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MG MGB Technical - Tuning MGB with Weber 45DCOE Carbs

Hi,

I have recently returned to my 79 mgb roadster Project after 7 years in Dry storage!!

History: I bought the car on Ebay and knew there was a spitting problem with the Carbs but never got around to fixing before losing interest!! What I did know was that cylinder 2 was down on pressure, 110 psi when all others were 155 psi.

Anyway got the car through MOT with minimal expense; clutch needed sorting, No hyd oil? Rear brake cylinder required, new off-side front shock and track-rod end, a few lamps, new ignition switch and bobs your uncle!

So, with engine running horribly I decided to sort the carb and after reading many articles on the WWW I didn't get anywhere. Changing Jets did little,altered timing (more advance)and then by luck I stumbled on another article that did help.

Basically, you need to start with the Speed-screw set at a maximum of 1 turn Open and the mixture screws should NOT have to be Open more than 1 1/2 turns max. If one, as many do are having to Open mixture screws 2 and a 1/2 turns or more to achive what seems like a ok IDLE then the idle circuit is too lean. By fitting say 60F8 Idle jets (Std 50F8, up 2 sizes) the need for the mixture screws to be so far Open changes dramatically!!!

Start with mixture screws about 3/4 open and adjust slowing until hearing the max idle speed increase and close off on Idle speed screw to reset idle between 850 - 1000 rpm.

I also found like many others that when tuning Weber carbs there does seem to be a need to increase the timing from 10 BTDC (18V847 engine) to about 15 Degs by strobe at 1000 rpm.

My car is now OK, but I will have to sort No 2 Compression. Might be exhaust valve, rings, bore who knows, but I will find out!

Note: Other Weber settings are:
Main Choke 36
Main Jet 145
Emulsion Tube F16
Air Correction 165
Pump Jet 45

And Air By-Pass screws (26) MUST be shut!!!!
RH MARSHALL

Hi RH, interesting info re your Weber. I have a DGEV 32/ 36. I'll try the timing at 15 degress. My engine is 846F (no overdrive).

/Moss
Moss

Moss, this is regarding the Weber 45 DCOE carburetor. A much different kettle of fish to calibrate. Almost everything is able to be swapped out to make the carb run properly for almost any engine that you can mate it up to. I have a whole box full of spare Weber parts that I went through before I finally got my engine running correctly. It only took me 2 years and a lot of patience. Plus, you need 2 of every part and nothing is cheap when it comes to Weber carbs. RAY
rjm RAY

And ,,,Excuse my ignorance, but isn't the Weber 45 DCOE different to the DGEV 32/36? The 45 a side draft, and the 32/36 downdraft? So even on the same engine they will have to be set up differently? And does the downdraft 32/36 refer to the bores of the carb body? ie it is twin bore with the second larger bore only opening at larger throttle openings? I have not got either of these carbs, and I may have all this completely wrong, I would like to know please. Mike
J.M. Doust

Mike the 32/36 is as you say a downdraft very similar to the two barrel on the last of the carbed falcons.
The 45 doce is a two barrel side draft without progressive linkage and very complex in design and tuning. You can fit a variety of choke tubes (venturi)etc to fit various size engines. I was never really happy with the DOCE. You could get great performance but it always seemed to be at the expense of economy although some people swear by them. A row of them look great.
Denis4

Yes, am aware of the different types ie. side draft and down draft. Needless to say the 15 deg btdc did work work :)
/Moss
Moss

Hi RH

I am a Weber fan and have been since the begining of the 70s. :)

I have generally found that the idle jet is always more than the rule of thumb value that many of us tend to use. I generally use a 50 to 55 on A series engines (1300cc) and often use 55 on a B series (1600 to 1800cc)and occasionally resort to a 60.

With that in mind you are I think spot on but what does worry me is the advancing of the timing?

Irrespective of Weber carb or not the B series timing should not exceed around 32/33 degrees at max mech advance (more than 4500RPM) advancing the timing at idle will inevitably increase the timing at high RPM beyond this value unless the dissy advance is very little. This must be checked and set accordingly

If the engine has NOT been fitted with vac advance then it is no suprise that the engine will "appear" better with more advance this of course does not mean it IS better! Indeed most engines when timed by ear at idle revs will ALWAYS sound better but will be a problem waiting to happen due to over advance at higher revs, do not do it!!

Vac advance must be used both to account for the design of the dissy curve and also for fuel economy, without it the fuel economy will suffer by between 10 and 15% and probably accounts for much of the urban myths surrounding poor economy of the weber.

If you already have vac advance fitted then ignore this statement.
Robert (Bob) Midget Turbo

Persuing this thread further,,, I built a Westfield SE in my earlier days with a Ford Kent engine. Lovely thing really. Anyway I bought a pair of Weber 40 DCOE from a chap in the pub ( back in England.) I didn't think it would have been 'jetted' anywhere near the actual engine spec, so I 'copied' the jetting arrangements of the ford Escort RS at the time I think??? The engine did run, but was way too rich, and would spit flames out of the intake, quite scary. Ahh, the things we did in our youth!! Mike
J.M. Doust

Hi Bob

The problem I always have is people fixated on idle advance when they do not state max advance. Distributors vary when new and when worn advance figures are nothing like the book. I tend to tell folk to set a max advance at 4000ish rpm no vac then see what it is at idle.

Peter
Peter Burgess Tuning

Sorry missed your reply Peter

Of course you tell them that and you are spot on Peter, :) just don't understand why some still describe timing as degrees static, totally useless figures except for initial start. on initial start up when I have had the dissy out I simply set it slightly BTDC and get on with setting it as described dynamically.
Robert (Bob) Midget Turbo

Hi there,
nice to see that folk view this web-page!

Further to my earlier post I have revised my views wrt
Tuning and hear are a few more thoughts.

1. keep to the idea of throttle plate being set at 1/2 turn Open and replace jets etc with this set in stone! Remember that you dont want to uncover Progression holes at idle!

2. Stock MBGs using a 45DCOE152 will run "better" if changing to a 34mm Main Choke; believe me mine does and its so noticeable through Idle and mid-range. Yes a purest will say one limits Top-End, but I am tuning my car to suit my driving needs. I am not a boy racer and if I was I would not drive a 79 MGB.

3. Adding a 4th progreesion-hole is easy enough, but doggie as you have to be accurate with the 1.2mm hole. Drilling in-situ sounds STUPID, but slow-speed and a vacuum cleaner sucking out debris will prevent 99% of foreign objects entering Intake. Easy to plug hole if tuning goes to Rats!

4. Vacuum Advance - I have now removed vacuum advance as previous ownwers had this coming off the Downstream side of the Butterfly which is not RIGHT as this advances the timing by 8-10 Degs at idle.
My 152 carb does not have the additional upstream plug as seen on the Internet (YouTube). I would return to vacuum advance if I find a way of tapping into the UPSTREAM vacuum.

Regards,

Rees



RH MARSHALL

Hi Guys
I promised myself I'd never do this again - but I can't see how the thing runs on 145 mains with 36mm chokes
The starting point for 36mm chokes is normally 165 - 170 it would be very lean with 145's I would think
Try a pair of 170's - you'll be surprised
Even if you went back to 34mm chokes which defeats the whole advantage of running a 45 you would still need at least 155-160 mains
I know all cars are different , but not that different
That's me done Willy

As you are aware the Weber doesn't normally have a vac. advance port but I have had fairly good results from having the ignition timing on the std 10 deg. mark and then running the vac advance hose from the dizzy straight to manifold vac.
The result is that it gets rid of that flat feeling down low on light throttle and stops the little spits and things on light throttle that webers seem to be known for
William Revit

For
William Revit, Tasmania, Australia

Hi Will, can't agree with your comment on size of main jet as the Rule Of Thumb (ROT) is 4 x Choke, hence
4 x 34 = 136 and 4 X 36 = 144 so a 145 on 36 is OK as 99% of Posts would suggest.
Once I get idle correct I will fit 135 Main Jets to suit my 34mm Choke. Downsizing to 34mm has improved my bottom-end, however I am having difficulty achieving Idle with Throttle stop-screw set at 1/2 turn Open and I need mixture screwsw at least 3 turns Open???

Another thought I have is that the ROT to leave the Throttle screw 1/2 Turn Open may be erroneous as the setting could be critical in relation to the Progression holes. I am going to remove the Progression Hole blank, increase throttle and mark side of card when I see the plate ON the hole! Replace blank and set 1/2 turn Open and note position at 1/2 Open to mark on side of Carb. My Theory is the throttle plate should be set at the point that progression starts and this I feel will alter the need for Mixture Screws to be more than 1 & 1/2 turns Open. I also leave Screw 26 Shut, but have read that it's there to increase air-flow and hence fuel if a good idel cannot be achieved with the screw 26 set at 1/2 turn Open. also, I am keen to see the change in vacuum (currently 15 inches) when I move the plate and expect this figure to fall, but will it?

Regards
RH MARSHALL

I am with Willy on this. The problem with Siamesed ports is trying to feed two ports or at least 1.5 x flow of one port, Chokes need to be large enough to feed at high rpms, viz 1 1/2" or 38 mm for SU per port. The siamesing tends to fairly large main jets with smaller than main jet air correctors to maintain the fuelling. Sizing is more reasonable with tiny 34 mm chokes but we tend to reckon a 155/165 main with say 140 or less airs may well be near the mark.


If you need 3 turns of the mix screw it implies your idle jet/main jets are too small. Is all this tuning seat of the pants or dyno?


Peter
Peter Burgess Tuning

Before installing a Moss supercharger, I ran a Weber 45 DCOE for 10 years. It took me 2 years just to get everything to work in harmony. My end results were 36 mm chokes, 5.0 auxiliary venturis, 170 mains and 60 idle jets. RAY
rjm RAY

for Peter Burgess

Hi Peter, what do you mean by siamesing? Also, your

"Sizing is more reasonable with tiny 34 mm chokes but we tend to reckon a 155/165 main with say 140 or less airs may well be near the mark" is against the grain of most posts, but believed as you are an engine tuner! Normally posts suggest Airs = Main + 50? Explain how your recommend is so different?

Rees
RH MARSHALL

The inlet ports on a B series engine are siamesed, one port feeds two cylinders via a siamesed section, you are feeding more than one ports worth of fuel and swapping flow from cylinder to cylinder. The choke sizing jet system you are using does not obtain with siamesed ports, that is why larger main jets and smaller air correctors than a single port set up are needed. Other experienced B owners are also trying to tell you this.

Do I assume you have not done all this fine tuning on a dyno?

Peter
Peter Burgess Tuning

Just been thinking, you will need idle jets in the 50-60 range.
Your post re main jet +50 is a big jump for air correction. We have found, when chokes are just-so on separate port/choke setups the air correctors are around main jet +20 eg 125 main 145 air. Italian standard production fitment sidedrafts tend to run small chokes with smallish mains and larger airs. I reckon this is to give good sound as the top end suffers somewhat.

You cannot get all you need from generalised books and threads. As I said the A and B series are different as they have the Siamesed inlet port, and a siamesed ex port to upset ex tuning too!

Peter
Peter Burgess Tuning

Thanks Peter your comments are excellent and much appreciated.
I have smaller Airs and Main jets so will experient at Home as I can't afford to get my car tuned by Dyno although you and othes will probably say I will save in the long run.
Final Q - what MPG is normally expected from an stock MGB? I am thinking 25-28 around town?

Rees
RH MARSHALL

Hiya

Probably less mpg with a Weber as the poor running below 2500 means the engine is only running well on two cylinders....another effect of siamesed inlet ports is charge robbing, basically you can pull 1 and 4 plug leads off at idle and the engine doesnt sound very different, 2 and 3 are nicking the petrol!

Are you using some sort of lambda sensor for measuring afr or co? Plug chop tests tend to only confirm whether the plug grade you have chosen is suitable.

With regard to tuning costs, we tend to drill and or solder jets which saves the cost of buying new jets.

Peter
Peter Burgess Tuning

Uuuum,

Peter, was the previous owner right in fitting the 45DCOE arb as I have the original SU HS4 carbs in a box?

I would prefer better MPG than top-end performance so what would you recommend?

Never really worried about Air Fuel Ratio (afr tbc) or CO but was told the emmissions were ok during MOT. that said I have de-glased bores; replaced rings and had Head "unleaded" post MOT and all Pots are now 150psi where No 2 was 95 psi pre work.

it makes sense to solder and re-drill, something that I have done a couple of times, but Jets don't seem expensive at £2.00

Just replaced my DIZZY after strip, clean and lubricate, all seems fine!

Thanks for all your help,

Rees
RH MARSHALL

Hi Rees

To be honest the SUs are brill, they give good throttle response and good part throttle economy. Personally I would fit a Weber for Track Day use but stick to twin SUs for road, especially with economy in mind.

Peter
Peter Burgess Tuning

Now I am confused, Weber or SU?

Any way have made a few changes based on what parts were in my top drawer.

Pre-posting Set-up

Main Choke 36
Main Jet 145
Emulsion Tube F16
Air Correction 165
Pump Jet 45


Now:

Main Choke 34
Main Jet 165
Emulsion Tube F16
Air Correction 170
Pump Jet 45

Basic setting 10 BTDC and vacuum attached so approx
22 Deg BTDC with Full vacuum at 1000 rpm.

Car still seem a little uneven, but perhaps I am asking for too much with my siamesed ports and Weber carb. Car's response is however better with the smaller 34mm choke and screw 26 (air by-pass) SHUT as I don't need to tweak 26 to achive idle with throttle Plate set at 1/2 turn Open. Idle Jet is on the large size at 65F8 and have tried 55 and 60 yet Mixture screws are still 2 & 1/2 turns Open!

Probably getting 20 mpg so a little unhappy with cost of running this old baby hence; wait for it, I have gone and bought a 2 previous owner, 33K AUDI 2.6E cab, mpg 28 if I am lucky. When will I learn?

Will probably take MGB to Dyno-man in Plymouth as Peter Burgess is too far away!
RH MARSHALL

Do Audis cost less than Peter's time? And a room for the night.
Is the resale value that bad?
How much is Audi parts?
How far is too far in a country that size and Peter is "in the middle"? Admitting that you seem to be nearly as far from "the middle" as possible.

Not giving you a hard time, but I am interested in perception of situations.

FRM
FR Millmore

Interesting thoughts FRM

Most Americans never set foot out of the States, why should you.

Why would I travel 200 miles to get my engine tuned especially when it always rains in the UK and my MBG is dry weather Only!

The AUDI is a future CLASSIC and not to be compaired to my MGB and a much safer, comfortable ride, but much more expensive and difficult to maintain.

Plymouth, Devon has a very qualified Rolling Road/Weber carb specialist who may or may not be as good as Peter.

I don't want to sell my Roadster, but I might have to althogh my daughter will be gutted as she thinks it's hers!

Rees
RH MARSHALL

Interesting reply, and implications regarding perception.

"Most Americans never set foot out of the States, why should you."
>>Not sure what to make of this, might be snarky, or...
Do you know whether I've been "out of the States"? And why or why not as the case may be?

"Why would I travel 200 miles to get my engine tuned .."
>>My perception is that any excuse that both results in a 400 mile round trip AND accomplishes something useful and/or fun is a blessing. All the more if the payoff continues after the trip. I routinely look for parts or events that give me the excuse to make up to 600 mile day trip drives - shipping costs saved usually pay for fuel. My car is sitting on wheels that I bought, drove 700 miles to fetch, saved $80 shipping, added another 400 miles for another set of wheels and a cylinder head and gearbox, saved another $150 or more = profit for driving, got to spend an evening with each of two seldom seen friends, near perfection in life.

"especially when it always rains in the UK..."
>>Perception = you are a Ninny; we got rain too. My wipers work and I'll bet you've even got a top and demisters and heater, I've had a lot of cars without, and for a while a motorcycle as my sole transport year round.

"and my MBG is dry weather Only!"
>> = that is your decision and perception, But yes, my perception, Ninny. I'd drive it in snow too (fun, aka, the whole point). I bought a 67 B a couple of years ago and drove it home 450 miles non stop in sub zero F weather, no heat, not much floor, lots of blankets. Heavy snow last hundred miles. Daughter was following and asked me to please keep it under 80 or so (no speedo, bad tach, no panel lights). Had a ball. Sadly it is too rusty to drive any more until total rebuild.

"The AUDI is a future CLASSIC and not to be compaired to my MGB and a much safer, comfortable ride, but much more expensive and difficult to maintain."
>>Sounds like the B is not for thee, perhaps you could find a nice young lady who would appreciate and deserve it.

"Plymouth, Devon has a very qualified Rolling Road/Weber carb specialist who may or may not be as good as Peter."
>>You are blessed. We have few to no rolling roads. Every time I hear of one it is out of business next time I look. Would be happy to find one within 500 miles that actually knew about these cars, and/or would let me do the tuning while he measured it. I only mentioned Peter because he's the only one I've heard of who knows tuning besides owning the machine. And as you see he is helpful. And right next door by my perception.

"I don't want to sell my Roadster, but I might have to althogh my daughter will be gutted as she thinks it's hers!
>>So don't be a cad. Sort it and give it to her. No doubt she will repay the favour by driving you to the bank, then the Audi parts & service emporium, frequently - unless you don't drive that either.

Note:
I admit to getting old. 500 miles in a day is now a decent day's drive, and while 1000 miles is not out of the question still, three or four in a row is getting old. Three years ago we did a 10,000 mile "around the country" in a bit under a month, for $1000 total for two people. We can't get to UK for that, never mind drive that much after, even if we swim home.

Do yer thing and have fun at it, whatever it may be!

FRM

FR Millmore

For FRM

ninny - a stupid foolish person. nincompoop, poop · simpleton, simple - a person lacking intelligence or common sense ...


Thanks, R U American by any chance!
RH MARSHALL

Peter, if only this forum had a quote button. SUs for the road DCOE weber for the track, one dual choke 45mm Weber flowing more than 2 SUs, but fixed jets even with all the twiddly bits evolved over the years never standing a chance against the brilliant constant depression design over a wife range of loads.
There is meaning in the Weber designations D - dual choke and I think also means they are simultaneous not progressive(ie track focused) and the O stands for ' orizontal or sidedraught. Err if you can "stand" for horizontal?
Stan Best

Thread seems to have strayed a bit...

A rolling road is the only way to get it "right". On the road may be ok for fine tuning a "correct" system, but it is simply not possible to measure things accurately on the road. "Seat of the pants" feeling is unreliable. Your local rolling road may be fine, but I'd check they are genuinely familiar with our ignition systems, and also have the skill and parts to change bits. Without this you will simply get some (expensive) print outs. How many webered/distributored cars are parked in their yard?

FWIW my local RR have a great bunch of guys but only the (old) boss understands a distributor.

Peter has these skills in spades as you can see above.

Distance/cost/effort/fun for a proper set up is a decision only you can make.
Michael Beswick

My my guys... some of you all had too many RedBulls or what? My concern nowadays is not Weber or SU, as it's clear to me over many trial years, for the street, SU is the ONLIEST way to go. Sure, Weber at the track idling at 2500 rpms and you can watch the fuel needle descend.
My concern these days is what --in the wide wide world of sports-- is Peter Burgess' or FRM's opinion on HIF over HS for the 18V souped up B mill.
FWIW 12 years ago when Doug Jackson built my cross flow after Peter flowed the MSX head and corrected the mating surface, I popped for the sidedrafts and manifold as everybody "in the know" insisted on as the only way to go "if you are serious". But I was not at all happy with the ride. So, 2 years later, a new set of 1 3/4 HIF SU from Burlen, bolted to a Burgess flowed manifold, and the car transformed: noticeably smoother and stronger than with the Webs. This engine spun past 7k pulling like a train all the way. Strongest B I've ever driven, including the SC's that spawned and came to dominate the power luster's quest for BHP. Cheers, VEM
vem myers

Hi Vem

Looks like you are laying down a gauntlet there....MSXrs vs SCers with SUrs and Webers mixed in there too.

Peter
Peter Burgess Tuning

VEM -
10-4 on the RB! Seems our UK pals don't listen to Car Talk, or engage in friendly joshing at the pub.

I think SU are preferable to Weber in most cases, for most drivers, maybe even in competition. Cheap, reliable, and effective, and you usually already have them.

HS are easier for DIY roadside repairs, and they don't usually have the float problem you have now discovered, nor the mysterious cold start device failure. The latter is not much problem once you know about it, but it does require removing the carbs, not necessary on HS.

The float configuration is the big advantage of the HIF, as it eliminates G force effects, so if somebody starts making decent floats...
Also, the thermostatic jet auto adjust should be a benefit, and would be if you could get or make different thermal coefficients for that. The HIF at least in the metric versions seems to have lots of "possibility" bosses and lumps, some of which might be utilized by a clever and motivated tuner - but I have never even seen an actual metric HIF; nothing but the HIF4 MGB version.

So HIF is theoretically and sometimes actually better, but parts are more intricate and more likely to become NLS or at least "not this month". I could actually make a whole HS if required, but an HIF is gettin' fiddley.

I might add that I think the ZS is a seriously maligned device.

(and here my computer disappeared the above and I was too bummed to redo. Then the computer just nicely shut down, all by hisself. So I restarted and this was lost, even to Firefox - not usual. 8 or so hours later it has miraculously reappeared!)

ZS were set up for wretched engines, but the carb per se is fine, and has the advantage of being not subject to vac chamber wear and damage - hey, that's just like the very first Skinner's Union leather diaphragms! And they are all over the place in 1 3/4" size, with a lot of variant doodads that could be used or trashed.

Anyway, SU is the girl you want to marry, and Weber could be if you are skilled and lucky and rich, but more like the weekend crazy chick. Weber wasted on siamese head though.
And I, still, want, a set of Webers on a XK or Aston or Fizzarri - Orgiastic music!
Never met, but thought of building, a set of the fabled twin choke SU, what is her song?

FRM
FR Millmore

Can't help but comment, having run a 1995cc/DCOE45 BGT alongside a stock 1800cc/HS4 Roadster.

1995cc/DCOE45 = thug, 1800cc/HS4 = charmer, preference depends on your driving style and mood. The soundtrack of the Weber is very special.

Vacuum advance (manifold, there is absolutely no advantage to upstream ported and you risk ruining your carb) gave sharper part throttle response and improved average imperial mpg from 24/25 to 28/29. No brainer.

A visit to PB's rolling road convinced me to try going from 36mm to 38mm chokes on account of the overbore. Great advice, with freer revving at the top end and no discernible loss lower down.

As for road manners of the DCOE, I drove the GT in London logjams, flat out on motorways, and more usually on a 60 mile a day country lane commute for ten years, and relished every moment. But I still love my Roadster.

SteveP

The Twin Choke SUs suffer from pulses which are difficult to overcome. We played with a pair of twin chokes on a race tr4 in the late 80's gained no power didnt run really sharp but wound up the opposition as the regs stated must be SU carbs, as the standard setup was a pair of SUs the pair of twins was also legal!

I attach a link to a youtube video of a pair of twin chokes on a Cooper Race Car on our rollers, turn the sound up and enjoy the glug glug of the fire up!

http://www.youtube.com/watch?v=G4lFNJGELmI&list=UUwQ6N03qXmwhmx_c1vYGqLA&index=2&feature=plcp

Peter
Peter Burgess Tuning

Well, I spent the hour+ it took to download that, and when it got to the end it disappeared. But, I had watched it over several times in bits. Quite splendid. Lovely song at all speeds and conditions.

Thanks Peter!

FRM

Anybody know if there is a way to capture these things?
FR Millmore

Peter, I also played with a pair of DU-6 on a 2.2 Triumph engine with lots of compression and cam. They worked, but I suspect, as you say, that they were no better and probably inferior to 45 DCOE.

I sold them to a TRS Le Mans owner.

Thanks for the great video!
Bill Spohn

The Metric HIF 44 is an excellent carburetor and doesn't suffer from the bi-metallic strip that raises and lowers the jet. It's what Moss choose for their supercharger setup. It has all of the good features of the HIF 4 without the drawbacks and is slightly larger. RAY
rjm RAY

Back to the original post - Mr Marshall - there is no way I'd use the DCOE for the street. As you are finding out, there can be endless fussing until you get it right and even once you do, any theoretical advantage will be up in RPM ranges where you rarely tread.

Put it on the shelf or look at the Weber and put those excellent SUs on your engine instead.

Bill Spohn

This thread was discussed between 18/05/2012 and 26/07/2012

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