MG MGB and MGB GT Tech Talk
I am looking for some input. I have the dual su carbs that came with the motor that I got for my new project but they are in need of a major rebuild. Should I be keeping them and putting them on the engine or put the money towards a webber conversion?? I am getting the motor rebuilt as well (30 over for sure) any input would be great. Thanks guys and gals
a weber is head and shoulders above the su...it is a set it and forget it system with an accelerator pump. SU does not have one, it is the difference between a charge of fuel being shot into the engine and a charge wandering into the engine with the SU. A 45 DCOE will be the difference between night and day. if you are having the engine rebuilt the weber and a 270 or 286 cam will give you some real performance.....
a 270 for flat top pistons, a 286 for slight dish pistons.....make sure they are alloy and not the old heavy kind with the long skirts......adds weight to the engine...also have them take out 3 or four pounds of useless metal from the flywheel. and be sure when the stuff is installed get the engine balenced. It is a beautiful thing.
I had a Weber downdraft and hated it. It started with just a touch of the starter, rain or shine, hot or cold. Never needed to ues the choke, got good gas mileage (34-36) and was as relaible as a Toyota. Never had to touch it. Spark plugs were perfect color all of the time. Couldn't ask for anything better, huh? Performance was down on the car by what seemed to be at least 20%. No accelleration. Car just ran like an old Toyota. No personality, either. Couldn't stand a fine British car running like a 70's Jap econobox. I pulled that thing off and replaced it with a set of good HIF4's and there was a difference like night and day! It was unbelievable! The BGT then ran like a Brit sportscar should. Even sounded better! Me, I like SU's, I like using the choke to start on cold mornings, I like the sound of the SU's sucking air, I like living with the shortcomings(?) of a pair of variable venturi carbs. Maybe I'm just nuts, because I know there are lots of other B owners who swear by their DGV 32/36's instead of swearing at them! But then, I don't like radios in my Bs either because I love the sounds my cars make when they run through the gears!
It depends on what you want. Overall, I'd say SUs are the best setup. The Weber 32/36 DGV is rock solid, never touch it again reliable, has the best economy, and can be ALMOST as powerful as a set of SUs. But it doesn't look as cool, sound as cool, and they're not as powerful.
The various sidedraft setups offer the most ultimate power, but at the expense of being kinda tricky to set up, thirsty, and drivability may suffer, especially when cold.
The 38/38 downdraft MAY be the sweet setup, but there's been no reports from the field yet.
So overall, I'd say the SUs offer the best combination of power, economy and britishness.
I love my S.U. s and they are THE best carb for a stock or slightly modified MGB.
It should take about 2 hours to take each carb apart and clean them. Then it should only take an hour to install the rebuild kits. Rebuild kits are $20 per car or $40 total. The other alternative is to send them to Tom Bedenbaugh at www.importandsports.com and he will rebuilt them to the highest standards.
The only other carb I would consider installing on my B would be the Webber 45 DCOE. They are great when combined with three angle seats, rimflow valves, and a port and polish job. But I don't have the money for all those nice things!
Go with the SU's. Since I have learned to set them up and rebuild them, I will NEVER go back to the 32/36 Weber. I have run both for several years and there is no comparison to the SU's.
The SUs don't have an accelerator pump, true enough. However, when you tromp it, the vacuum picks up the needle quickly and you get a stronger dose of gas right away. Shortly after that, the needle settles down as the vacuum drops and the carb balances out. This mimics the accerator pump action.
David, Love that picture! Sure am glad that it is not an old MG abandoned in that garden!
I have nothing to compare it with. My MG came with a weber downdraft. It was set wayyy to rich when i got it. My MG mechanic tuned it better, and put a K+N filter on it. Quite impressed now. That is, once the car is warmed up it drives like a dream. Good power. And I race (autocross) this car. Certainly hasn't let me down as much as the stock suspension has. I am pleased with the turn-key,start, drive reliability.
Again, have never driven another MG to compare it to. I'll tell ya...the K+N filter rocks..sounds like I have a blower on the car. Seriously...I thought I had an exhaust leak after I drove it the first time with it. I stopped the car..popped the hood ...didnt see any leak..blipped the throttle...the air filter let out a very sweet (loud) noise of air rushing into the carb.
I am pleased..
Paul
I have a 32/36 Weber DGV with manual choke on my Bea and it has been flawless for nearly 10 years. I ran one with an electric choke on a 1976 Honda Civic 1200 for 14 years with no problems.
Click on search in the bar above, select "All Dates" and type in "SU problem, problems, trouble", or something like that. Reading past posts about SUs will put you on the Weber trail if nothing else does.
I wouldn't replace a set of SU's with a DGV Weber. The SU's are evry bit as dependable and trouble free as the Weber. The main problem with SU's is the back yard mechanic screwing with them. If they are set up correctly they will give you as good or better preformance as the DGV Weber. The DCOE is a different animal. If your talking about it you need a $5000 modified engine for it to fully funtion to it's potential. I wouldn't use the $20 kits David talked about. If you rebuild the use real OEM SU kits, replace the throttle shafts, and floats.
Tom B does the finest rebuilds of SUs in the country, but he has Webers on both the MGs he has driven daily for the past 23 years or so.
They could use an SU set-up too, but they don't. I know what he's got and his DCOEs are right for what he's done to his engines and I still wouldn't have a pair of SUs up my ass if I had room for the ship they came over on.
from the ridiculous to the sublime....
by the way JDW, I'll take any SU's you might end up with, but only after they've been sanitized.
Search brings up the wrong stuff by typing in what I said to. It brings up this thread because I put those words in it, but poke around with other words related to SU and the negative posts will show up.
OK....maybe I would....if my wife wouldn't find out....(I'm not THAT old....yet....
I went with the DCOE because they didn't offer the HS6 convertion when I did my GT. Had they been avalible I would have went with the SU's. As for the Midget they still don't offer bigger SU convertion for them. The engines I built need mor CFM's than the stock SU's could supply.
I would have to think about that one. I might even turn loose of both the MG's in that trade. With her money I could be driving ,well anything I wanted to.
If I had her money, I could afford to drive....a chrome bumper MGB :-)
Well, as you can see, there's a lot of opinions. I'm one of the few who's owned ALL the carbs commonly available on a B... and like I said, it depends on what you want out of the car. The DGV, DCOE and SUs are all good setups, but they're all different setups... you decide what you want and go that direction.
So the better question is... I want my MG for X, what's the best carb?
For MAXIMUM reliability and economy, get the downdraft.
For MAXIMUM performance (and you'll have to build an engine that take advantage of it) get the Weber.
For a good all-round setup, get the SUs.
BTW, I think the 40 dcoe is better suited to the B motor, but all the kits come with a 45. Only because I think a lot of Bs run better on a 34 throat, and 45s can't go that small.
John, you are entitled to any opinion that you want, but if you have never run or tuned SU's, I don't have a clue how you could have an educated opinion!! Wouldn't that be like me saying that Farraris are crap?? Never owned one, never tuned one, never drove one but man they must be crap because it is hard to spell!! :o) Know what I mean??????
I plan on rebuilding the heads and motor as well so I need something that would give the most power fir the rebuild..
I've got to echo a comment above: with the 32/36 downdraught, my current B ran like an old toyota; leached the charm right out of the car. I've had SU's twice before and have never had to touch them. I'm much happier since I traded the Weber for the SU's. Even with leaky throttle shafts, they gave me more power off the line and sounded sweet!
Unless you have a Zenith carb or are planning to go racing, the SU setup is the one to have.
Just my 2¢ -- I've driven a friend's '76 B with a Weber downdraft, and my own with SU HS4s. I think the SUs give better low- and mid-range power. That's just seat-of-the-pants -- no instruments were harmed in the making of these measurments....
I can't speak for sidedraft Webers, as I have no experience. However, I'm told that they are difficult to set up initially, and are wretched overkill for a stock or mildly tuned engine.
As for trouble with SUs -- certainly. The trouble is that the SUs receiving the complaints are at least 30 years old and are worn out and/or bodged by previous owners. I think new or compentently rebuilt SUs are every bit as reliable as a new Weber -- at least mine have been! FWIW, YMMV, etc, etc....
Cheers!
Thats what I'm affraid of. Will I need a better carb if I do up the engine??
It will take a heap of 'doing up' to run the SUs out of breath. Get some richer needles and try them if you feel you need to. The needle sets are relatively cheap and greatly cheaper than a weber + manifold.
Rebuilding the motor & punching out the bores do not, by themselves, increase the breathing capacity of the engine. The siamesed port head plus casting flaws are the biggest airflow limiter. The biggest change you can make to your breathing capacity is a port/polish job by an experienced craftsman with a flow bench. This results in improved torque response, and that's what you drive with.
You didn't say what type of SUs you have. If it is HIFs, you might want to consider replacing the butterflys with HS4 butterflys. Whichever carb it is, do a rebuild as Tom B. suggested (above).
You will have to do a lot of mods to warrant replacing the stock SU with anything else. SU's are great carbs. Many people curse them because they have never taken the time to even learn how they work. There is just no simplier carb than a SU. Sure, carb bodies were out around the throttle shafts. And adjustment screws vibrate loose if you use the nuts to lock them in place instead of springs. Most of them are over 25 years old now, so that helps give people some reason to complain. Run a downdraft Weber for 25 years and tell me if it performs like it did when it was new. I'd bet it won't. Get a good set of SUs, either HS or HIF, take the time to learn how they work and set them up right, keep the oil in the dashpots and they will reward you with good service. SU naysayers are full of xxxx! Americans are too used to vacumm robing butterflies that require an accellerator pump to make up for the loss of fuel when the vacumm drops to zero. Design a carb right and you don't need an accellerator pump! Now, on the other hand, the ZS just may deserve all the bad press it gets, and some people that I have heard bitching about "those damned British sidedrafts" didn't even know what carn they were bitching about. because they say "they're all the same" Guess you can tell I'm biased, Huh?
Don't quote me on this, but it seems like I've heard the 1.5 SUs are good for up to about 120 horses. That's a fairly serious B motor. There will be MANY things limiting performance before the SUs become the bottleneck.
If you're after ULTIMATE performance, then go to either 1.75 SUs or the DCOE... on the other hand, by that point you're spending money that puts you well on the way to a supercharger kit, which makes the whole carb question somewhat moot.
Bax.....the DCOE 45 will take a 34mm choke. That's what mine has. It probably give up some top end on a mildly warmed over engine like mine, but I think it's gives a better low and mid range - and that's where a street car spends most of it's life anyway. Unless you've got an engine like Tom B's, 36mm or larger is a losing proposition....
Hmmm... I thought a 36 was the smallest you can go on the 45... well then it's pretty much a wash, isn't it? I stand corrected.
You're smart not running more than 34, I think.. I suspect that's where people run into trouble with the 45... they try to run 36s or larger, and they just don't have the motor for them. It ends up bogging, losing all it's torque, and generally running cruddy.
Me, I've got 32s, which are probably too small. I'll get around to upping to 34s one of these days.
Depending on how much you do. My engines are damn near racing engines. I put Omega high compression pistons, had the head, and block shaved .030. That bumped the compression up to 235lbs. Needless to say I have to put octane booster on top of high test gas. I also used high ratio roller rockers, with a hot street billet steel cam. I port,and polished the head and used rim flow anti reversion waisted stem valves. I have lighten and polished rods. The engine was balanced, stress relieved, and blue printed. To build an engine for a customer with the same specks it would run $6,000+.
So Tom, what do you run? 36? 38? Like I said, on 32s I've got no complaints about drivability (especially now that I've finally got the thing tuned well), but I do feel I'm strangling it a bit, even with a VERY mildly built engine. I think Joe's about right for most of us, with 34s. But when you're on a serious engine, as you are, what do you run?
Gerry said it best. The SU is the only carb setup on that belongs on a box standard chrome bumper car.
I too, many years ago on a 73 MGB, opted for the Weber set-up. After a LOT of screwing around (AKA "tuning") to eliminate the bad flat spot on acceration, which involved different idle speed jets and modifications to the secondary linkage, it ran quite well, but was less economical than the worn out SUs that I removed. And was noiser to boot! On top of that the Weber does not have a good PCV setup for the B. I ended up using a Smiths "mushroom" PCV from an earlier car. Connecting the front side cover breather to the bottom of the air cleaner does not provide effective ventilation.
I'll keep the SUs, thanks. The are one of the most eloquent carb designs ever in the pre- electronic engine management and fuel injection era.
Hey Tom,
Whats involved with me sending my carbs to you for overhaul? I am sending the engine out in a month or so for it's rebuild. Would you like to know what is being done to the motor for your rebuild of the carbs. Also, what kinda costs am I looking at for this?? I live in Ontario Canada.
Thanks
Dezi....the SU's were good enough that the Japanese copyed them! Were it me, I'd change them out for HIF-4's only because I like the integrated fuel bowls
Didn't copy them, Tony. Built 'em under license. They had permission to copy them.
Could you get a bigger signature??Anyway, it doesn't matter how the Japs did it, but they did it!! They never copied a Webber to my knowledge!!
Sure they did! Mikuni made a virtual copy of the DCOE, as did Dellorto in Italy...
Makuni....had one...they're better than Weber's...no dead spot....& there's a guy in South Carolina who can still get parts!
You mean there's supposed to be a dead spot in my DCOE? I musta missed it.....I'll see if I can find it later :-)
ooops, reread one of my earlier posts....Mikuni is what it should've said....
....Joe, when I put the Mikuni on my car it improved its performance; when I put the Weber it was more slugish & had a dead spot on hard take-offs around 1800rpm...when I went back to the HIF-4's, the performance was as good as the Mikuni & the dead spot of the weber was gone!
All this talk about sluggish performance and dead spot. I don't understand.
I have the downdraft weber. Unless the engine is stone cold, I get no hesitation, bogging down, or any bad performance. Sure, when cold, if I hit the gas, there is some bogging down, but once the engine is at operating temp (few minutes at most), it is gone. When I have the Wheelbarrel full of cash to redo the engine, maybe I will go to the SU...naaa...wait to get the Moss Supercharger...lol
Again, I have zero experience with any MG other than mine, but so far so good.
BTW I took first place in FSP at the local autocross. Beat 2 other cars (Accord) that I have previously lost to.
Oh yea, got to love the sound the K+N filter makes under full throttle.
Paul
It's all in you dose the set up. If you set them up correctly, there is no dead spot. It is super critical on the DCOE.
