HS6 Carb Conversion

[Tim30]

I have a 1966 TR4A which has been running on Strombergs. I've recently acquired a pair of HS6 SUs. The carbs came off a working TR7 not long ago and seem to be in reasonable condition. I'm gathering various parts to give them an overhall, new throttle butterflies (wihtout anti-run on valve), converting waxstats to standard jets, new gaskets etc. The one area I'm struggling to know what to do with is the metering needles. I've spoken to a few of the leading suppliers of parts for carbs, but all I'm told is that they can't recommend one, a test would need to be done on a rolling road. I accept this but would like to start in the right ballpark! These HS6s have sprung needles, unlike the original HS6s fitted to the late 4a's. My engine was rebuilt recently with a fast road cam, and 87mm pistons. Aside from that, I'll be fitting K&Ns but the rest is standard.

 

Has anyone any experience of needles for this carb? I've no idea if the standard needles used in TR7's would suit. I've been told by Burlen Fuel Systems that I can convert them to fixed needles. Any pointers would be much appreciated. I know I won't be able to get the ideal solution without going to a rolling road. If it makes a difference, I'm interested more in performance than economy!!

Thanks

Tim

[BrianC]

Many years ago I bought a pair of HS6s for my standard 1991cc engine in my previous TR3A and subsequently transferred them to my TR4A with standard 2138cc engine. As far as a can remember, the needles were SM and worked fine on the standard engines with cast iron manifolds. However, on my current TR3A, when I had the head converted by SAH to Stage 3 with their 4-branch manifold, I had to change needles in the carbs to RF (although I think these are H6s - will check next time I'm in the garage ). I assume the needle profiles are the same irrespective of whether the carbs are H6 or HS6, but I'm sure someone with far more knowledge will correct me if necessary.

[67_gt6]

http://www.mintylamb.co.uk/suneedle/

 

I use TW fixed needles on a standard 2138cc with K&Ns

 

SM is a richer taper to begin with, and also has a richer curve in the midrange than TW

 

I thought the TR7 had a shorter dashpot than the HS6s on a TR but I may be wrong. Not against rebuilding carbs etc but if you have gone to the expense of rebuilding your engine I would have thought it might be worth getting a good rebuilt pair from Andrew Turner for a couple of hundred quid and knowing that you are starting off on a solid basis.

 

Andy

Edited March 11, 2010 by 67_gt6

[Roly]

My HS6s had TW needles in when I bought the car.

On the rolling road the mixture was seen to be seriously weak, so while I finished the running in BDL needles were fitted.

When I returned to ther rolling road after finishing my running in ( about 1200 miles had been covered since liners and pistons were replaced) the BDL needles were changed for BAJ and now the car seems to run wonderfully.

It seems that the rolling road is the way to go to have ones own car tuned.

My HS6s have the spring loaded needles.

 

Roly

[Ade-TR4]

There are different needles for fixed/ biased. I think mine are TW (fixed) on stage-2 head/ engine.

 

Check out http://www.mintylamb.co.uk/?page=auto.htm for their SU-Needle comparison program (only nerds need apply...)

 

Ade

Edited March 13, 2010 by Ade-TR4

[Roly]

There are different needles for fixed/ biased.

Ade

 

 

Well well, that's interesting.

Where can I find more info about that?

I don't see that info on minty.

 

Cheers

 

Roly

[Chris Hubball]

Hi Tim

 

I have a TR4 with HS6 carbs. The engine is modified Stage3 Head Road Cam Extractor Manifold & K&N Filters. I had the car set up on a Rolling Road,the guy fitted SL Needles and obtained 110bhp at the rear wheels.If you are rebuilding/modifying TR7 carbs make sure the float chambers are at the correct angle ie vertical when the carbs are fitted to the manifoid on the engine. I worked for SU Carburetters for 32 years not as an engineer but I gained some technical knowledge.I can provide dimensions for most relevant needle profiles if you need them.

 

Chris Hubball.

[Ade-TR4]

Hi Roly,

 

The later HS6s had needles that are positioned by a spring, the earier, fixed type don't.

 

Moss' catalogue has a description and gives some equivalences for the 2 types.

 

mintylamb does have the all the needles so you can compare one against the other.

 

hours of fun...

Ade

[pampa]

My 4a has rebuilt motor 87mm pistons,balanced, ported head,mild street cam,free flow exhaust,123 Elect.Dizzy and cold aibox with K&N filter. Tried to run it with original TW needles ran ? but died to a spluttering crawl at any sign of a slope.No rolling roads locally so had it tuned on an electronic scope machine which imediatly showed it was very lean at anything off idle,He changed to RF which it ran very well on but he said were a little rich up top but he prefered that. Drove for some time but fuel milage wasn't the best,so recently went to a car show in Melbourne and stayed over and put the TR on a rolling road. It imediatly showed double the required exhaust gas levels and rich right thru the rev range,he tried four or five needles before settling on RG which gave levels he was happy with,Then he started on the timing,he altered the Dizzy curve,ran the car then played with the static timing until he was happy.Car certainly runs smoother and MPG improved by 5-6 MPG on the trip home. He said as well as the slightly rich needles the timing being set incorrectly also would have been contributing to fuel consumption.I was after smooth running and reasonable fuel consumption not outright power. His main business is Carby rebuilding and he showed me a pair he had just finished Made mine look rather second hand even though they were done by a professional. Two bits of advice he gave me was use only genuine SU parts when working on carbys,he has had a lot of trouble with non genuine kits etc. and don't know what your fuel is like over there but here in Oz hsaid to use only BP and Mobil(both from same refinery)the rest is like using cats ****. Leave the old needles in an get to a rolling road or your mods will be wasted and so will your enjoyment.

[Badfrog]

Hi,

 

From my experience, biased needles are performing less well than fixed ones.

The ideal fixed needle for a standard or stage II/III engine with standard or uprated HS6 (mods on venturi and piston) will be among these:

 

standard:TW SL SM

uprated: TS RF RH RG KD

 

For a start, TW is lean, SM/SL is the best bet. Then try TS, then the richer ones.

 

I run TS needles on a pair of HS8 (eight) with 0.100 jets, standard head, double reinforced valve springs (Revington), a sort of fast road cam (set at 103°), TR4-race 45D4 dizzy with Pertronix innards and 87 mm bore with 2" 4 in 1 and pipe.

 

Cheers,

 

Badfrog

Edited March 15, 2010 by Badfrog

[Tim30]

Thanks all, much appreciated! That answers my queries and gives me a good place to start from. Inevitably a little trial and error but hopefully will get it to run reasonably well before taking to the rolling road.

 

Cheers

Tim