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Recentering the jets?

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Luke_Healey

Luke_Healey

Jedi Trainee
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I keep running into "recenter the jets" when reading about SU carbs like mine that might be fuel-fowling due to a needle physically touching the jet.

However, I have not found any instructions on how to do it? Some vague reference to loosening the top nut under the jet, but that's about it.

Any help would be appreciated!

For some reason, the way the bolts on the carb-to-manifold mount stick out, my rear dashpot cover binds up and is not easy to pull up and away from the carb and I fear that may have bent the needle ever so slightly.
 
Hi Luke, IMOP the best way to align the jet is to get an alignment tool from the usual suspects. Otherwise it can be a frustrating first time exercise.---Fwiw---Keoke
 
Hello Luke,

to centre the jet, adjust the jet to it topmost position, i.e. flush with the bridge. This is easier done with the dashpot\piston assembly removed so you can view the jet. While the piston is out, check that the needle is installed so that the shoulder is flush with the bottom of the piston. Replace the dashpot and piston. Slacken the jet tube clamping nut, which is above the mixture adjusting nut and while pressing down on the piston (remove the damper) re-tighten the jet tube clamping nut. Reset the mixture nut 12 flats down, which is the nominal mixture setting. Lift the piston and release and it should drop slowly and come to a stop with a click. If so that is a good job.

(by the way it is not fuel fouling that is the problem but that the piston will not rise and fall as it should due to the dragging. Later Hs carburettors had a spring biased needle which was always in contact with the jet tube, I don't know why this was done?)

I don't understand why the mounting bolts (studs?) foul the dashpot when you remove it, that is not how it should be. If you have bent the needle you probably will need to replace it?

Alec
 
I'll give everything a looksee. The top nut on the rear carb was physically loose when I first got the car. Also, the lower nut was turned out about 8 turns.

So things were out of whack from the getgo.
 
Keep in mind you may adjust this 10 or 20 times to get it perfect.
 
Or get the "anvil" tool and do it once.
 
OK folks, with the piston down and the seat loose, slowly tighten the seat droping the piston each turn and only tighting with the needle/piston down.

If you are careful it can be done first time every time.

Not rocket science guys, just a bit of slight of hand.
 
well, I'm fairly mechanically talented and I can't get it. Each time I tighten the jet nut the whole piston freezes into place. There is obviously a sweet spot where it will fall down and clunk like it's supposed to, but as soon as I tighten the nut it locks up. It's now starting to make sense why the previous owner left that nut looe.

What gives?

I pulled the needle out of the piston and rolled it on glass and it's straight as an arrow.

I did notice that the paper gaskets on the dashpot caps are missing (verified looking at my other car.) Might that slight gap difference be causing my headaches?
 
No, Luke, the gaskets have nothing to do with it. The damper shouldn't even be in there when you center the jet. It could be that the jet bearing is worn, or you may just have to keep fiddling with it.
Are you following the book procedure?
Jeff
 
Well DR E and Keoke told you to get the alignment tool and quit frustrating you self so!---Keoke /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
 
I don't have a book that mentions how to do it, but I read about it online today. The online thing I read is not working (jlaird's method, basically.)

I assume this is the tool?

su_centering_pin_lores.jpg


It looks like it fits down into the jet with the choke engaged as an alignment pin?

I will have to hunt one down.
 
Yup-yup. Or one similar. It does exactly that: fits into the jet 'sleeve' to hold it centered as you tighten the nut. With piston and chamber snugly in place to "fix" the alignment correctly. The "SU Tuning Kit" VB or Moss sells has one in it. Needle shoulder O.D. at one end, jet sleeve I.D. (less a thou or two) on the other.
That one tool alone in the kit is worth the money, IMO. Makes the whole process a two minute exercise.
 
hehe, but you got to mess while waiting for the tool.

Only tighten 1/8th of a turn at a time and wiggle it center each time.

It really does work, have been doing them off and on since 1963 that way.
 
If once you get the alignment tool, you still have the same problem, heed Jeff's advice on the jet bearing, sometimes they wear funny where they seat against the carb body and no amount of special tools or any other tricks will cure the problem. If you have any spares carb lying around, you can swirch jet bearings to see if that does it.
 
I have a footnote to centering the jets that may help a bit (later).

I paint mark the ears where the suction chambers attach to the carb bodies. This is more important after you get the jet centered properly. In subsequent servicing, the paint mark assures you put the parts back together in the same orientation and not 180 degrees out of phase... possibly loosing that jet centering.
 
Dr E -

when you say the jet centering tool comes in the su tuning kit, do you mean the carb rebuild kits that moss/vb sells?

about to order a rebuild kit to do my carbs and want to make sure i get the tool.

Thanks,
Ed
 
I couldn't find that VB sells the kit, but Moss does:

BAC-438.gif


Item B: SU Tool Kit, part number 386-300


I already have mine in the mail. I tried Jlaird's method with the 1/8th and wiggle and it jsut would not go. It always seats in exactly the same spot no matter which way I apply pressure to the thing.

Here's to hoping the alignment tool will work. I may have to buy a new jet bearing set.
 
For sure they can be a pain.
 
Hello Luke,

is the jet screwed up to the top while you tighten the jet tube and are you checking with it screwed down two turns? There is no need to keep lifting the piston as you tighten, use a screwdriver or similar to push the piston down as you tighten the jet tube.
As it happens I have never heard of the 'special tool' or ever used one needless to say.

Alec
 
The few time I have had issues with centering jets, I slowly tighten the centering nut while repeatedly lifting and dropping the piston a short distance. This allows the needle to continually work to re-center the jet as the nut is tightened.

As mentioned by Piman, all this has to be done with the jet tube up and flush with the bridge. If there is a tiny (and I mean tiny) bit of binding when the jet tube is all the way up, remember that the running position (2+ full turns down) will have clearance... and it's probably centered well enough.
 
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