S.U. Carbs…

[DeadCat]

Just retuning the favour for some of the stories I have read here, hope you enjoy it….

It may be news to some but I very recently sold my originally painted burgundy ’68 MGB with twin S.U. Carburettors before we moved to Germany…

(insert picture of man on knees on garage floor crying like a baby). – Be sure to include oil on garage floor and unused hydraulic fluid on back shelf …

Sniff, now I purchased this vehicle about 5 years ago and I think I got a pretty good deal out of it. It initially ran pretty rough, the radio didn’t tune and the clutch was dodgy. I didn’t worry too much about the radio and replacing a clutch on a car you just got a good deal on isn’t always a bad idea, especially when the vehicle has good bodywork and the vehicle is mostly original.

The first thing I did was to take it to a garage with the following list: (OK, First I emailed Shell with the vehicles make/model, year, compression readings, mileage, engine no. They got back to me within a week with the recommend modern Shell oil requirements)

- Drain and replace gearbox oil
- Drain and replace diff oil
- Drain and replace engine oil
- Drain and replace hydraulic system oils (it also runs the overdrive switch)
- Inspect, repack wheel bearings.
- Electricals back to factory spec.

However, after this the mechanics told me that the rough running seemed to point to a problem with the carbs, “fuck, I thought”. (It was about AUD$500 per carb to get them tuned/serviced).

Well, it was still drivable and within about a month of owning it, (it wasn’t an everyday car), it did an alternator in on a trip from Sydney to Canberra, no prob I thought, get a new alternator. Done, but furthermore, the new alternator seemed to resolve some of the running issues the car was experiencing (i.e. very rough when cold, fine when warm).

OK I thought, seems we have an electrical problem somewhere. I already had a good source for MG parts so I acquired the following…

- Plugs
- Points
- Leads (screw-in, not pop on)
- Condenser
- Coil
- Capacitor

I then got a standard Shell garage to fit them along with a tech sheet explaining what setting I wanted the electricals at. (Dwell, ignition advance, etc… With the constant flow characteristics of the S.U.’s and a high rev limit (7,000 rpm) they don’t respond very well to electronic tunning. You tend to either get good performance at high or low rpm, not both)

And viola, the car ran like a dream, heaps of grunt, even the radio worked, (though with an engine sounding like it did the radio only got used when the engine was off, lol).

To carry-on, the car continued to run like a dream for many years with an oil-change, electrical tune and wheel bearing re-pack every 10,000 miles at a standard shell garage. “Bring it back to factory spec” I’d say, everything was fine.

Anyway, after about 20,000 miles the plugs and points really needed replacing. The car was still running really well and I didn’t want it to be tinkered with, though I knew the points were going to die sooner rather than later.

So out of convenience (was very busy at the time) I took her to a place in the city near my office and instructed them many times over on what I wanted to be done. “Plugs, points and timing to these particular settings, DO NOT touch the carburettors”

I pick her up 2 days later and she seems to be running ok with a little flat spot coming out of low rpm. I park it at home and let the car cool down for a few days, the next time I try to run her she won’t start. (I was thinking, WHAT THE FUCK DID THEY DO!!!!)?

After we finally get her started (with the help of the NRMA, the local road boys), it had a massive and I mean massive flat spot when cold. You could easily stall the engine in this flat spot; you had to practically floor it to revive the engine.

I had little choice and as Aust law stipulates I must try and get the problem fixed by the original garage before I can take any legal action. (I was pissed off and wanted retribution) So I took it back to the garage and explained the problem, at this time they reiterated that they did not touch the settings on the carbs and that they thought it was an electrical problem. Arghhhh!!!!

After taking back to the same shop 3 times, I gave up on them….. (litigation would’ve been useless so I just put that behind me)

The car was still running like a right old pile of shit when it was cold, it was barely drivable. The valve noise had increased and it had no top end power.

One mechanic after another and they couldn’t find the problem, I knew it was the carbies but I wanted someone to just say “Yeah, it’s your carbies, I can fix it by tomorrow… (I even took it to a Jaguar garage (being a British car and all) where one of the old timer mechanics said “sure I can tune S.U. carbs”, and I then watched him attempt to tune my MG by ear. No luck there. Thou they did have the right oil for her suspension)

Anyway, by this stage I had had enough of being fecked around. It was time to take things into my own hands and fix this problem myself.

After minimal research I found this, with the engine at idle, place a tube (like a vacuum cleaner tube) over the inlet of each carb and listen to the airflow going into the engine. This was a major wakeup call as one carb had a lot of airflow and the other practically none. (It was clearly a carbie problem)

So it was now serious research time, on the Internet I found 6 or 7 different ways of tuning S.U Carbs (the most commonly used word was repeat) and various documents explaining their operation, there was also a basic procedure in the owners manual. I also threw a few questions at the MG club (most of them knew squat or had forgotten what they did know or just said throw on a Webber), and spoke to some other older retired mechanics about the procedure.

I then produced my own procedure from all of the above research. The outlook was looking better; at least I knew what the problem was.

I then took a standard 486DX CPU fan and hooked it up to a Fluke 87 multimeter; this was my cfm meter, it worked like a charm.

The procedure (somewhat in abbreviation) was as follows:

- Warm engine.
- Disconnect choke.
- Unlink the two carbs so that they can be operated separately.
- Adjust the idle screw on the two carbs so that they have the same airflow at 750rpm. (use cfm meter)
- Adjust the mixture on carb “A” so that when the piston is pressed manually the engine drops about 100rpm for half a second and returns to idle.
- Adjust the idle screw on the two carbs to have the same airflow at 750rpm. (use cfm meter)
- Adjust the mixture on carb “B” so that when the piston is pressed manually the engine drops about 100rpm for 1 second and returns to idle.
- Repeat from step 4
- Repeat from step 4
- Repeat from step 4
- Repeat from step 4
- Repeat from step 4
- Repeat (keep repeating till no adjustments necessary, took me about 8)
- If you get lost; revert to 2 ½ turns on mixture and start again
- When no more adjustments are required, re-link the carbs
- Test
- Repeat above procedure again (and again)
- Reconnect the choke to allow full open on start-up (this is an S.U thing)

All I can say is that it worked a lot better than expected. (Even if it took up a whole weekend to complete).

But one thing I must admit, it was the absolute best feeling to be able to floor the throttle on a well-tuned vehicle that I had tuned myself….

Anyone else had any experiences with S.U. Carbies???

(DeadCat)

P.S. Internet from home should be up on the 26th March, and we will be interfering in the FPOTM…

[rick1776]

Deadcat,

Nice little story. Did you need to adjust the variable main jets?? There are little pendulums that slide up and down effectively altering the main jets diameter.

My Nephew had a Centura with variable jets. It was running as rough as guts and getting 10mpg. I managed to find an old factory manual at the local library of all places dating back to 1975. Changed the specs to factory settings and voila, 18mpg. Good for that model car with 245ci motor.

My Holley on the other hand is a piece of crap which Im just about to give up on.

cheers
rick1776

[El Sid]

Yeah,

Great story DeceasedCat!. Even if I know stuff-all on S.U.'s I have to agree with rick, this is a nice story.

Incidentally, just the other day I was thinking of proposing a thread with a name of, say : "A Neverending Story" where all the different stories can be either told or C&P'd to. This one is the sort of material I had in mind.

Howz about it? Shall we try

I know it's early but I need a "fixer-upper"

[Henrik]

El Sid,

I think we had something like that a long time ago, but I'm not sure where it was at. I can try and dig it up, because there were some good stories in there.

But agree with you, it is a great idea, and I love reading stories that shamu mambers contribute!

About the S.U., I don't know much about them either. Had my S.U. replaced by a Nikki in my Mini, which was certainly an improvement!

[Henrik]

El Sid,

Check under the other motorsport/karting thread now. I have moved up two story threads for you....

[daSilva]

Get rid of the SU's and get some nice sidedraught Webers!

I got to work on an E-type 3.4 with three SU's, what a nightmare to setup!!! But when they run properly they are quite nice, I'll take the Weber's anyday of the week though.

[DeadCat]

Rick,

If we’re talking about the replacement/resizing of the tapered pins/needles (not sure of the correct terminology) that controls the amount of fuel being picked up by the air flowing across the valve, no.

If we’re talking about the placement and adjustment of the existing tapered pins that control the amount of fuel being picked up by the air flowing across the valve, yes.

From what I read, the tapered pins “do” wear and will eventually cause uneven mixture at different throttle levels and thus eventually leading to an un-tuneable engine

There was also the question of the piston return springs that control the damper piston and basically how the carbs react to throttle changes and effectively the responsiveness of the engine. The carbs I was working with had a specification of a “red” spring type and from what I understood; you’re pretty damn lucky when/if you can find two springs that behave the same way.

However, I was fortunate enough to know (and trust) that the 2 crabs I had did used to run balanced together and which ruled out probably the two hardest things to tune. (The tapered pins and the piston return spring).

(I also read/heard some horror stories where an owner would bring in an engine for tuning only to have the shop replace the nice factory new tapered pins with old pins and then tune the vehicle knowing that the carbs will probably need a full overhaul in under 12 months, the human race does suck at times.) – Though that fear quickly resided when I discovered that the majority of mechs knew stuff all about S.U. Carbies…

Anyway, back to placement, on the bottom of the carb is a large valve adjustment nut that controls just how far up the tapper the pin sits in the fuel chamber. This is what I understood controlled the mixture and this is what I adjusted to ensure the engine would idle as normal even when the damper piston was manually lifted, (too rich and revs would rise, too lean and revs would drop).

An interesting point to note is that the choke (if you could call it that) on the S.U.’s is nothing like I’d seen before. Firstly, there’s no butterfly choke as per your conventional/semi-modern carbs. But, basically there’s fine choke adjustment that just lowers the tapered pins a fraction, (just like the throttle really, I guess it’s to give more throttle movement). And then there’s the real choke that literally removes the tapered pins altogether and just floods the engine with fuel…. (You can actually see the main valve drop from the base of the carbies by about ¾ of an inch, guess starting these phuckers on a cold winters morning was a bit of a prob….)

“Back to factory specs”, I think I mentioned this before; factory specs are wonderful things for old cars that don’t respond too well to electronic tuning. Good to see you didn’t have to phuck around too much with that old Centura, (you do have a reputation to uphold, oh, sorry, that must have been someone else….lol)

On the Holly, I had a 350 Holly on me old Cruiser, check in the first car thread, what a great invention….

Arrrrr, DeSilva,

“Get rid of the SU's and get some nice sidedraught Webers”

Spoken like a true backyard mechanic, (lol, just jokes). Call me sentimental but I just didn’t have it me to butcher such a brilliant piece of British engineering, cough, cough. Actually, I enjoyed the challenge along with some of the looks from the younger mechanics (<50 years) faces when I popped the hood and they saw two great big piston chambers attached to the carburettors.

Oh, and the vehicle was worth more with more original components, (since the rest of vehicle was practically original.)

On 3 S.U. carbs and on a Jaguar of all things, (excuse me whilst I just let of some steam, AAAARRRRRGGGGHHHHH), 2 was bad enough. Now 2 to the power of 2 is 4, which means 2 possible variations (01, 10). But 2 to the power of 3 is 8, which means 6 possible variations (001, 010, 011, 100, 101, 110). In layman’s terms that’s 8 times the work for 1 extra carbie…. (I’m sure some maths guru will correct me, never did like variations in maths, thou I enjoyed limits)

(DeadCat)