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SU HS6 on GT6


Richard Heath
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Hi

I have a Mk2 GT6 with a 2.5 engine and fast road cam. The car has been running on Stromberg 150 carbs, which are clearly not right for the engine, and a V3186 manifold (not sure what that's from). I have acquired a pair of Dolomite Sprint SU HS6 carbs that I intend to fit, but wanted to check a couple of points before making any expensive mistakes! The Strombergs pretty much touch the bonnet, so the SUs wouldn't fit the current manifold without fouling. What is my best option? My understanding is that the standard GT6 manifold would work clearance-wise if adapted to the larger carbs, but I gather they aren't well balanced, and there also don't seem to be many around. Are there other options? Any advice on needles and air filter also gratefully received!

Thanks.

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You need the GT6/vitesse inlet manifold. Sorry, no way round it. Though a picture of yours would help identify what you have 

I have done a couple of manifolds, I used a cone-shaped burr in a drill to open the manifold out to match the carbs. I have mounted the carbs in 3 ways too. (1) use the adapter plates from Moss (2) blank the drilled threads with some threaded ali bar, file flat and then redrill to suit the carbs. (3) slot the carb mounting holes. The last is the easiest and possibly best.

My filters were K+N elements mounted inside a std GT6 airbox that had been modified to suit the carbs. I also used stub stacks, very worthwhile. Do not underestimate to advantages of a cold air supply.

Needles, Sprint are a very good starting point. A trip to a rolling road will be needed to get everything spot on though. 

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Hi Richard,

Clive has laid out the options for fitting HS6s to a GT6. I used a home-brewed version of his Option 1 when fitting Dolomite Sprint carbs to my 2.5 litre GT6.

I made a pair of adaptors from 10mm alloy plate to fit to the original GT6 inlet manifold. The adaptors attach to the original Stromberg mounting points using countersunk Allen headed screws. The adaptors are drilled and tapped to take Dolomite Sprint rubber carb mounts, onto which the carbs are bolted. I made the adaptors so the carbs would sit a fraction lower and further rearwards, giving the front carb clearance of 15mm under the bonnet, which has proved fine. The mouths of the adaptors and the entrance to the inlet manifold have been opened out gradually for the larger choke size of the HS6s.

Throttle and choke linkages are homemade, using SU cranks on the ends of stainless rod and a throttle lever bought on eBay. The airbox is also homemade and conceals a pair of stub stacks. The snout on the front of the airbox draws air via an angled cone K&N filter, which is far enough forward to avoid the heat around the exhaust manifold. Without the airbox, mixture was so weak that the car was almost undriveable.

My engine has a modestly gas-flowed cylinder head and Chris Witor's CW3021 fast road cam. I started with BDQ needles and yellow dashpot springs from the Sprint but found hesitation around 3,000rpm and roughness on part throttle, as if the mixture was too weak. On Chris's advice, I've fitted BFZ needles with red springs and it now runs beautifully throughout the range of rpm and throttle openings. I'm sure a rolling road session could find a little more performance, so I plan to do that sometime.

Hope this helps.

Nigel

DSC_5106.JPG

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Clive, Nigel

Thanks both for the very comprehensive and helpful replies. Just need to find a GT6 manifold! Here's a picture of the current setup as requested Clive. Given the comments you both make, I am wondering how much of the current running problems are down to the K+N filters sucking in hot air. That might explain why I seem to be able to get it revving OK with the bonnet up, but things going downhill when it's on the road.

IMG_2824.thumb.jpg.e412cfc435b40447953b2174755cb56c.jpg

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looks like a 2000 saloon manifold to me. 

So yes, gt6/vitesse manifold is required. 

Regarding running, with the bonnet up (I hope) you are not driving the car so no load on the engine, and it won't really notice if the mixture is way off. And I suspect that is teh issue, though do not discount the timing (ie is teh distributer giving plenty of advance with revs??)

My 2.5 vitesse had MASSIVE issues when I first ran it. I rebuilt the engine, carbs etc. It all came from a 2500s (I know, the low power 2.5) but it would not run above 3000rpm, and struggled at over 2500rpm. Turned out the 2500s airbox was so restrictive the engine was fitted with very weak needles. Using a modified vitesse airbox, which allowed far more airflow, meant I needed richer needles. As it turned out, dolomite sprint needles, which was handy as I had some! So depending on teh source of teh carbs, teh needles may be way off ideal. But afraid I have no idea what stromberg needles would be required.

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Clive

I am running Stromberg 175CD2s on my 2L GT6 Mk2, which were already on the car when I bought it in 1977.

They were fitted with steel SAH/TT adaptors with I have since removed, preferring to have the flanges of the manifold built up in ally and drilled to accept the 4 stud pattern of the 175s. This has been a successful mod and I feel provides a more positive fixing than the adaptors. See pic. Having said that, the adaptors are available should you be interested and will be be suitable for the HS6s as they have the same fixing pattern as the Stroms.

I have opened out the original GT6 manifold and had the internal passages reworked to correct the imbalance on #2 and #5.   The original manifold has a casting number of V3135. see pic

A cold air box incorporating the K&N filters, similar to Nigel's is next on my project list

I also have one of the later long branch manifold with a pair of HS6s, but that would need modification to remove the inbuilt 7 deg angle required for the saloon.

Ian

DSC_7868.JPG

DSC_7869.JPG

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2 hours ago, Ian Foster said:

Clive

I am running Stromberg 175CD2s on my 2L GT6 Mk2, which were already on the car when I bought it in 1977.

They were fitted with steel SAH/TT adaptors with I have since removed, preferring to have the flanges of the manifold built up in ally and drilled to accept the 4 stud pattern of the 175s. This has been a successful mod and I feel provides a more positive fixing than the adaptors. See pic. Having said that, the adaptors are available should you be interested and will be be suitable for the HS6s as they have the same fixing pattern as the Stroms.

I have opened out the original GT6 manifold and had the internal passages reworked to correct the imbalance on #2 and #5.   The original manifold has a casting number of V3135. see pic

A cold air box incorporating the K&N filters, similar to Nigel's is next on my project list

I also have one of the later long branch manifold with a pair of HS6s, but that would need modification to remove the inbuilt 7 deg angle required for the saloon.

Ian

DSC_7868.JPG

DSC_7869.JPG

That's a neat installation with the big Strombergs. It's always stated that Strombergs have fewer needle choices than SUs, so wonder how easy it was to find the correct needle.

The saloon long branch inlet manifold needs more than just the 7 degree angle eliminating. The carb flanges are higher in relation to the cylinder head than on a GT6/Vitesse manifold. Even when cut and rewelded to bring the carbs to horizontal, the front carb is too high for the GT6 bonnet to close. I found that out the hard way!

13 hours ago, clive said:

looks like a 2000 saloon manifold to me. 

So yes, gt6/vitesse manifold is required. 

Regarding running, with the bonnet up (I hope) you are not driving the car so no load on the engine, and it won't really notice if the mixture is way off. And I suspect that is teh issue, though do not discount the timing (ie is teh distributer giving plenty of advance with revs??)

My 2.5 vitesse had MASSIVE issues when I first ran it. I rebuilt the engine, carbs etc. It all came from a 2500s (I know, the low power 2.5) but it would not run above 3000rpm, and struggled at over 2500rpm. Turned out the 2500s airbox was so restrictive the engine was fitted with very weak needles. Using a modified vitesse airbox, which allowed far more airflow, meant I needed richer needles. As it turned out, dolomite sprint needles, which was handy as I had some! So depending on teh source of teh carbs, teh needles may be way off ideal. But afraid I have no idea what stromberg needles would be required.

Agreed, that does look like an early Mk2 saloon manifold.

As for the 2500S engine running weak without an airbox and filters, Chris Witor - the guru when it comes to big saloons - has commented that without an airbox, the engine runs very weak and has such a massive flat spot as to be undrivable. I would expect finding suitable needles for SU HS6s will be less of a challenge than setting up Stromberg CD175s.

Nigel

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Nigel et al

Not much info is available to assist needle selection on the 175s and probably fewer options than with SUs, but I do seem to have dialed in an OK set up, with good delivery across the rev range and good mpg (35+ on both the 2016 and 2018 RBRRs). The car came with 2D needles (bit rich)  and I ran 2Hs for many years (I think a Triumph Tune Stage1/2 recommendation), but currently running 2Es.

Rolling road and/or wideband monitoring would be beneficial I am sure.

Sounds as though I need to put that long branch manifold and HS6s up for sale!

Ian

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On 29/09/2020 at 22:37, clive said:

looks like a 2000 saloon manifold to me. 

So yes, gt6/vitesse manifold is required. 

Isn't one of the saloon manifolds slanted? I seem to remember this from my own GT6 which had a saloon engine way back, but I'm not sure if it applied to the big 6 engines or just the four cylinders, and could be remembering incorrectly.

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On 28/09/2020 at 22:13, clive said:

I have done a couple of manifolds, I used a cone-shaped burr in a drill to open the manifold out to match the carbs.

Would something like this do the job?

https://www.ebay.co.uk/itm/Carbide-burr-12-X-25mm-Pointed-Cone/143028610775?hash=item214d2b66d7:g:MXYAAOSws9dc7UFI

Slightly nervous of messing this up, so any advice appreciated!

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Richard

Shout if you are interested in the TT adapters. If opening out the manifold, you would fit the adapter a blend to that.

An insulation spacer is usual between carb and manifold. Burlen have several thicknesses available.

You might want to check clearance to the wheelarch before committing as with K&Ns on my GT6, it was a bit tight.

Ian

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4 hours ago, Colin Lindsay said:

Isn't one of the saloon manifolds slanted? I seem to remember this from my own GT6 which had a saloon engine way back, but I'm not sure if it applied to the big 6 engines or just the four cylinders, and could be remembering incorrectly.

Yes, the 2500S etc manifold was teh slanted one, nice long runners and probably the best inlet. Also used on US versions of the TR6 (I bought one, and despite the internet rumours, that too was slanted)

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1 hour ago, Ian Foster said:

Richard

Shout if you are interested in the TT adapters. If opening out the manifold, you would fit the adapter a blend to that.

An insulation spacer is usual between carb and manifold. Burlen have several thicknesses available.

You might want to check clearance to the wheelarch before committing as with K&Ns on my GT6, it was a bit tight.

Ian

What's the thickness of the adapters Ian? I haven't sorted filters yet. Would a standard airbox (modified for the SUs) still fit?

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Richard,

If you looking for a manifold I've got a couple shortly to go on Ebay - but I'd be happy to sell direct.

I've got;

311749/V3235 'GT6 type' - with central mounting for accelerator cable bracket

308671/v3153 'Vitesse' type - for bracket mounted on rear end of manifold

Do message if you are still needful of a manifold.

Cheers

Chris

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To me looks like the two adapters are the same so I reckon there to adapt HS2 SU carb which has vertical mounting studs to the Stromberg RH offset stud mountings.

Don't know if they would fit a Strom 150CD mount, what's the dimensions diagonally across similar mounting holes.

 

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12 minutes ago, Ian Foster said:

Colin

I don't recognize those. Would need measuring against various carbs to confirm.

Ian

The hole in the centre matches a Stromberg 125; too small for the 150. The mounting holes also line up with the Stromberg.

The other two line up with the mounting holes for a SU taken from a Morris Minor, which would make it an HS2 AUD13, also 1.25 inch. 

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