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TR6-Converting US stromberg carbs to SU HS6 carbs


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My first post here.

Can anyone point me to a definitive guide on converting a 1976 US spec TR6 with Stromberg carbs to SU HS6 carbs? It is actually work I carried out some 25 years ago (pre-internet) but I would like to compare with someone more knowledgeable than myself.

Thanks.

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The Zenith-Strombergs have a vacuum-draw port on their front (SU's do not have this feature) that is used to relieve pressure from the engine, via the rocker cover nipple.

 

Relieving pressure from the engine, in this fashion, prevents engine pressure from finding new/undesirable escapes; for example- the rocker cover gasket or the oil filler cap.

Or, the head gasket.

 

SU's have no power advantage over the Z-S's; and do have this disadvantage.

 

The Z-S carb vacuum port also draws raw petrol vapor/fumes from the carbon canister so that your garage doesn't get all stinky.

post-13442-0-61125800-1509126538_thumb.jpg

Edited by Sapphire72
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Hi Lancerman

There are various companies that will supply you with a kit such as Andrew Turner or Burlen fuel systems. It is an easy job to do providing you have a fairly standard engine speciation,.

Having said that I did my car 20 years ago and was never 100% happy with the results, last year I realised my new SU carburetors were worn out (no surprise really after 2 decades). So I rebuilt my old Strombergs which was quite successful, since then I found a pair of new Strombergs and would not go back to SUs on the TR6

 

George

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The Zenith-Strombergs have a vacuum-draw port on their front (SU's do not have this feature) that is used to relieve pressure from the engine, via the rocker cover nipple.

 

Ahh but they do ..... See photos in post 7 of this thread << http://www.tr-register.co.uk/forums/index.php?/topic/63909-carburettors-and-manifolds/ >> These are of course later construction type that were fitted exotic cars like Austin Allegro or Volvo of the early 1970's. Tri Spitfire in UK & MG things also used this 'suck the crap from the engine and burn it' ventilation system.

 

Peter W

 

Relieving pressure from the engine, in this fashion, prevents engine pressure from finding new/undesirable escapes; for example- the rocker cover gasket or the oil filler cap.

Or, the head gasket.

 

SU's have no power advantage over the Z-S's; and do have this disadvantage.

 

The Z-S carb vacuum port also draws raw petrol vapor/fumes from the carbon canister so that your garage doesn't get all stinky.

Ahh but they do ..... See post 7 in this thread

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Strombergs as fitted to U.S. TR6s effected mixture control very well, resulting in very high mileage before rings / cylinders would wear. Deviation from this fitment is almost certain to degrade this ( sound like the warnings in the manuals? ) so without the prospect of performance improvement, and with the stigma of non-originality the case for S.U.s on these is hard to make...for the engine in original form.

 

And if the engine is modified to CP spec or higher, triple Weber DCOEs wins hands down across the board over all comers -_-^_^ .

 

Cheers,

Tom

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Strombergs as fitted to U.S. TR6s effected mixture control very well, resulting in very high mileage before rings / cylinders would wear. Deviation from this fitment is almost certain to degrade this ( sound like the warnings in the manuals? ) so without the prospect of performance improvement, and with the stigma of non-originality the case for S.U.s on these is hard to make...for the engine in original form.

 

And if the engine is modified to CP spec or higher, triple Weber DCOEs wins hands down across the board over all comers -_-^_^ .

 

Cheers,

Tom

+1

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What happens a lot of times with the ZS carbs on the TR6 is that certain 'rubbery' components such as diaphragms and seals come to the end of a useful life.

Failing to recognize this can lead to a lot of tampering with the various adjustments (and there are a few) in futile and frustrating attempts to compensate.

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Thanks guys, loads of good info to get my brain working again. Luckily the Strombergs failed to sell on e-bay recently so I have a Plan B if the I cant get the HS6's properly balanced. The engine is running 'lumpy' and always has since fitting them. I had to remove one of the vacuum lines from one of the carbs as petrol was being forced back through it's K&N air filter!

I will get the brakes working first then turn my attention to the carbs. I do recall seeing several references to TR6s running on SUs over the years so have to believe it is possible.

I will keep you posted with progress.

 

Lanceman

 

 

May 1976 LHD Triumph TR6. Comm No. CF55718U/ Engine CF45998UF/ Silver plate T38130CF/ Originally fitted with Zenith 175 CD-2 carbs. Now fitted with SU HS6 carbs.

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Thanks guys, loads of good info to get my brain working again. Luckily the Strombergs failed to sell on e-bay recently so I have a Plan B if the I cant get the HS6's properly balanced. The engine is running 'lumpy' and always has since fitting them. I had to remove one of the vacuum lines from one of the carbs as petrol was being forced back through it's K&N air filter!

I will get the brakes working first then turn my attention to the carbs. I do recall seeing several references to TR6s running on SUs over the years so have to believe it is possible.

I will keep you posted with progress.

 

Lanceman

 

 

May 1976 LHD Triumph TR6. Comm No. CF55718U/ Engine CF45998UF/ Silver plate T38130CF/ Originally fitted with Zenith 175 CD-2 carbs. Now fitted with SU HS6 carbs.

It certainly is, mine is running on SU's and runs very well now. You need to spend time tuning it yourself or take it to a rolling road though.

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The engine is running 'lumpy' and always has since fitting them. I had to remove one of the vacuum lines from one of the carbs as petrol was being forced back through it's K&N air filter!

 

 

Lanceman

 

 

May 1976 LHD Triumph TR6. Comm No. CF55718U/ Engine CF45998UF/ Silver plate T38130CF/ Originally fitted with Zenith 175 CD-2 carbs. Now fitted with SU HS6 carbs.

Classic symptom of a stuck float chamber needle valve....rarer but possible is a float that's taken on fuel and lost it's bouyancy.

Edited by poolboy
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Hi Lancerman

There are various companies that will supply you with a kit such as Andrew Turner or Burlen fuel systems. It is an easy job to do providing you have a fairly standard engine speciation,.

Having said that I did my car 20 years ago and was never 100% happy with the results, last year I realised my new SU carburetors were worn out (no surprise really after 2 decades). So I rebuilt my old Strombergs which was quite successful, since then I found a pair of new Strombergs and would not go back to SUs on the TR6

 

George

 

It's also an easy job if you have digressed from a fairly standard engine specification. It's getting it right that takes time and effort.

 

Balancing between appropriate needle, correct tuning and best ignition profile seems to be far more of a challenge... but a rewarding voyage of discovery for a dope like me.

 

Paul

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Good stuff thank you all. Breaks are fixed now (new front hose, cleaned discs and new pads, bled all round) so I am back on the road and ready to attack the carbs. Reckon it would pass the MOT as is. :rolleyes:

Lance

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Turns out when I said I had to disconnect one of the vacuum lines to keep the engine running I was lying. It is the fuel return line (on the carb nearest the bulkhead) that I have to leave disconnected in order for the engine to run. The fuel return line on the outer carb stays connected. I assume this line returns straight to the fuel tank. Do i have some sort of pressure release problem going on?

post-15286-0-82892800-1510075431_thumb.jpg

Edited by Lanceman
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Those are not fuel return lines. The carbs do not have such a thing. Those are vents and need to be open to atmospheric pressure.

Connecting one to the other is not the way, unless an opening in the hose to atmosphere is provided that is accessible to both carbs.

Otherwise one or both of the carbs will be deprived of adequate fuel.

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Poolboy thank you, I have struggled to understand what function these hoses provide. The rubber hoses from each carb are joined at a T-junction and then a single pipe disappears under the car, I had assumed to terminate in the fuel tank. This would also have been the original set up with the strombergs too I think, as I doubt if I would have modified that area although it was a long time ago.

 

When I look at the exploded diagrams of the TR6 fuel system, which I have to say leave a lot to be desired, there does appear to be a line returning to the top of the fuel tank. If I am understanding this correctly then this is also open to the atmosphere along with the 'ports' on each of the carbs. This will be easy to try and will be my next move.

Lance

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The ZS carbs, up until the ones on the 73 cars, had their float chamber vent port located in the carbs' air box mounting flange.

There was no nipple and no hose to deal with.

In 73 an Anti Run-on Valve was incorporated into the 'plumbing'. To accommodate the ARV an additional float chamber vent port was added to the ZS carbs, which did have a nipple and hose on the left hand side of the carbs.

However, that vent arrangement was only open to atmosphere when the engine was idling...at other speed the float chamber was vented to atmosphere thru the same port as the pre 73 cars.

There was a gas tank vent line from the top of the tank. Originally the line ran along side of the fuel line for a good ways, then it ran under the front of the engine, emerging on the forward right hand side of the engine bay where it was connected to the Carbon Canister.

It's only connection other than an inline reducer was to a nipple on the top of the Carbon Canister.

Regardless of where you find it now, do not cap that vent line or the result could be starving the fuel pump.

Edited by poolboy
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All good with both the hoses removed from the float chambers and the fuel tank vent line left open the engine is running much better. Still a bit lumpy but I think some serious re-balancing of the carbs should sort that out. I think it is down to cosmetics now, how to make those vent ports not look like something is missing, I am thinking just a small length of hose maybe pointing down to avoid anything being ingested there. The same applies to the fuel vent line which at the moment points upwards and is just waiting for something to fall into it!

Many thanks for your help poolman.

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Actually the more I think about that fuel vent line, petrol fumes being fed into the engine bay, sparks from the ignition.... Perhaps it should be removed altogether and vented back at the fuel tank. :o

Surely you could track down a carbon canister, somewhere somebodies got one for offer.

 

Run the fuel vent line to the canister, then join another hose to the carb suckers,

and you've got a system that burns off the fumes-

just as Triumph engineers figured out 40 some years ago.

 

Keeps your garage air nice & fresh smelling.

post-13442-0-27404400-1509629505_thumb.jpg

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If converting from Strombers to SUs its critical to ensure that the crankcase is properly vented through the valve cover. In order to do so a slight vacuum is needed to help purge the crankcase pressure. Failure to do so will result in oil leaks from the valve cover, in particular on the left side of the engine by the engine number stamping. Heres the configuration that I have used on my HS6s, which have a vent nipple to provide the necessary vacuum. Not all SUs have this (these are off of a Volvo) in which case the only other option is to vent them to the intake side of the carbs, typically through an attachment to the air cleaner. Also to note, Ive routed the float bowl overflow lines forward to exit below the canister, better than dripping onto a hot exhaust manifold!

 

[CarbVent.jpg

Edited by yycdave
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Dave,

That looks nice, looks a very similar set up to mine, I think my carbs came from a Jag, but not entirely sure as I purchased from a 'breaker' in the last millenium (literally). I had the same concern deciding where to run those vent lines, and yes noticed the left carb would vent any fuel onto the exhaust manifold. Not good. So I will follow best practice discussed here, find a cannister, mount on the inner wheel arch and attach the vent line. I just got it through the MOT here (annual roadworthy test), it wouldn't pass the emissions test, but then it never has, it does pass if you remove the probe from the exhaust pipe!

Thanks

Lanceman

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