0 CFM

@ 6500 RPM ( " ) = 221 CFM " " " = 110.5 CFM

What we have done above is divide the TAC* by 2 to show the TAC* for one carburetor (flow dynamics aside).

HS4 or HIF4 carburetors fitted with a good aftermarket intake manifold such as Maniflo P/N SUB4-2 which was developed to give a good 5HP increase, complemented with air horns and air filters, would have more than enough adequate CFM* flow rate through the intake tract (providing we have not screwed up our cyl/head work and, what's more, we should have a good high velocity density of charge).

SUMMARY If we can provide adequate CFM* through these carburetors, when combined with the intake manifold and air cleaner assembly(s), to satisfy engine demands, then we are better off fitting these sized carburetors. Remember, by sucking air through the smaller diameter HS4/HIF4 carbs, as opposed to the larger diameter HS6/HIF6 carbs, the engine has to suck harder and potentially the velocity (FT/SEC*) of the mixture charge should be higher.


1963-1974 1/2
P/N E2400 element (2) will fit directly into the existing OEM snorkel type air cleaner canisters, unfortunately, this type of set up is inefficient, however, it is better than retaining the OEM filter elements.

K&N P/N E2400 can also be found as part of Moss P/N 222-910. This kit utilizes an aluminum black and silver MG medallion retaining plate and requires that the original OEM adapter plates be fitted. Air horns (2) Moss P/N 222-970 (which are superior) cannot be used with this set up. However, K&N stub stacks, Moss P/N 222-910, can be fitted inside the air filter element assembly. This would require the 4-spacer tubes to be shortened to the appropriate length. At this time, I do not know whether or not this substitution would result in a CFM* flow rate improvement.

K&N European P/N 56-9095, Moss P/N 222-950 (2). These individual filters contain element P/N E3190 5.87" x 4.75" x 3.25" and will allow the fitment of air horn (2) Moss P/N 222-970 (TWM P/N 2700-4050). This filter set up is K&N European, and not to be confused with K&N USA P/N 56-1390 which contains element E3180 5.87" x 4.5" x 1.75" and would require the fitment of the original OEM adapter plates or K&N stub stacks, as outlined in the above paragraph; P/N 56-1390 is a 2-bolt mounting style assembly and, even with the substitution of P/N E3180 with E3190, the fitment of air horn (2) Moss P/N 222-970 is impossible.


ITG P/N JC40 blank backplate (has to be drilled for adaptation) with element P/N JC40/65 and air horns (2) Moss P/N 222-970 (TWM P/N 2700-4050). This set up allows for unrestricted breathing, as the filter design incorporates no front filter plate (i.e. superior). Although there are no CFM* flow rate figures available for this set up, the manufacturer states this air filter assembly will supply enough air to satisfy an engine making at least 200 BHP.

1975 Onwards
P/N 56-9320(USA), 855-5039 (Europe), Moss P/N 222-985, with K&N stub stacks P/N SS51 (KNSS51) Moss P/N 374-580. Caution: Contact could occur between the rear air filter assembly and the brake servo unit. It is important that the engine mount be in good condition and/or replaced if necessary. Be sure that the two engine mount spacer plates are installed.

With the fitment of the above air filter assemblies, the following metering needle profiles are recommended:

HS4 - Use #6 (Fixed) P/N AUD1005, Moss P/N 374-170
------ Or #7 " P/N AUD1006, no Moss P/N
HS4/HIF4 - Use AAU (biased) P/N CUD1018, Moss P/N 374-380

Use damper spring P/N AUC1167 (yellow), Moss P/N 021-081. We have found this spring to be better than the recommended P/N AUC 4387 (red) Moss P/N 370-520 and also recommend these springs regardless of which other air filter assemblies you have fitted.


Street & Street Performance
Trying to strike a balance between maximum BHP output, acceptable gas mileage, and excessive exhaust emissions, can be very difficult. Numerous publications have indicated a variety of recommended metering needle profiles such as KP (AUD1541, Moss P/N 374-025), SY (AUD1338, Moss P/N 374-045) or TE (AUD1347, Moss P/N 374-035), all fixed, BAP (CUD1113, no Moss P/N, BBW (CUD1144, no Moss P/N), or BDR (NZX8016, no Moss P/N), all biased. Based upon our limited dyno work we believe SL (AUD1327, Moss P/N 374-1990), SM (AUD1328, Moss P/N 374-200) or SW (AUD1337, Moss P/N 374-225) all fixed, to be better choices. No engine dyno work, with biased metering needles type carburetors, was carried out. However, in both instances, it may be a question of metering needle availability.


K&N (European) P/N 56-9098, Moss P/N 222-960(2). These individual filters contain K&N element P/N E3190 5.87" x 4.75" x 3.25" and will allow the fitment of full radius air horns (2) Moss P/N 222-975 (TWM P/N 2700-4550). K&N stub stacks, Moss P/N 374-590, can also be used with this application.


K&N (USA) P/N 56-1400 contains K&N element P/N E3180 5.87" x 4.50" x 1.75" use with K&N stub stacks, Moss P/N 374-590 or retain the OEM adapters. Alternative K&N elements are E3190, 5.87" x 4.50" x 3.25" and E3210, 5.87" x 4.50" x 3.50", with K&N stub stacks, Moss P/N 374-590 or retain the OEM adapters. Full radius air horns (2) Moss P/N 222-975 (TWM P/N 2700-4550) cannot be used with this application. *

Note: To accomplish the fitment of stub stacks or OEM adapters, we need to purchase Moss P/N 328-315 spacer tube, and cut to the appropriate length (K&N element P/N's E3180, E3190 or E3210).

ITG P/N 14JC20 blank backplate with element P/N JC40/100 and air horns (2) Moss P/N 222-975 (TWM P/N 2700-4550). This set up allows for unrestricted breathing, as the filter design incorporates no front filter plate (i.e.. superior). Although there are no CFM* flow rate figures available for this set up, the manufacturer states this air filter assembly will supply enough air to satisfy an engine making at least 200 BHP.


If I have overstated my enthusiasm for retaining the HS4/HIF4 carburetors on your MGB engine, it is because a "good behaving" set of these carburetors, when rebuilt properly, is very hard to beat.

With the exception of broken or damaged carburetor bodies and/or distorted throttle plate bores, HS4/HIF4 carburetors can be successfully rebuilt and should include new components as follows:

Throttle shafts/bushes (standard or oversize)
Needle & seats (Grose Jet recommended)
Late style plastic floats (level set to specifications)
Metering needles
Damper springs
Jet tubes
All gaskets, seals and O-rings

The above should eliminate any "bad behavior" such as misfires, hesitations, surging etc. etc. (when carburetor mixture is set correctly), however, if these symptoms still exist, the blame will lie elsewhere, such as, engine tune-up work required, ignition timing not set correctly, air leaks, low manifold HG* etc. etc.

Low Manifold HG* (engine idle)
Basically, there are four (4) reasons for this problem:

  1. retarded ignition timing
  2. lean and overly rich carburetor mixtures
  3. intake manifold/carburetor air leaks
  4. high duration camshaft

Items 1 through 3 can be easily rectified; unfortunately, #4 is a different matter altogether:

Camshaft Duration
The installation of higher duration camshafts (OEM 214 deg. duration) will result in a greater intake/exhaust valve (open) overlap period with a reduction in engine idle HG*; the higher the camshaft duration the greater the problem, resulting in stumble and backfire through the carburetors upon acceleration, especially when accelerating from low engine RPM*, and also lean mixture surging at cruise engine RPM*. These two problems can usually be rectified by running excessively rich idle mixtures which, unfortunately, could result in poor idle quality and low engine HG*. From our experience, we consider a camshaft of 225-deg. duration to be the maximum for street/street performance. Usually, we opt for camshafts in the 220-222 deg. duration range and carburetor changes, such as, substituting damper springs with P/N AUC1167 (yellow), Moss P/M 021-081 and installing richer metering needles (see previous text).

SU Sensitivities
Remember HS4/HIF4 SU carburetors, especially HIF4, are sensitive to ambient and coolant temperatures and really require some sort of heat exchanger or preheat device to assist in drivability problems during the engine "warm-up" period. This holds true for high under hood temperatures where air should be directed from an outside source to provide a denser air charge.


A good addition, that will result in an increase of approximately 5HP, is the fitment of an improved CFM* flow intake manifold (Maniflow) available under P/N SUB4-2. This intake manifold is designed with a crossover balance tube for good idle characteristics, however, there are no provisions for the fitment of a PCV valve (MGB 1965-68), or a gulp valve assembly (MGB 1968 onwards). However, it is my recommendation that the PCV valve system should be changed to the carburetor control system, as found on 1969-1974 1/2 MGB's (requires the fitment of these carburetors). One could also use and modify 1968 carburetors (see "SU Carburetors (HS4/HIF4)").

The crossover balance tube should be drilled and tapped for the inclusion of a fitting for distributor vacuum advance purposes on engines with 10:1 GCR* or less.

UPDATE 04/21/98

With the fitment of part # SUB4-2 it will be necessary to install #6 metering needles part # AUD1005 Moss part # 374-170 on both HS4 and HIF4 carbs fitted with biased metering needles. We have found that the recommended metering needle AAU part # CUD1018 Moss part # 374-380 to be to lean. We can provide you with the necessary adapter to convert to fixed needle.

Further richer fixed needles are available. #7 and CS1. With high compression engines and modified cylinder heads this may be necessary.

We found that the AAU metering needle, when installed along with the K&N air filter assemblies, to be the correct needle profile for this application, it wasn't until we installed the SUB4-2 intake manifold that we started to run into lean mixture driveabilty problems, I guess a testimonial to the more efficient SUB4-2 intake manifold.

P/N SUB6-2 is also available for HS6/HIF6 carburetors.


CFM: Cubic feet per minute
WOT: Wide open throttle
HG: Inches of mercury
H2o: Inches of water
PTO: Partial throttle open
TAC: Theoretical air consumption
BHP: Brake horse power
RPM: Revolutions per minute
ft/sec: Feet per second
GCR: Geometric compression ratio


Configurations for the various air horns discussed in this text are shown below: