How do I adjust dual carbs?

Post by **katy** » Fri Jun 12, 2009 4:13 am

Our recently purchased '61 Morris Minor has a 1275 cc engine with dual carbs and runs terribly rich. How does one adjust these carbs?
I am a Canadian and am familiar with north american carbs, which are pretty simple, but these british jobs are a real puzzle to me, I don't have a clue as to where to start.
The workshop manual I have talks about screwing the jet adjustment nut up or down, but I can't even find it.
Also I would like to get them balanced so all cylinders are working evenly.
Maybe they're balanced now, but then they're both awfully rich.
Any help/advice would be greatly appreciated, or pointing me in the right direction to find it.
BTW, this car needs lots of work before I can get it saftied and licensed.
TIA, Ken<br>

<br>

Post by **mike.perry** » Fri Jun 12, 2009 7:40 am

I have a 1275 MG engine fitted with twin SU carbs. I would advice purchasing the SU Reference Catalogue and Workshop Manual from Burlen Fuel Systems. email info@burlen.co.uk, www.burlen.co.uk.
I would recommend reading the instructions carefully several times to familiarise yourself with the workings. The checking and setting up is too involved to list here. Incidentally the mixture adjustment is the large nut under the carb. Wind it up weakens the mixture, down richens it. Before you start to tune the carbs they need to be carefully checked as instructed in the book

Post by **bmcecosse** » Fri Jun 12, 2009 9:19 am

Lots of advice on the internet - google for it!
But basically - take off the air cleaners and check them out - if they are blocked it will run rich. Then check there is some oil in the dampers - unscrew these little black knurled knobs and lift out the little piston - you need some thin oil in there. Then try to lift the pistons in each carb with finger in the inlet - should rise easily and fall back with a nice 'clunk' - if ok, refit the little dampers. Start the car - and get it up to temperature - make sure the choke mech is going off completely on each carb. To check for balance - listen to the inlet of each carb through a length of hose pipe - held in the same spot at each carb entry - the 'hiss' should be the same. if not - you need to adjust the idle screw for each carb until they are the same. Ideally you need to loosen the linkage between the carbs to do this - but there should be enough free play in the linkage for small adjustment. Once they sound the same - you can now worry about the idling mixture. On each carb at the side near the entry there is a little 'lift pin' - you lift this with a finger so the piston is raised about 1/8" - if the engine speeds up it's too rich, if it falters and slows then it's too weak. If nothing much happens - it's just right! The mixture on each carb is indeed adjusted by winding the nut on the base of the carb - wind up to weaken and down to enrich. Do this on each carb in turn - back and forth - with possible re-adjustment of the idle speed as you go along - until it's perfect!
When the car is running on the road - the colour of the plugs will tell you the REAL state of the mixture.
PM me if you want the download for a free Workshop Manual.

Post by **katy** » Fri Jun 12, 2009 3:59 pm

bmcecosse, thanks for the info and PM, much appreciated.
mike p, thanks for the info and link

Post by **katy** » Fri Jun 12, 2009 5:27 pm

I just remembered that I have a spare manifold with 2 carbs on it that came with an assortment of used parts that I purchased, (seperately from the car), that will give me something to study and try to figger out.

Post by **aupickup** » Fri Jun 12, 2009 6:17 pm

and listen very carefully to both carbs on the air sucking and u will get it right as bm says

thats what we used to do

Post by **eastona** » Fri Jun 12, 2009 9:05 pm

As said above, there's no mystery in it, just following a logical progression and start from the basics.

Although they're not really used much now, I like twin carbs because

a) they sound nice
b) they look good

Whenever I struggled to balance or tune them on my MG it was always because I'd either tried to rush it, miss steps out or they were worn.

Where are you? In UK or canada?

Andrew

Post by **katy** » Sat Jun 13, 2009 2:55 am

I'm in western Canada, Alberta, to be exact.
I went to sucarb.co.uk and downloaded some info, then I disassembled, cleaned, then reassembled one of the spares.
I think I now understand how they work, so that makes me more confident for adjusting them. I'll give it a shot tomorrow.
We bought the car from a place that was basically at sea level but we live at aprox 3,000 ft ASL, so maybe the thinner air has a bit to do with it running rich.
Another thing is, I suspect that some previous owner may have used the car for racing as the front end was lowered and it had homemade traction bars on the back end.
If that's the case maybe it was enriched for more power?

Post by **bmcecosse** » Sat Jun 13, 2009 8:29 am

The 3000ft shouldn't affect it too much - the SU is an extremelly clever device -and meters the fuel according to the air flow through the carb - by adjusting the height the piston rises. Yes - at 3000 ft the car will be down on power - but it should run near enough correct mixture, assuming the needles in the carbs were correct at sea level of course!

Post by **katy** » Sun Jun 14, 2009 4:29 pm

Here's the update:
I pulled the tops off the float chambers and cleaned all the rusty looking sediment and crap out of them, the plastic (nylon?) floats and the needles looked OK. Then I removed the dashpots, suction chambers, the springs and the suction pistons and cleaned them all up. While the float chamber tops were off and the suction pistons w/needles out I blew compressed air backwards through the jets to the float chambers to clean the jets and connecting tubes.
After re-assembling all, with fresh oil in the dahpots I started it up and proceeded to try and adjust the carbs. Listening with a chunk of hose as bmcecosse had recommmended, I found one carb 'hissing' fairly good and zilch on t'other one.
Upon checking I found that the idle speed adjusting screw was missing from the suspect carb, it was only running (idling) on one carb! and that was way rich.
Borrowed a screw and spring from one of the spare carbs and set the idle up so both carbs 'hissed' the same.
So far so good.
I was able to adjust the jet on the rear carb to a semblance of proper mixture, but could not get the front one to lean out enough, even w/the adjusting nut turned all the way up (and the jet w/it).
I suspect that the fuel level in the front carb may be set too high, so I'm going to borrow a float from one of the spare carbs and try that.
The floats that are in the carbs have no obvious means of adjustment as they are all one piece of plastic, but the spares have the float on a metal arm which can be manipulated to adjust the fuel level.
BTW, should the float chambers be vertical? I assume they should be but both chambers are about 8 or 10 degrees off. Gona try and correct that.
Also, I see that the jet needles are not firm in the pistons, they are kinda loose in there and if I push it into the piston there seems to be a small spring behind it. The setscrews are tight but don't seat against the shoulder on the needle as shown in the manual. I assume this is some variation from the norm, possibly a later improvement(?).
If it means anything, the carb numbers are: AUD549F and AUD549R
The saga continues.

How would this engine run if I replaced the manifold with a single carb on it? Any suggestions?

Post by **Matt** » Sun Jun 14, 2009 5:40 pm

Single carb would be fine. Do the needles in both carbs have the same code stamped in them at the top? (you will need to remove the needles to see it)

In this country we would use a HIF44 or an HS6

Both have a 1 3/4 venturi.

Post by **bmcecosse** » Sun Jun 14, 2009 7:14 pm

A single carb is actually FAR better. And certainly easier to set up correctly. It may be the float valve in the front carb is leaking a little fuel all the time - so swapping to the older style floats and tops may well give you a cure. The needles are mounted in the spring seat (known as 'swing needles') - to eliminate the need for precise centering of the needle/jet assembly. It is much better - except that the needle is always rubbing on the jet - and eventually causes the jet hole to elongate slightly - eventually it's no longer possible to get the correct mixture when idling - this also could be the problem. You don't say what size the carbs are ? I suspect 2 x 1.25" - in which case even a single 1.5" SU carb will actually be better - and as Matt says above - a single 1.75" carb will be even better still!

Post by **eastona** » Sun Jun 14, 2009 7:15 pm

As Matt says, check the needles and float levels.

A good idea is to raise the jets on both carbs (turn clockwise) until they are flat with the top of the "bridge" (either look down the venturi or take the dashpots off to make sure the jets are flush.)

Then richen (turn anti clockwise) both by the same amount so the jets are both at the same starting point (normally 2 full turns down IIRC).

To richen or weaken the engine, both should be turned by the same amount the same way, that way they are the same mixture strength. (providing they have the same needles in!)

With those spring mounted needles, (which are later, it means you don't have to centre them) the jets can wear, so new jets does make it a lot easier to tune them. Worn jets can make it really difficult to get a lean idle (i.e it always appears rich). But once you try to drive it, it appears lean when the throttle opens.

HTH

Andrew

Post by **Matt** » Sun Jun 14, 2009 7:29 pm

Twin carbs keep the port velocities higher so the torque is higher, so there is an advantage, however you are very unlikely to notice the difference between twin 1.25s and a single 1.75 in practice

Post by **bmcecosse** » Sun Jun 14, 2009 7:44 pm

Doubt that Matt! The twin 1.25" carbs are VERY restrictive - and the 'ports' in the head remain the same no matter what carbs are fitted. The large SUs work very well indeed - Rover soon realised that - starting with the Clubman 1275GT which came with single large carb - and then that spread down the line to 998 Mini and then all 1275 Minis. Forgot to comment on earlier posting - yes the float bowls MUST be vertical. I suspect you have the correct low angle manifold for a Spridget engine - BUT the carbs are designed for the higher angle Mini engine, and so on that manifold the carbs are tilted backwards by ~ 10 degrees. Check the 'spare' carbs - they may be the correct ones for that manifold. Otherwise - with some filing - it is possible to straighten up the float bowls - take one to bits and the method is quite obvious.

Post by **mike.perry** » Sun Jun 14, 2009 10:18 pm

AUD 549F&R are the correct carbs for a USA spec Midget Mk3, the needles should be ABC. I suspect that might not be the best set up as USA cars tended to be de tuned for emissions

Post by **bmcecosse** » Sun Jun 14, 2009 11:03 pm

Which would suggest running weak - not rich!

Post by **katy** » Mon Jun 15, 2009 4:04 pm

Well, I managed to get the float chambers straightened up, just a matter of loosening the mounting bolts, re-orientimng the chambers and then retightening the bolts.

The float from the spare was quite a bit different from the existing one so I made a slight addition to the original to lower the fuel level about 1/8". It seemed to help some but that carb still wants to run rich.

At this point I'm gonna go through the ignition system to make sure that it's up to snuff. Also going to do a compression test to see how things look in that department. I probably should have done all that first, but a 'simple' adjustment of the carbs to get it to idle better was all I wanted to acomplish when I started on this rocky road.

Quote: "Do the needles in both carbs have the same code stamped in them at the top?"
I will have to see about getting the needles out w/out danaging something, I removed the set screw but nothing wanted to move. I'm thinking that the split sleeve that they're in has to come out.

Are all the jets the same size? Just different needles?

I have a spare set of manifolds with a 'single' intake. The port on it is 1 1/4", I'm guessing that to use a 1 1/2" or 1 3/4" carb I would need a different manifold?

BTW, the dual carbs are both 1 1/4" as are the spares.

TIA, Ken

Post by **MarkyB** » Mon Jun 15, 2009 6:40 pm

S.U. carbs with sprung needles like you have tendency to wear both needle and jet over time where they rub.
This might explain the rich mixture.
From memory (always dodgy) the needle and spring are in a sort of tube arrangement in the middle of the piston.

Post by **bmcecosse** » Mon Jun 15, 2009 11:25 pm

The single 1.25" manifold would work - but the power will be very much restricted. You do indeed need a bigger 'ported' manifold to take a larger carb - although that standard manifold can be opened out to take the 1.5" carb - it's very much NOT the best manifold to use!