Technical Information (MGB 19)
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
WEBER 45DCOE (MGB 1963-74 1/2)
WEBER 45DCOE (MGB 1975 ON)
MIKUNI 44PHH (All MGB's
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.
CFM FLOW RATES
WEBER 32/36 DGV
*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:
Actual Air Consumption
K&N AIR FILTERS
At maximum brake horsepower (5500 RPM):
Engine Size 1800cc A = 110 x 5500 =
Engine Size 1924cc A = 117.5 x 5500 =
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.
AIR FILTER RECOMMENDATIONS
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
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
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)
Mikuni 44PHH (1963-1974 1/2) Short 3
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
MIKUNI 44PHH (MGB 1975 ON)
SU CARBURETORS (HS4/HIF)
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
All carburetors tested at 1.5" (HG) equivalent to 20.4" (H2o). Readings are per carburetor:
* 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.
Fixed Metering Needle v. Biased Metering
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:
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.
AIR FILTER RECOMMENDATIONS (HS4/HIF4)
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.
NOTE: PART 222-950 SUPPLIED BY MOSS MOTORS MAY BE THE K&N USA AIR FILTER ASSEMBLY. BE SURE TO VERIFY WITH YOUR SALESMAN.
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.
With the fitment of the above air filter
assemblies, the following metering needle profiles are recommended:
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.
SU CARBURETORS (HS6/HIF6)
Street & Street Performance
AIR CLEANER RECOMMENDATIONS
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.
NOTE: BE SURE MOSS PART 222-960 IS THE EUROPEAN STYLE. CHECK WITH YOUR SALESMAN
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.
HS4/HIF4 CARBURETOR REBUILD
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:
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)
Items 1 through 3 can be easily rectified; unfortunately, #4 is a different matter altogether:
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.
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
Further richer fixed needles are available.
#7 and CS1. With high compression engines and modified cylinder heads this may
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
Configurations for the various air horns discussed in this text are shown below: