NADA HS8 Question

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petergh3

New Member
Hello,

I've just done a minor rebuild of the HS8 carbs on my NADA 1968 TC--just cleaning, replacing jets, needles, and gaskets. The HS8 isn't covered in the WSM, but I have a special supplement for US cars.

These HS8s have a "jet compensator" that keeps the jet from going too high, a primitive form of emission control. It's basically a big washer. Assuming the Feds don't read this, can I remove the compensators and tune them as one would other HS8s?

That said, I went to Moss Motors, and learned that no other HS8s have the jet-adjustment setup of the TC. All others apparently are adjusted via a big nut on the jet, not by screws, as on the TC. This makes it difficult for a dullard like me to translate instructions, such as from one of the SU books. All instructions I've seen for centering the jet involve adjusting the non-existent big nut.

Any advice or philosophical guidance welcome.

Gordon.
 
Obviously a dud of a topic, but I have found that the XJ6 of the early '70s used the same HS8, including the compensator. I've decided to keep the compensators because the manual supplement I have assumes them as part of tuning, etc. I'll post a picture here, if that's allowed.

Gordon.
 
Here is a view of the jet prior to replacement. Note that the adjustment is done remotely, but a screw up top, rather than directly via a big nut, as on most HS carbs. You can also see the compensator, that big disk just above the spring. By the way, only some Rolls Royces and some Jaguars use this kind of spring, as far as I've been able to tell.

The second picture is of the more common kind, with the nut.

Gordon.

HS8Jet.jpg


Hs-type.jpg
 
Probably they engineered this solution for mixture setting because it is impossible to reach under the carbs in a TC!
 
Hi Gordon, Probably best to stick with the factory settings, changing jets and trying to match needle profiles is a real pain. the lever and spring set up is standard on TCs its almost impossible to get fingers or tools under or around the jet openings at the base of the carbs. instead of the big nut you have a jet adjuster screw to raise or lower the jet in relation to the bridge (easiest to do this with the dash pot off) follow instructions in the WSM and bobs your uncle.
Im sure youv'e already done this but give the fuel bowls a really good clean and check the floats, mine had tiny pin holes and had some fuel in them causing them to sink!! Needle valves can stick too.

Regards

Marcus
 
marcusk said:
instead of the big nut you have a jet adjuster screw to raise or lower the jet in relation to the bridge (easiest to do this with the dash pot off) follow instructions in the WSM and bobs your uncle.
Click to expand...

IIRC the WM is wrong on the adjustment. Whatever way it tells you to turn the adjuster to richen or weaken the mixture, it actually works the opposite way around. Worth checking before you start, as I'm sure that came to light when I was looking at a carb with the book in front of me.

All I would say is the ones I was doing weren't NADA, just the ordinary HS8's if that makes any difference.
 
Thanks for your replies. For what it's worth, I've been able to center the jets by loosening the bolts that hold the guide in and moving the guide around until the pistons fall freely.

It's been a surprise and a disappointment to learn how rare are the parts for these carburettors. My usual Rover parts guy doesn't have them, Moss just laughs at me, and even Joe Curto, the acknowledged US SU guru, had nothing, even something as simple as throttle return springs.

Gordon.
 
Hi Gordon
Must be really frustrating, when i rebuilt the carbs on my 1968 TC last year i got all my parts and quite a bit of useful info from a company here called Burlen Ltd should be easy to find online, they have a really usefull seach tool where you can find parts by carb no. which can normally be found on a small tag, normally on the float chamber (AUD....) or by vehicle, some good pics as well ,look at ( view full spares list)
Hope this is of some help.

Marcus
 
Thanks for the lead. I thought my 3-Litre was the rare one here, but at least it has a plain old HD8.

Gordon.
 
Here is an update on my carb situation. I installed them easily enough, and the car started right up, which is unlike it. The new problem is that the idle is around 2400, and none of the adjustments have any effect, i.e. the jet and idle adjustments do nothing. It's possible that I re-assembled the carbs improperly, although I used another set as a guide.

The odd part to remember is that I have a NADA model with primitive emissions arrangements, as noted in previous posts.

Any thoughts welcome.

Gordon.
 
At that speed, it definitely sounds like one or both of your throttles are stuck open, or their closure is obstructed. First move for me would be to get the dashpots and pistons out, and peer at the throttle discs to see how and where they are obstructed.
An air leak might cause a higher idle, but not that fast. It needs air and fuel, not just excesses of air, so it definitely sounds like something is awry with the actual throttles.

Michael
 
If you removed the throttle discs and shafts, if you didn't align them properly during reassembly they are not able to shut off completely.
 
Thanks for your responses. The throttles were fine, so I narrowed it down to the jets not coming up high enough (over an eighth of an inch below flush). I solved the problem by removing something called the "compensator tab" from each carb. Although it's pictured in the NADA supplement, no other TC guys here had heard of it, and it just seems to be a primitive emission-control thing gone wrong.

I still have to fine tune and synchronize, but at least my idle is in the appropriate range.

Gordon.
 
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