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Weber (MG World article 1999.)

{short description of image} Initial Purchase, Fitting & Set-Up
Difference's and Workings

See Many Weber Powered vehicles on our project pages.
Weber FAQ Page
The following is a recent contribution to MG World magazine, with regards to the Weber 500

MG WORLD Contribution
Roger Williams. "Prospective Titles"


Background and Plan

Funny how some things are meant to be. I had had my Holley 4150 (390 cfm List 8007) carburettor for years and it had given excellent service atop my Rover powered MGB. Nevertheless I felt it time to try a Weber, the only practical alternative that offers the essential facility of an accelerator pump and corresponding performance. Simultaneously, while researching for a second book (entitled “How to Improve your MGB, MGB V8 & MGC” to be published late summer), I found it difficult to find any objective comparisons between the Holley and the Weber.
They are the two most popular carburettors used by enthusiasts fitting Rover or Buick V8 engines to their MGBs. My research led me to approach Chris Crane at RPI Engineering. He focuses his whole business on the Rover V8 engine and has an excellent reputation. I was also very influenced by RPI’s knowledge of and commitment to a wide range of V8 performance parts, many from the USA. I was particularly impressed to learn that Chris imported about 500 to 600 Edelbrock carburettors alone each year. It transpired that he was also keen to objectively compare each carburettor's performance. Consequently we set up these experiments and I would therefore like to thank Chris for his help, personal interest, the workshop facilities and the excellent personnel support that made the following tests practical.

We planned I would initially drove up to Norwich on my Holley carburettor, carefully doing a fuel comparison check - which would preclude much by way of spirited driving. Clive Athowe Tuning would carry out a “base-line” Rolling-Road test whereupon RPI Engineering would fit an Edelbrock 1404 badged version of the Weber carburettor. After RPI Engineering had fitted and tuned the Edelbrock it would be back to Clive for a comparative rolling-road test while the journey home would be used to carry out the comparative fuel consumption test. At this point I should be armed with the raw statistics, although we expected there would other important if subjective issues to report upon, and this indeed proved the case. Consequently I planned to drive the car (and Edelbrock) for a few hundred miles to enable me to assess its characteristics against the Holley.

So that was the game plan. Let's find out how it worked out but first, by way of additional information that may save confusion at a later date, could I also explain that Weber make a range of 4-barrel carburettors and market them under the names of Weber, Carter and Edelbrock. Apart from the label, it is generally impossible to tell one brand from the other. For the Rover V8 engine range it is important to select a Weber 500 cfm carburettor regardless of the brand name & engine specification. In fact my engine specification is modest. I rebuilt the engine some 35000 miles/9 years ago retaining the original 9.35:1 compression ratio pistons. I fitting new rings, a new standard SD1 camshaft, cam followers etc, but basically sort an engine with plenty of smooth low down torque similar to that fitted to the SD1 V8s. Consequently the inlet manifold selected back in 1990 was a dual port Offenhauser. I fitted a Low Rider 14” x 2” air filter and these parts were to be retained during these comparisons since the concept required we change the very minimum of ancillary items. We wanted to isolate the effect of each carburettor on the engine's performance.

Initial Purchase, Fitting & Set-Up

Obviously the price I paid for my Holley 4150 is now quite irrelevant. I found that there is in fact very little difference between the initial cost of the Holley 4160 & the Weber/Edelbrock 1404 carburettors today. However the Weber comes ready to go on the car whereas the Holley requires you purchase a secondary metering plate, secondary jets & new primary jets for a further £40/50 and to upgrade your purchase to the 4150 specification, which of course makes it the more expensive choice. The Holley was the easier to bolt to its manifold & connect controls to. As I recall I only had to make a small bracket to hold the outer throttle cable. The Weber needed the same type of bracket but also some minor modifications to its accelerator arm. Steve, RPI’s Workshop Manager, had to bend the lower part of the arm by 45 to clear the manifolds and cut a top “leg” off to allow the MGB’s throttle cable to move freely. No big deal. The Holley proved to be the taller of the two carburettors - by 0.25” or 6mm. Also no big deal I hear you say. Well, it could be in the confined under-bonnet space of an MGB. The Weber might just allow you to get the bonnet to close!

The Holley 4160 I had initially purchased proved an absolute nightmare to set-up initially. In fact I still remember I could hardly get the car to run and the bore wash was so great that I had to change the engine oil twice before the engine had run for more than 15 minutes. I ended up trailering the car to Oselli in Oxford and, as described in my first book “How to give your MGB V8 Power”, they really had to make a number of changes to the Holley on top of upgrading it to a Model 4150. While expensive at the time, their work has stood the test of time. I have had 9 years of immense pleasure and reliable service from the resulting carburettor that is justification in itself, but furthermore Clive Atthowe told me after his first Rolling-Road run that it was the most effective and powerful Holley carburettor he had ever seen. The Weber however started first twist, ran smoothly straight away and would have allowed the car to be driven straight away. True the carburettor was set-up with 1441 Metering rods (“needles” to many readers) and 1421 jets - a combination that in RPI’s experience suited my specification 3500cc engine. Certainly the settings were then refined using Crypton equipment to correct the omissions - but if you had fitted the Weber at home the car would been perfectly drivable to your local garage for this diagnostic work to be carried out with the minimum of fuss & cost. To give you an idea as to how small the adjustments on the Crypton were, we started with the Holley’s timing at 2.9 BTDC and advanced that by 3 (5.9 BTDC). The initial CO figure was 9.9%, which of course is high but this was a “straight from the box” figure. Five minutes later we were showing 3.3%. Hydrocarbons were not too bad “from the box” at 600 ppm but Steve quickly had the un-burnt fuel down to 300 ppm. Carbon Dioxide was 11.8% & Oxygen at 0.9%. I did not time Steve, but I doubt it took him 15 minutes to carry out these adjustments. In fact my maintenance deficiencies took longer to correct (Steve’s sharp eyes noticed a split water hose!).

RPI’s experience was such that the jets and metering rods they selected were perfect for the Weber on my car. However, they demonstrated the ease with which they could be changed (10 minutes) if you got something wrong or wanted to use your Weber on a different capacity (say upgrading from a 3500cc to 4600cc) engine. The Edelbrock range of carburettors come with a very helpful 36 page Owners Manual. It covers the operational theory of the carburettors over 9 pages and explains the metering, transient control and effect of external fixtures. Section 2 of the booklet covers the turning of the carburettor, revising it and includes calibration charts, rod/jet reference charts, exploded diagrams and trouble-shooting/solution pages. I don’t recall any supporting information when I bought my Holley 9 years ago although I have since been able to buy a very helpful book ("Holley 4150 & 4160 Carburettor Handbook") from John Woolfe Racing.


Full marks to both carburettors for their cold starting & indeed to the Holley for its hot starting capabilities. The Weber has proved slightly reluctant to fire-up unobtrusively when hot. I resort to putting the pedal to the floor to get her going - - - which is effective but she starts with a major boy-racer style roar! The Weber idle is, however the superior. The engine ticks over at 550 rpm smoothly & quietly. This is more of an advantage than it may sound since the lower idle significantly increased the time it took to trigger my electric fans in traffic. The Holley idled at 950 rpm in spite of numerous unsuccessfully attempts to slow it down.

Tolerance to slow speed pick-up from say 1000 rpm (I am so lazy at times!) is marginally better with the Holley - which was effortless & fuss free. I feel the Weber is saying to me "you should not really do this, but I will do my best"! At first the Holley also seemed better at mid range rpm where its "pick-up" had been faultless. The Weber initially had a split second hesitation before the power came in. The characteristic was particularly marked after a spell on trailing-throttle, say when easing-off on a motorway or running up to a roundabout. I felt the hesitation and subsequent power-punch uncomfortable and asked RPI if they could smooth the take up of power from a closed throttle. In fact Steve managed to improve the performance of the Weber still further and reduced the hesitation somewhat but, we agreed, the problem was due to a stretched timing chain and worn timing gears in my engine. The extra aggressiveness and punch of the Weber had highlighted a problem that I had been vaguely aware of for some time. So I will have to live with the occasional hesitation until I can replace the timing gears with, probably, the upgraded duplex chain and hardened gear set that RPI sell in preference to the standard Rover single chain and plastic gear. Consequently I am delighted with the additional responsiveness and enjoyment the Weber gives and consequently asked Chris Crane if we could "do a deal" enabling me to retain it!


No question, Weber also wins this comparison easily. Its fuel consumption seems to be 15% to 20% better than the Holley based on just the basic statistics I gathered. However there are some extenuating circumstances which probably reduce the differential to about 10%. The Holley had done some 35000 miles and may have been in need of some maintenance. I had recently fitted Peter Burgess “Econotune” cylinder heads & noticed that there seemed to be more carbon in the heads and pistons that I removed than I would have expected. On reflection this may have been due to a long-standing float-level/cut-off valve problem. This possibility was endorsed by a problem that in fact came to a head just at the wrong moment. Don’t they always, I hear you say. Well, not always, but certainly I wasn’t amused when the Holley’s secondary jets started to flood fuel into the cylinders quite uncontrollably on the Norwich ring-road shortly after the first Rolling-Road test run! Maybe the high level of fuel flow during the test run had lodged a spec of dirt under the float cut-off seat or maybe it had been weeping for some time. I do suspect the latter. A second extenuating circumstance definitely effected the Holley’s fuel consumption test run. The test runs were similar in that they were both a mixture of motorway, fast “A” roads, slow “A” roads & slow urban driving. Obviously one has no control over traffic conditions but the Holley’s 203 mile run was marred by two traffic jams that necessitated stop/start crawling for maybe a total of 10 miles. The Weber run necessitated only about 1 mile of start/stop driving. So possibly the difference isn’t quite so great as the following statistics suggest:-

Holley 390 Weber 500
Distance Covered 325 kilometres (203 miles) 328 kilometres (205 miles)
Elapsed Time 3h 15min 3h 10mins
Fuel Used (95 RON,Unleaded) 35.9 litres (7.9 Imperial gals) 29.9 litres (6.6 Imperial gals)
Fuel Consumption 9.06 kpl (25.7 mpg) 10.98 kpl 31.0 mpg
Average Speed 100Kph (62.5 mpg) 104 Kph (64.7 mpg)


From the cold statistics provided by the accompanying charts, the performance of the two carburettors would appear remarkably similar. The Weber carburettor did show a very small advantage from 4500 rpm as the Graphs 1 and 2 show - perhaps 4 bhp on power & maybe 6lb ft on torque. However bearing in mind how infrequently one uses 4500 plus rpm anyway and that the torque and power curves both drop away quite predictably once one gets above 5000 rpm, then the difference is of no practical consequence. Certainly one cannot choose between the carburettors on the basis of the graphed performance below 4500 rpm. Consequently an interesting and originally unplanned experiment was introduced by RPI, who suggested we evaluate one of their imported Mallory twin points distributors on the rolling road. The benefit of the Mallory’s different characteristics, particularly its earlier advance curve, are clearly shown in Graphs 3 and 4. There were a few inconsequential bhp to be had in the mid-range as shown in Graph 3, but the improvements in torque that resulted from the change of distributor can be clearly seen in Graph 4. The maximum benefit is in fact 8 lbs ft but its real benefit comes from its position in the rpm range. It is placed along the frequently used low-down to mid-range rpm and even has a peak benefit as low as 2150 rpm. The resulting 5% or so improvement will translate into easier driving, less frequent gear-changes and better acceleration. All very desirable and I also made a mental note to change my standard SD1 electronic Lucas Opus dizzy as soon as practical!

What the cold statistics do not tell you is that the Weber does provide much more by way of acceleration and throttle response right up the rev range. Although totally subjective, I have absolutely no doubt that the car is quicker off the mark with the Weber, although the graphs would give little hint of this

Overall Assessment You will note I have weighted the marks in favour of the ongoing benefits that Economy and Performance provide. Consequently I assess the carburettors as follows:
Assessments/Marks (Maximum) Holley 390 Weber 500
Initial Set-up (15) 3 14
Starting/Drivability (15) 11 14
Economy (30) 25 30
Performance (30) 24 30
Totals (90) 63 (70%) 88 (98%)
We can Summarize by saying that the Weber seems to my mind to be far easier to fit than the Holley and to offer better fuel economy and performance, so I am please to report that a deal was indeed struck and I am sticking with the Weber!

RPI Engineering
RPI Engineering was established some 15 years & have focused their business on Rover V8 engines all that time. Chris Crane is the proprietor but has been ably assisted for the life of the business by Steve Chambers, Workshop Manager. Today they have nine supporting members of office, workshop & stores staff. The business still revolves (!) around the Rover V8 but has expanded into several new but related technologies. It is still based some 5 miles north west of Norwich and still, says Chris Crane, tries to do properly first time what others don't do quite so conscientiously. Chris told me he has plans for an additional three-bay building facing straight on to Holt Road in Horsfold & indeed started clearing the ground when I was on site. The expansion is a consequence of the breadth & popularity of the businesses work on Rover engines. There appears to be no limit on the type of vehicle RPI will handle -so long as a Rover V8 is in evidence. There were several Range Rovers (both carburettor and EFI models) in for LPG/Petrol dual-fuel conversions, an RV8 in for a 4.6 litre engine conversion and re-chipped ECU and a one-off “Mud-Plugger” (which is in fact one of the leading Hill Rally vehicles) four wheel drive was delivered for an engine upgrade. Not, of course, to mention a certain MGB V8 in for carburation changes! Chris felt this mixture was quite typical, sometimes augmented with several V8 top end overhauls or Stage I, II and Stage III cylinder head up-grades. Chris explained that most automotive products are a compromise & frequently RPI are able to improve the original company's product performance as a result. The RV8 in the workshop was a case in point for Steve fitted an RPI re-chipped ECU while we were at the Rolling-Road to release some 20 bhp “at the wheels”. Chris believed the Rover ECU will have been originally mapped to cope with Tokyo’s traffic, Johannesburg’s 6000ft altitude, Australia’s heat and the UK’s cooler, damper climatic conditions. Since RPI’s client was unlikely to require all these extremes, the RPI chip could optimize the car's performance and generate the extra power. Chris also emphasized that RPI were committed to LPG gas conversions and saw much of their future growth coming from this market. He told me he believed Low Pressure Gas fuel conversions would become more and more common place as Europe became "Greener", that there was even a possibility that such conversions could be compulsory one day as the emissions were so very clean and that Weber carburettors could accommodate LPG fuel!

Roger Williams
A retired Engineering Manufacturing Consultant & Company Doctor. Roger has built, restored & enjoyed motor cars as a hobby all his life. His first "interesting" car was aluminium clad space-framed Ford 10 Williams Special he built during his apprenticeship in the late 1950's. He has restored numerous MGB's but feels a need for speed - - - hence his especial interest in V8 engined MGBs. His wife maintains a balance within the household via her beautifully restored Triumph TR6 which she uses whenever possible.

Differences and Workings
In very simple terms, Holley carburettors are assembled from three sub-assembles that are split vertically; a main body and two (pannier style) float bowls bolted fore and aft. This construction means there are assembly joints below the level of the fuel and although there are gaskets incorporated into the design, I must report I found it essential to tighten the eight-float bowl/chamber retaining bolts every few thousand miles if I was to avoid a smell of fuel in the car. The Weber is built from two main sub-assembles split horizontally above the fuel level thus dramatically reducing the possibility of a fuel leak. Both carburettors use twin float bowls or chambers, although the Holley only utilizes the secondary bowl after you have fitted the secondary metering plate & a longer cross fuel-transfer tube (thus upgrading from a model 4160 to model 4150). The Holley controls the volume of fuel fed to the engine via the primary chokes (or venturi) by jets which have different carefully controlled hole sizes. These metering orifices control the fuel flowing into the discharge nozzles which are clearly visible at the top of each primary choke. The secondary chokes or venturi are held shut by a spring when the engine is on light load. However an increase in vacuum within the primary chokes (brought about by high engine loads) displaces the secondary's diaphragm against its spring and thus opens the secondary throttle plates appropriately. The Weber, also in the simplest of terms, uses vacuum controlled needles (similar to SU carburettors) to control the primary system. Obviously these needles (called Metering Rods really) can be changed to suit each engine's particular characteristics just as the Holley's jet sizes can be altered at will. The Weber's needles move up and down in response to manifold vacuum, just like the SU. When the engine load is high (wide throttle opening) manifold vacuum is low and the needle's control spring is able to lift the needle out of its seat and extra fuel flows. On low engine load the vacuum is high and closes the fuel control needle against its spring. The Weber's secondary system only delivers fuel when the throttle position AND the airflow through the carburettor warrant extra fuel. Both carburettors have supplementary controls such as accelerator pumps and chokes. The Holley choke is automatic and operated electrically whereas the Weber enrichment is in my mind simpler, manually operated and thus more desirable. You can buy a manual choke kit for the Holley at modest but nevertheless extra cost

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