Fitting twin SU’s on smaller capacity engines – easy to tune?
Hi
I just want to confirm my logic is correct regarding tuning of SU carbs, I suppose some of you may be so young to not even have used these
I have a 2.25ltr petrol landy which currently has a terribly restrictive single downdraft weber and and equally lame manifold. The plan is for some
fun to make a couple of twin manifolds and fit 2 SU carbs I have from a 3.5 rover v8 to get some real power out the beast .
My thinking with SU’s is that the only difference in fitting them to a smaller capacity engine than before would be that the carbs pistons would
unlikely raise to the top of their stroke (tbh running out of stroke I’d have thought would be more of an issue). I’d imagine that they don’t need the
annoying and expensive tuning of jets like Webbers? Just bolt on and fine tune the mixture screw, but am I right?
I’m not looking at increasing the top end rpm (might fit a limiter anyway though it has a forged 5 bearing crank) but these engines are extremely
restricted by the factory spec manifolds despite actually having very large 45mm inlet valves and I’ve ported the head too as Landrover made an arse
of that. Going from info on the web I should see it go from less than 60bhp to around 100bhp or more (oh my poor drum brakes!)
Yes there is only one jet, and its size is fixed, for example all HS8s have 0.1" main jets, HD8s have 1.25" Main jets, I know this because
I have lots of them for the jag engines, and have done a lot of research.
The variable with these carbs is the needle, you can get needles with all sorts of different tapers, to regulate the mixture at the differing piston
heights.
You say you might not get the piston to rise fully, the problem here is you are reducing the amount of needle which is used to control the mixture
across the rev range.
So the first thing is to make sure the carbs match the engine, and you do get good piston lift across the rev range.
Then tune the mixture by changing the needles, there is a handbook available from Burlen, which gives details of needle tapers.
The 2 x 1 3/4" SUs might be a bit big for you relatively slow revving engine, and probably wont generate the vacuum and air speed to get the
pistons to the top of their stroke, 2 x 1.5" would be nearer the mark.
I'm sure there is an SU carbs book in the Haynes range, and Des Hamils wrote one I think, I do have his Jaguar XK tuning book, which covers
quitte a bit about SUs and needles, I can scan the relavent bits for you into a pdf next week if you like.
It will not make any difference to the performance of the engine fitting whatever carbs you like, it will still be a petrol guzzling antique with the
acceleration of an overweight tortoise. Fit a 200tdi engine and diffs and have something you can use and enjoy. Other than that SU carbs are
relatively simple but needle choice is critical and they are not cheap to buy if you want a few to experiment with. A friend of mine who put them on
his old Ford engine tried modifying the needles by filing them to alter the profile where he needed more fuel and it worked. I helped with a series
IIA build a few years ago and after it went on the road with the petrol engine it was a matter of months (with fuel consumption sometimes getting as
high as 15mpg but usually in single figures) that it had a diesel transplant.
[Edited on 21/2/14 by Peteff]
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
Hi i have run 2 1.5s 0n 970 cooper s we raced many years ago so dont see the prob with 2.25 landy engine main thing is to get them balanced well has
landy got siamese ports you should be able to get it going not bad by adjusting nut on bottom moving jet up and down on needle in an ideal world i
would use 1.5s but if you already have 1.75s why not good luck interested in outcome.
Cheers Ali
Hi as said above you can get different jets the photo above is my 2l pinto with two su carbs off 1000cc minis and a home made manifold
It took quite a time to get them to run right and set up
I had them fitted for about 2 years then changed them for bike carbs which works on the same principle as the SU's
Graham
A single 1.75 SU won't be restrictive until you start approaching 100bhp, so would be more than adequate for the old 2.25L Landy boat anchor.
Twin 1.75" SUs will be overkill, if you must have twin carbs then 1.5" SU's would be more suitable.
quote:Originally posted by MikeRJ
A single 1.75 SU won't be restrictive until you start approaching 100bhp, so would be more than adequate for the old 2.25L Landy boat anchor.
Twin 1.75" SUs will be overkill, if you must have twin carbs then 1.5" SU's would be more suitable.
Yeah that seems to be the general feeling of everyone and I'm now thinking the same, plus the twin manifolds are very easy for me to fabricate
otherwise I'd have to butcher the orginal landy manifold which is a piece of sh*t anyaway.
The landys engine is actually a lot more powerful than the 60bhp makes it sound, it just doesn't rev very fast but even my little 88 can haul a
3.5 ton trailer at over 50mph. It should be easily smoking the tires once I've finished with it
it picks up slowly because its got a long stroke crank and a heavy flywheel,all the things that are important on an off road vehicle - everything is
done for torque
you may get it breathing better with porting but it wont change the character of it - and ignition is as, if not more, important
whats the point in putting loads of fuel in if the valves,cam and ign is the same
i was talking to someone earlier whos helping rebuild and tune a gt6 engine
thrown a fortune at it - i mean thousands - and at the end of the day its still an old long stroke 60s pig,no matter whats bolted to it
my last 2 1/4 landy had the weber carb upgrade,it was more economical,ran better and although no more powerful - it felt it because it ran properly
if it were mine ide go 200tdi or the perkins prima from the montego td
both will leave the weezey petrol standing on power and do 30mpg
if your adamant on petrol,stick a ford v6 in it
if you want to stick with what you have , sell your SUs and buy a nice new weber,jetted for that engine and an electronic ign kit
obviously all in my humble opinion
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posted on 21/2/14 at 09:38 PM
SU is a far superior carburettor compared to a weber, correctly set up they are great.
You will probably get quite close out the box, twin SU's will be a bit wasted as the engine is at such a low state of tune, I would take the
original manifold, chop the top and bottom off the boxy bit including the heat bowl then weld an adaptor on the side, you can grind the insides to
quite a good shape.
You will need to chop off the top of the exhaust manifold and cap to create a decent air gap, I did this with good success using a mig welder and a
3mm mild steel plate you do need to heat in a barbecue, quickly weld and pop back so it cools as the charcoal burns down or the welds will crack. The
carb will also need a heat shield from a bit of old tin or the fuel will boil.
tbh I always liked the SU over a weber, its a much neater and smarter design also flows so well when fully open
I know the in thing at the moment is to fit the 200tdi but having been in a series with one fitted I thought it was just a horrid racket. My landy is
not my daily car its a fun toy when the weather is nice, hood off, screen down nothing could spoil it more than a noisy diesel, mpg? frankly I
couldn't give a monkeys.
however it does sound like the v8's su's will be to large for that engine and I'll have a look for a couple of smaller ones. jacko,
your set up looks very like what I'm thinking including the manifold design, good idea using mini ones
quote:Originally posted by Mr Whippy
nothing could spoil it more than a noisy diesel
quote:Originally posted by Mr Whippy
There is also zero chance of me fitting a clattering stinky diesel in my landy, can't stand them, they just sound like a horrible old tractor
quote:Originally posted by Mr Whippy
nothing could spoil it more than a noisy diesel
quote:Originally posted by Mr Whippy
There is also zero chance of me fitting a clattering stinky diesel in my landy, can't stand them, they just sound like a horrible old tractor
You can also change the damper springs in the su carbs. IIRC there are 3 different strengths, dependant on carb model I expect.
Soft springs will obviouslyvallow the damper/piston to rise more.
Try the "mintylamb" website where you will find a few guides, including one where you can compare needle profiles.
There used to be a programming kicking around the internet that could predict needle choice based on a raft of inputs from you. Winsu or something.
Or try the landy forums. I am never afraid to pinch/borrow other peoples research/work.
Really you don't want to go down the twin SU route
The SU was a brilliant design in 1920. Single SUs were OK until the end of the carbe era but they were unreliable the pistons stick, the jets and
needles wear, the float valves wore out and leaked, on the HS variants the plastic tube between the float chamber used to fail and spray petrol
over the exhaust. and a lot of the 1970s and 1980s sub-variants especially those with the Waxstat jet were crap.
The best SU models were the ones with the annular float chambers built into the main body of the carb, similar to the the Zenith-Stromberg CD and
Bing
Twin SUs are a constant fiddle to keep running properly even when jetted perfectly.
A single 1.75 SU would be more than adequate for your engine and a lot easier to get running properly
Also if you fabricate your own manifold don't forget to build in a balance tube to join the two inlets.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
. 
obviously you are a diesel lover... 
