Hi Terry, 60 years of driving cars with SU carbs has told me that they are a fine , simple device for many makes of Brit stuff, MG, Riiey, Austin Morris etc etc…I have had other cars,really old stuff, and mounted SUs to experiment, for example, on my 1927 Rover nine, with an up draught carb…no internet then, no on line info, no one knew how I should jet it, but, eventually found the carb from a Singer Le Mans, made up a ninety degree flanged pipe bend, rigged up a choke cable and in ran right first time…luck…the next mod, a side valve ten horse Ford out of a Y Type, running on a Solex…I think…might have been a Zenith…found a secondhand downdraught SU, no knowledge of the original application, much bigger bore than either of the two inlet ports, two ninety degree water pipes flanged for me, pretty sure it was a four bolt SU, pipes about three quarters of an inch internal, but never kept data, and it ran, with a comforting smooth hiss…now, back to the HD8s, as you know , there is a annulure groove around the top for setting the depth…where do you set yours ?? I found instead of using a feeler gauge to accurately “lose” the groove into the body, I now expose the groove, in effect, weakening the mixture, this small move and the radius work no doubt improved the slow running with the UO needles…with no jet adjustment other to WEAKEN again to see how low I could go…but, as I know you are all aware, that is for my application only, I know for a fact that even with a bog standard pair of XK engines, you will not, generally, be able to jet them exactly the same for Webers, well, not if they are worn, I don’t know about straight from the factory…
How does this differ from just moving the jet? I can see that the range of mixture adjustment via the jet changes with needle position, but I’ve never run out of range–not even close to it.
Well, it just could be that the exposed groove has some effect, I don’t know, one of those areas where you can’t see what is happening, only surmise, mixture on Amals , set with needle position, I suppose thats why I like to play around with needle depth rather than move the jet, not scientific I know, and goes against the book, but it works…one other thing, with the Amal,the fuel bowl level is constant with the top of the fixed main jet,or very near, but constant… with the SU, my float settings, just one turn down from the bridge, caused fuel to well up, maybe more by luck than judgement the tops of my main jets are now exactly right for fuel level, omitting the rich "stumble "I had before…
Keith I agree that SU’s are a great carb for their time. They have limitations - for example going to higher lift cams with greater duration doesn’t agree with them, but with a stock XK engine they do work superbly. The manuals I’m familiar with all suggest that the needle is to be mounted in the piston just enough that you can’t see the groove, and no more. I don’t know why lowering it as you do would have any effect on idling, for if you set the jets as specified in the manual they will simply be lower than they would if the needle was a bit higher, cancelling each other out. I say this assuming that your carbs are otherwise in good operating condition and your floats are properly set. The level of the fuel in the jet is the same as in the float chamber and if has an effect on operation. All these things are interrelated - the depression over the bridge is set by SU as to raise a certain quantity of fuel for a given position of the piston (regulates the size of the venturi), a too low (or high) fuel level will have a significant effect on this. By the by all the 1/8" needles (U series) are essentially the same thickness in the idling position including the UO needle.
Your description of turning down the jet one turn from the bridge and getting an up welling of gas sounds like the level may be too high. As I recall (I’m away from my manuals) Jaguar recommends turning the jets down quite a bit more, and even then the gas is still below the top of the jet. The fuel level should never be higher than the top of the jet.
It’s difficult for SU’s to give you a “rich” stumble if they are operating correctly. Because of the mechanics of a SU’s operation (no accelerator pump) it can’t deliver a lot of fuel in a hurry, like an American carb or Weber can. Usually it’s a lean stumble. Not easy to tell the difference
I have been digitising my old beat-up copy
Chapter 8, 5, and 10 refer directly and only to carb mods.
my question; Has anyone done his main recommendation, which is to change to the lightest oil and damper springs possible, and customise the needles so there is no flat spots?
If so, how has that been?
(a recent poster mentioned he had done this on his V12 E-type to positive effect, but I dont think they had SU as standard)
to paraphrase (Hammill) words, this gives greater acceleration, at the expense of fuel economy
the needle profile needs to be sanded down, otherwise there would be flat spots when the piston flys up that bit faster, emulating the accelerator pump in other carbs
Everything you say is correct, a couple of years ago, I started chasing a problem that only occurred as the engine warmed up, then cleared, pretty sure it was a spark problem so went over the soidered wires in the Edis to get temporary respite, then the problem came back…here’s a scenario…start up from cold///pop bang, runs on four for a while, then picks up on one and two…solder the join on the one/six coil pack, forty miles on, pull in for gas, restart, pop bang, runs on four , gradually picks up on the other two…got it home then whipped off the ITG filter to see fuel running into the inlet tract from the one/two carb…I had just collected the car from Etype fabs in Newcastle for brake work, Uryk and I had both concluded ignition and had done the soldering there, but the drive home to the west was miserable, getting a max of 80mph with foot to the floor…fitted new needles and valves, I have no doubt the levels are a little high since then, hence my tinkering, but you know, it runs absolutely spot on at the moment, I often arrive at my destination in a convoluted way, story of my life really …my cams are high lift longer duration by the way, can’t recall the spec, not regrinds with top hats, new Kent billet cams from Rob Beere, similar to D profile I recall, ok for traffic, considering the big valves and porting, its pretty good in traffic, and responds to a heavy throttle from low rpm, better than my Webers ever did !!1
Actually, it’s a lot easier to file or stone a flat on one side of the needles. This makes it a LOT easier to check that you’ve got the same profile on all of them. You can change the profile with a lathe, but it’s a pretty small bit of metal, even with a taper attachment you will fiddle endlessly to get smooth transitions on the first needle, then you’ll spend forever trying to repeat that on the others.
With the flat, you can lay them side by side and your eyes pick up the differences immediately. The blending from station to station is also easier.
I mount the needles so that the flats all end up in the same orientation, but I suspect that this doesn’t really make much difference.
I must say I’ve never understood Hammel’s methods here. Weber’s and US carbs have an acceleration pump that dumps fuel in on sudden application of the throttle. SU’s do it by slowing piston rise with the oil and spring. This increases the air speed over the bridge more than it would if the piston rose instantly which leads to greater depression over the jet which sucks in more fuel. Lighter or no oil etc leads to leaner fueling which he tries to compensate for by filing flats in the “right” place on the needle. He provides a method of determining what position the needle is at by the construction of a small post that rides inside the piston damper and is marked with positions. To read this while you accelerate you need to be on a dyno, and because the piston rises very quickly reading the markings at different positions is difficult to say the least. Hammel says that when done properly the SU will function more like a Weber. There is no doubt that the initial push to WOT on a Weber setup is explosive compared to an SU, something I attribute to the pump, but try as I have, both in a race car and in my autocross car to find an actual performance increase with Webers leaves me in doubt that they actually change anything. Hammels advice for SU mods is in my opinion for a racer who is looking for everything.The mods are difficult to do, and without a dyno probably nonproductive.
I agree with you to a point, but when a lap time increase of one quarter second could make the difference between first and second, these guys go for it…there is a E dragster known as Red E that goes back years over here, have snuck up on them to watch them switching needles between runs,no Webers, atmospheric pressure plays a daily part, …the way a Weber introduces a jet of raw fuel in one promotes bore wash, unlike an SU ,and fixed jets at set sizes complicate a system where sometimes, as I have seen, strands of copper wire are utilised to slightly reduce an orifice…I am looking forward to seeing some tests on this latest range of twin SUs that Burlen are developing, DesHammill is still wandering the back street auto repair area of this town, his favourite phrase ?? the Jaguar XK engine is not compression sensitive, …
^ this.
I’ll go one step farther. Without extensive knowledge of how a motor works, even if you theorise correctly and adjust/modify with the utmost care and precision… unless you put the car on a dyno and actually collect empirical data you might as well just take a stack of money and grind it to a pulp…
Well.I’ll go one step further still, I have a friend who grew up with me in the forties and fifties, without any help from a dyno, by sitting, chatting, theorising and tinkering, we put together a three cylinder auto union two stroke which ran to 10500 rpm and beat everything on the stock car track here in the UK, two strokes banned the next season because of its success…I have a grandson who has degrees and masters etc etc…I expect one day he will have a mortgage and pay it off, he is pedantic and methodical, my friend and I had a basic education and own several million quids worth of land and property, we have had stacks of money and ground it up, but we have had the satisfaction of doing our stuff out of our own heads, I don’t think you guys get it at all…must be a Brit thing…
Could be.
I believe there is also a techno-generational gap between you, and your grandchildren… the immediate, ready for use, throwaway world they live in would be one major difference.
In a nutshell…I think one day it will all turn around, still a few good boys out there , an eight year old boy I met twenty years ago, his mother saw my spaniel and asked if i was a shooting man, took the boy under my and my single brothers wing and at eighteen set him up with a forty five grand unsecured loan for a fishing boat…he paid us back eighteen months later, now has two trawlers, a house, a new baby, a bi turbo Mercedes, most successful skipper in the southwest…just needed pointing in the right direction…knows his Caterpillar and Cummins diesels backwards…lets hope he passes it on…
No: it’s not a “Brit thing:” it is a function of time (pre-electronics) and a position of not having all the techy stuff (Prony brake, fluid dyno, A/F mixture gauge, etc), which I think all of us, at least those of us of “a certain age,” had to contend with.
I only ever used a garden hose (for SU balance), checked against a Uni-Syn till my ear was calibrated, ear, again to set timing, then confirmed by a timing light, and the list goes on.
We on this side of the pond share all that with you tuff Brits…
Heh heh bet you didn’t have wind up windscreens for frosty days !!!
Hello Terry, I have been reading all your comments. Please can you tell me how I tell the difference between a lean or rich stumble. My carbs are 2x HD8’s on a 3.4 XK120. The needles are UO’s but I have also tried UVO’s. I am using standard SU SAE 20 oil in the bells.The fuel bowls are set with 7/16 bar, the Jets down at 0.06, but I have tried higher up and lower down. The air volume screws are out by 5 turns to obtain a 700rpm idle, timing 10BTDC. I have the Rob Beere Cam’s designed for SU’s, which are not long duration but slightly higher lift at 0.41. The stumble has got worse since the carbs have had new bushes and new butterfly’s that close completely as they should on an HD8. The stumble is such that on level ground if I let the clutch in without any throttle the car stalls, or I have to give it throttle and juggle the clutch correctly to stop it stalling.
In neutral the engine does rev well and does not stumble, plus once going at above 1,000 rpm it pulls very well. One last thing, is that if I connect a manifold gauge to the manifold I only get about 14 inches, but the compressions are 170lbs cold. Your thoughts much appreciated on the stumble ?
Des Hammill states to test tuning with a series of “snap the throttle open by hand”…ie free rev tests
he states that even the slightest hesitation in this process means lean
he has a very detailed method outlined for altered tuning cars
Cant recommend his book enough
Hi John As SU’s don’t have an accelerator pump a stumble, if it’s a carb problem is almost always a lean stumble. You can have somebody stand behind your car as you start off. If it’s too rich and stumbles you should be able to see a puff of black smoke from the exhaust. There will be no like symptoms if it’s too lean. I looked at your cams on Beere’s site – they are quite unusual in my experience, in having 235 degrees duration as opposed to 252 stock. In theory less duration gives higher intake vacuum, so 14 inches seems a bit low. I’d contact Rob and ask him what it should be. You seem to have eliminated retarded timing, a vacuum leak, and bad compression, so the next step is to check valve timing – in particular the intake. Bear in mind that there have been incidents recorded where the notch cut in the cam sprocket into which the timing tool fits was not correctly cut. A quick visual check can be made at TDC on #6 on the firing stroke – the lobs should be equally “flopped” over, each pointing outwards.
You’ve got a unique carb and cam set up so advise is hard, but here’s my thoughts. First I’ve never had an equivalent problem to yours, that is, a problem only when you start off, but I do have an ongoing stumble/flat spot when running at normal speeds, that manifests itself with a small jerk when I initially apply power. In my case I believe that the piston rises too fast leading to a drop in vacuum over the bridge. Going to gear oil really helped – that may be extreme, but it’s not as heavy as you think. The object is to slow the initial rise – heavy oil will do that without affecting the ultimate position the piston rises to. (My problem, I believe is that the bore in the piston rod is worn and even with new dampers offers little resistance with light oil.) You might try 20-50 and see what happens.
Five full turns on the air volume screws seems excessive – typically my experience suggests half that, but I confess I don’t know what’s appropriate for your combination. Your engine at 3.4l will need to flow more air and fuel through each of 2 carbs than a 3.8 or 4.2l would with each of 3. This should yield a higher air speed over the bridge, and a greater vacuum there, so should it be a leaner needle??? Are you running some form of air cleaner? A UO needle is a racing needle and very rich at higher rpm in an E Type – don’t know how it would affect your engine carb combo. Are you using it for a particular reason?
The problem can also be ignition – you might try a one stop hotter plug.
I’ll keep thinking on it.
Fr
om:
Johnball via Jag-lovers Forums
Ian Ruffley, who built SU’s for racing, told me that the Healey people who were racing got him to bore 2" SU’s out to 2 1/8" and claimed it wa s beneficial. And SS 100 here that was set up by the factory for competition, had among other mods, the 1/2" carbs bored out to 1 5/8"
It depends on circumstances. Alcohol fuels need more volume so H8s had larger foat bowls,2 1/2" SUs had optional twin 3" float bowls… found that th3/8" BSPF banjos used on hydraulic machinery interchanged but gave more petrol flow { 4 holes instead of 2 as standard and wider banjo.] Did it help? who knows.
Some years ago, I fixed a lean stumble on a Stromburg-equipped XJ6 by simply installing tiny “E” clips above the brass damper pistons, eliminating virtually all lag between initial piston rise and the damping effect…this caused the damping action to “come in” as soon as the carb piston started to rise…worked very well.