SU experts, please chime in...

[No Quarter]

I think I know the SU carb, I've read the Haynes SU book, and worked on SUs on XJs and a Mk2 without problems. But today I put fuel throught the 3 SUs on my E-Type for the first time, and all 3 leaked seriously.

Background, this car was bought in pieces more than 15 years ago by the seller. He bought a socalled complete car, so 3 SUs were included, but I know for sure that at least one of the SUs have never sat on an E-Type. I spent weeks and almost €1500 getting parts for proper fuel line and choke and throttle, so from the outside, they look right and have functioning choke etc.

Today they all leaked fuel, it dripped from large hole (no air filter on yet). If I'm right, that means I don't have a leaking float chamber needle, but the fuel flows through the float chamber arms, up through the diaphram/jet? Is this a mismatched jet and needle, so that they don't close when piston is all the down? If so, is this carb tuned by changing needle, jet or both? And if this is the problem, then I need to buy new jets/needles, what sizes should I buy for a 63 3.8?

Another theory is that I have adjusted needle/jet wrong. I've adjusted jet to be flat with floor (piston out), and later 2½ turn clockwise.

Any other ideas, on these SUs?

[mgcjag]

Sounds like the float chamber needles arnt closeing so the fuel overflows up the jet....could be the floats not adjusted correctly or you have floats that are filling up with fuel and not floating or the needle valves arnt sealing correctly...Steve.....note that you should be able to remove the su cover and piston...then with your floats adjusted and working correctly the fuel level should be a bit below the top of the jet

[No Quarter]

Ah ok, so if float chamber all in all works correctly, fuel leve should stay so low that nothing flows up (high enough) in jet to run over. I'll have a look, thanks

[mgcjag]

Yes..correct..fuel should be about 5/16in down the jet.......Steve

[No Quarter]

Ha, fixed. A bit embarrasing maybe, but we're all friends here , so I'll share.

The float chambers were of course full of air at the start. That air is supposed to escape at the top through serrated gasket, and until it does the fuel/float can't get high enough to close needle. And that just took time. I helped things along by loosening all the top float chamber thingies so air could escape, and now all carbs are leak free

[mgcjag]

Great...Steve

[DWW]

maybe the float needle are installed up side down.

[rfs1957]

I had my fingers poised over the keyboard to predict just such a problem, but you got there before me !

I had a similar problem on a DB5 over 40 years ago, when I was very wet behind the ears, and it was a lesson that has stood me in good stead ever since.

Yes, those scalloped washers are there to enable the air to get out down the SU breather and overflow pipes.

No air-way exit and the fuel pressure just generates compressed air in the float chambers, so the fuel squirts up through the main jets and drowns everything.

[No Quarter]

I bought and was about to install 3 new dashpot dampers for the 3 SU HD8s. I noticed that the new dampers have no vent holes, and the old ones, 2 had a hole, 1 had not. What's right?

Reading the Haynes book on SUs, they write that there should be no hole if "suction chamber is vented with an internal drilling." No picture or drawing. How do I find out if the 3 HD8s' suction chambers are internally drilled?

All 3 pistons go up and down fine when tested by hand, engine off.

[DWW]

The vents are on the top of the bell

[No Quarter]

Perfect Danny, and I've now had all three suction chambers off. They all had that drilling, which I guess is quite interesting since two of them came with dampers with a hole in, so they have had double ventilation, not good for lifting pistons...

I'm into the details now, but on carbs, the details can matter

Filling with damper oil. My Haynes SU book says the following:
-Fill damper with engine(?) oil to 13mm below top of hollow piston rod
-If non-dustproofed=hole in damper top, fill 13mm above top of hollow piston rod
So on mine, 13mm BELOW. And I plan to use the special SU damper oil I bought long time ago

Finally, the new dampers I bought, no vent hole, so that's fine, but there's only 1-2mm thread on them. In the suction chamber the threads go far down, so why so little thread on the damper head. And If I use the gaskets, I'll have only about 1mm of thread. I'm tempted to go without that gasket, and keep an eye on things. Or do I have wrong dampers? Other dampers sold online look the same IMO. Are brass better (another €150)?

[Geoff Allam]

Do your old dampers have more thread than the new ones? If they do I would just plug the holes in them with epoxy. and reuse them. If you really want to use the new ones make up gaskets out of very thin plastic sheeting (oil resistant) to get maximum thread engagement.

[Heuer]

Up until December 1962 the dash-pot caps were made of brass with the letter 'O' stamped in them. After this date the dash-pot caps were made of black plastic. The brass caps were stamped with an "O" to indicate they had their pistons reduced in length from .378" to .308" to improve performance. The SU part number for these modified damper caps was AUC8114. The damper change was quite common as can be seen from this MG service bulletin:



The brass caps were cadmium plated to give them a silver colour, they were not originally the bright polished brass now seen on almost all restorations. A concours correct car would have plated damper caps although there is evidence to suggest later cars (mid 1962 onwards) had plain brass caps

[No Quarter]

Development, had the engine started today for the first time :-)

It ran erratically, now a lot better after adjusting the SUs. In an effort to get it 100% right I read the how-to on SU's homepage. And it led me to these questions:
1. On our 3.8 with triple SUs, is it correct that we don't have "throttle adjusting screws" and that all idle adjustment is therefore done on the "slow-running valves"?
2. All drawings on that webpage, showing multiple carbs, show the throttle shafts clamped together directly. But on our cars, they're not clamped together, and they are only aligned by the linkages up to the throttle slave shaft, correct? Which makes aligning the three throttle plates less accurate, right?

Anyway, that's what I got in the box when I bought it. Just want to make sure it's right for a 3.8...

[bitsobrits]

1. correct
but I would add there is the fast idle adjustment screw used when the choke lever is engaged.

2. If I understand your question, yes, the throttle plates can be tricky to get exactly synchronized due to the inherent free play in the individual drop links from the throttle shaft to carbs. But it should be possible to get them dead on given enough tries.

[Philk]

1. correct
but I would add there is the fast idle adjustment screw used when the choke lever is engaged.

2. If I understand your question, yes, the throttle plates can be tricky to get exactly synchronized due to the inherent free play in the individual drop links from the throttle shaft to carbs. But it should be possible to get them dead on given enough tries.

... and absolutely essential that, when the choke is set to off, there is a sheet of paper's thickness gap between the E-Type's choke adjustment "cam" and the the bottom of the fast idle adjustment screw on each carburettor so it does not interfere with normal idle speeds.