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MG MGB Technical - carbs
|When I built my V8 roadster 12 years ago, I bought most of it as a non started project. One of the parts was a JWR Offenhauser inlet, so i bought a Webber to bolt on. I have never been able to iron out a flat spot, despite many hours spent with a jet and needle kit etc.. I gave up this year and converted to HIF 6's. |
What a difference!! Smooth progressive power with no hint of hesitation. Just goes to prove what a wonderful piece of engineering the SU is.
Good to hear you have found something that works for you but I'm interested to know what the Webber was as in two barrel or four and what size (cfm)
4 barrel Edelbrock 1400 series/Weber 500. This roadster has a Rover Vitesse 3.5I but I also have twin HIF 6's on a 3.9 V8. Neither show any signs of high end starvation, the 3.9 producing 220+bhp. I can understand the racers ("boy" and track) using these high flowing carbs on highly tuned engines, when most of their driving time is spent high up the rev range on the secondary chokes, but I consider that the complication of the transitions between the 3 pairs of jets, the two needles/springs and the accelerator pump makes them too difficult to tune for smooth progress under normal driving.
You have made a good choice going to SUs for a road car
The Edelbrock 1400 is a loose copy of an American Carter
They are sensitive to engine size- and if the carb is a bit big there will be THAT flatspot that you will never get rid of--- Unfortunately that 500 won't work on a 3.5 without engine mods like heads ,cam ,exhaust etc and even then would be borderline too big for a normal road car
I had a 500 on a standard tune 4.2 and like you played with needles and jets for ages and couldn't get rid of the stumble completely .When I went to 4.4 litres with a bit of a camshaft it went like a jet in a straight line--Different thing alltogether - but around corners it wasn't much chop, as soon as you cornered fairly hard it suffered real bad fuel surge which again I tried for ages to cure as it would have been a real goer but no--Ended up with a centre swing Holley and all was well
A good guide for picking max. carb size for a standard or mildly tuned road car is double the cubic inch capacity of the engine
200 cubic inch engine---400cfm carb
Lots of people use max revs for calculating carb size but honestly, are they going to drive around at 6000 revs all day - I don't think so - If they were honest that they probably drive mostly up to around half that for road use they would calculate half the sized carb which will actually do a better job and will actually be responsive to tuning
Enjoy your SU's
|Willy, I've just checked the Edelbrock, it's a 1404. The heads are worked, I have decent headers (RV8 type) exiting through the flitches coupled to a good free flow exhaust, also a Viper Hurricane cam. According to RPI the carb should suit the engine, but there again engines are different!! I'm looking at my record of the changes made from the stock set up which ran rich. I definitely finished up, in stages, at the lean end. It did go really well and was extremely fast but the pick up stumble from a trailing throttle was really annoying. I have also re-built the dissy and played with the advance curve quite a bit as well as coupling it to good amplifier. |
Maybe the pump isn't up to bridging the fuel gap as the butterflies open and running the idle and primaries rich would have helped the progression!
Thanks for the reply.
Sounds like a nice setup you've got going there
As the carby is on the larger size for a road car, but not so much with the mods you have done-- it's close Possibly slightly larger idle jets to help with the progression onto the mains --maybe
A trip with a gas analyzer would be the only real way of sorting it-------ooorrrrr---SU's
A mate had a pair of 2" SU's on a 3.9 in a B and it was an absolute weapon on the road
|Weber's are good if you dont mind replacing them with new units from time to time. SU's are imminently repairable. I have had a number of V8 powered British cars equipped with the GM Rochester 4 barrel carb and Buick intake manifold on a Buick block. The Edelbrock stuff is crap unless you put on very few miles, the Webers need too much attention and tend to flat spots in power. I would rate well matched SU's up against the Rochester carbs as the best available for those (GM or Rover V8's)engines. The Rochester is easier to tune and will go much longer between service, but SU up to the task, also. An advantage to the GM carbs is the secondary circuit, that can be wired shut if you prefer making fuel mileage over giddy-up.|
|The Strombergs on the Range Rover 3500 were simple and effective. S.Us of course are a notch better.|
This thread was discussed between 22/11/2016 and 26/12/2016
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