10 replies to this topic

[cal844]

Hi folks,

Car is a 1980 clubman Estate 998cc A+ with a stage one kit and Rc40 Twin box exhaust.

Is it possible to recentre the jet on a HS4 carb without having to remove the carb from the car?

Jet is at roughly 4 or 5 mm below the bridge on an AAU needle.

I don't want to use the mixture nut to lift the jet unless I really have to!
As the car is currently driveable


The issue I have is that the engine won't run sweet under cruise (60 mph is our usual club cruise Speed) it'll run nicely at this but if I drop to 56(behind a lorry) or go upto 65 it judders like its running lean BUT Co at idle is 6.0% (I'd like to get this to 4.5 or 3.0% as per the MOT emissions limits).

So, to recap can I raise the jet whilst not altering how the car drives? I feel like if the jet was recentred it will run better either side of cruise...

Cheers folks

Cal

Edited by cal844, 24 September 2021 - 12:43 PM.

[ACDodd]

It needs tuning with a re profiled needle, you are describing part throttle
Lean misfire. Centring is for older carbs. Later HS4 carbs used the bias spring needle design which means centring is not needed. Congratulations for correctly understanding what’s going On most people don’t have a clue how the su carb works and that includes a lot of people on this forum. It is entirely possible to modify the needle such that at idle emissions will pass MOT and then go on to fuel perfectly across the rev range. Unfortunately this is not usually possible with off the shelf needles. Sometimes you can get lucky, but usually a custom made needle is the order of the day. Avoid filing flats on needles that demonstrates rank amateurism.



Ac

Edited by ACDodd, 24 September 2021 - 03:48 PM.

[PoolGuy]

I’m guessing you saw the pathetic post on the other channel 

[ACDodd]

No.

Ac

[cal844]

Thanks AC!

So I just need to book a rolling road and tune the needle in a pillar drill (as per SU carb manual, which I have on the shelf)

It's very confusing as I drove the car today and it runs well, as long as I use more throttle to get the needle to change.

As an additional question, what is the next richest Needle (so I can purchase one for the Rolling road) to the AAU?

I have a decent RR operator within 20 miles of me so will see what I can come up with!

Thanks and regards

Cal

[ACDodd]

A decent rolling road would have needles! There is unlikely a need to change yours it simply needs a touch of Reprofiling to fuel properly.

Ac

Edited by ACDodd, 25 September 2021 - 06:04 AM.

[cal844]

A decent rolling road would have needles! There is unlikely a need to change yours it simply needs a touch of Reprofiling to fuel properly.

Ac

Brilliant thanks for the advice 😁

Regards

Cal

[PoolGuy]

No.

Ac

That’s a shame, you might have found it interesting. 

[johnv]

 

No.

Ac

That’s a shame, you might have found it interesting. 

 

Link then?

[PoolGuy]

Link then?

 

 

 

Sorry, I wasn't sure if it's allowed, but as you've asked http://mk1-forum.net...=286150#p286150

[ACDodd]

Yes I was already aware of that thread. I always like people who think they know
Better, and can’t even get their facts correct. There are 86 needles for bias spring 0.090” jet applications and precisely because of this there is not one for every occasion. There are also 165 needles for the bias sprung 0.100” applications and again there is not any that I could use for every application. If there were I would buy and use them as it would save me time. These profiles were developed for every type of production car and not all for the a series applications. It’s simply a fallacy to say a profile Will be available.

Secondly the production tolerances on the needles are much wider than the usual mods needed, so frankly I can’t even begin to Comprehend the statement that there is a needle off the shelf for every application.


Ac

Edited by ACDodd, 25 September 2021 - 10:12 PM.