View Full Version : TR6 Are SU HD8s to big for a 71 TR6?

Paul Slice
05-25-2006, 07:12 PM
I asked this on another forum but am interested in another opinion.

I just purchased a 71 TR6 that I knew had been converted to SUs with a 73 twin exhaust manifold and 2 pipe system. When I drove it the 100 miles home it ran poorly and surged so bad at highway speed that I had to feather it all the way home. It had been restored around 13 years ago and I had planned to do a complete tune up as well as brakes, clutch system and full suspension rebuild to make it right. Also when I met the owner I knew that he had driven it very little during the 13 years he had it and when it started to run rough it was time to get rid of it as he did not have a clue, if you know what I mean..
So, the natural thing to do was to order lots of parts and the tuneup was the first thing on the agenda. After the Autolite plugs were trash canned for NGKs on went new wires, cap and roter as well as a Pertronix Ignitor to get rid of the points. She ran better and with the timing advanced pulled strong but still surges slightly when you level off at speed (say 70 on the interstate). The SUs have pancake filters on them and I wanted to see if the standard Stromburg airbox would fit on the SUs. When I removed the filters and held the rear airbox plate against the carbs I realized that these HDs were 2 inch carbs not 1 3/4.

Here is my question, should I go back to the Stroms or even get some SU HS6 carbs?

Are 2 inch carbs to big for a TR motor that is stock?

And could this be causing the surging?

I have syncronized the HDs but have not touched the mixture screws and it idles ok at 950 RPM.

here are some pictures of the 71 6 after polishing and waxing

All input welcome,





05-25-2006, 07:14 PM
Paul, can't help with the SUs, but I did want to say that that is a very pretty TR6. Love the blue.

Andy Blackley
05-25-2006, 07:59 PM
The big SU are more carb than needed. The surging is probably due to a mismatch of the metering needle. Finding the right needle for a each engine application was the result of a lot of testing by Triumph way back when , but it can be done with some trial and error.
The larger venturi openings are not pulling the same amount of vacuum as the stock size for a given station (height) on the needle, i.e. the larger opening has less velocity, hence less pitot effect over the jet. The HD series used on Jags (assuming thats where they came from) use 0.125" jets and a whole different series of needles, so using a needle recommended for an HS6 might not work. The Haynes Weber/SU Stromberg manual lists the sizes.

05-25-2006, 08:56 PM
Paul, can't help with the SUs, but I did want to say that that is a very pretty TR6. Love the blue.

[/ QUOTE ]

I was gonna say the same exact thing. /ubbthreads/images/graemlins/grin.gif Very nice looking car.

05-26-2006, 08:33 AM
I sure hope you work out the problems and find the 2"er's work out. I all ways hear "more than needed" or "more than can be used" but i think they will enhance any mod you do like exhuast, compression increase, electronic ignition, etc. I still remember t he days when you could put a 4bbl manifold and carb on a 2bbl V8 and get considerbly more power, noise and fun out of the motor with out changing the exhuast or compression. If the OEM's raise the pistons completely, it can use more carburation. This is gut feelings, not empirical testing.... size does matter!

05-26-2006, 09:09 AM
Great looking car!

I've read that the 2" carbs are too much for the TR6. For the same reasons Andy mentioned above. HS6's are popular replacements for the 175 Strombergs. My brother runs HS6's on his 250. He's happy with the performance.
By the way, HD8's seem to bring a good price on EBay. If you clean them and polish the tops you may recover a fair percentage of the cost of a set of fully restored HS6's.

Dave Russell
05-26-2006, 12:55 PM
Hi Paul,
Great pics, very nice looking car.

I agree with Andy. The SU is a demand type carb. If the carb is larger than needed for the car, the piston will just not rise as far for a given engine load. It's likely that the HD8 is only opening around half to 3/4 of it's full travel at maximum output on your engine.

This situation would put the metering needle position somewhere in it's mid range calibration instead of at the normal richer, lower on the needle, power positions.

To correct, find a needle profile that is richer in the midranges of the HD8. Probable needle stations 6 through 10. It might also be richer on the 10 & up stations but this won't matter, since the carb piston is likely not opening that far.

For example; A UM needle on station 6 would be .1104" diameter. A UO needle on station 6 would be .108" diameter. The UO would give a 15% richer mixture on station 6.

It may take a couple of tries to get the right combination. You will need to compare prospective needle profiles with your existing needles.

Stations are measured at 1/8" intervals starting at the needle shoulder. ie, station 6 would start at 6/8" down from the shoulder.

If you don't have one, there are several good SU needle charts such as the "Haynes Techbook, Weber Carburetor Manual" which includes Zenith & SU carbs. There are also a couple of "on line" needle comparison programs.

05-26-2006, 01:50 PM
I'm not sure sure if it's the size of the HD8 or if the pistons are slow to fall, vacuum leak or something else. I would think that Joe Curto would be able to advise you what to do before you start replacing parts. Needles & jets for the HD8 are available. Give Joe a call, Nice guy & he knows his stuff. If you need a list of HD8 needles let me know & I'll get one to you.

05-26-2006, 02:30 PM
Hi Paul,

Great looking car!

Unless you do a lot of performance modifications and vastly increase the engine's displacement, those 2" SUs are way too big for the car. I doubt all the jetting and fiddling you might try will ever get them to run right. Contrary to popular American thinking, bigger is not necessarily better.

As others have said, the problem is that SU (and ZS) rely on vacuum to operate the piston, which controls the needle and fuel/air mixture. With oversize 2" venturis, gas speed into the engine is reduced quite a bit, and vacuum along with it.

The car will likely never perform all that well with the big carbs. I'd swap to a set of HS6 - which can easily handle a 2.5L engine with many sorts of performance mods.

Joe Curto might be a good source and may do a swap with you. Also it's likely he can pre-jet a set of HS6 carbs pretty close to what your car needs, just be sure to give him a list of modifications.

If it were me, I'd choose SUs over ZS. The reasons are simple: SU parts are easier to get and I find them easier to service myself. In terms of performance, SU and ZS are very equal.

Enjoy your car!


05-26-2006, 03:53 PM
Somebody educate me here. How is it that if you run triple 1.75" carbs on the TR6 engine, it's generally not considered too much carburation...or is it?? If you figure out the bore area sizes (pi*r squared) on the 1.75 carbs (2.4") and multiply by 3 you get 7.2". If you do the same with the two 2" carbs, you get 6.28". It looks to me that triple 1.75" carbs is more carburation then two 2" carbs. If the triples can be made to work, why can't the HD8s??

Another question...how were they mounted up to the manifold?? Maybe that's where the problem lies.

05-26-2006, 05:00 PM
it's not the total bore area that matters. it's the area per cylinder. You could just as easily have 6 Mikunis or something and you're still good to go if the openings are the same.

Throwing out the slight "mixing factor" of 2 or 3 cyls. sharing the same carb, each cylinder is only using one carb when it is using it. They happen to be connected to 2 or 3 at a time but only intake is sucking at any given point (pretty much... yes there's some overlap - just bear with me here)

so when one intake opens and sucks from one carb the velocity of the fuel is determined by the diameter of the opening... for the one carb/intake combo that is using it at that instant. Put a bigger opening and you've got lower velocity.

The only way your figures would make a difference would be if all 6 intakes opened at the same time.

Clear as mud????

Simon TR4a
05-26-2006, 05:10 PM
My understanding of this question is that as the crank rotates the cylinders reach their induction cycles in sequence, so a carb is only filling one cylinder at a time.

It would make no difference, then, whether you used 2 carbs each feeding mixture to 3 cylinders, or 3 carbs each filling 2 cylinders.

The benefit of using triple carbs is to get equal length, and straighter, inlet manifold runs, not to provide bigger flow capacity.

There may be other factors which play a minor role as well; and I would be very interested to get some of the experts to give us a more detailed explanation, or correct me if I have misunderstood.
Thanks, Simon.

05-26-2006, 05:25 PM
See, that's why I asked someone to educate me. Clear as mud?? No, it's much clearer now. jsneddon, and Simon TR4A your explanations were very good. I understand alot more then I did a half an hour ago.


05-26-2006, 07:12 PM
A little more light may help:
Area of
2 2.0” =~6.28
3 175’s=~7.25
3 weber’s=?? Is my math close?

Dave Russell
05-26-2006, 09:51 PM
it's not the total bore area that matters. it's the area per cylinder. You could just as easily have 6 Mikunis or something and you're still good to go if the openings are the same.

Throwing out the slight "mixing factor" of 2 or 3 cyls. sharing the same carb, each cylinder is only using one carb when it is using it. They happen to be connected to 2 or 3 at a time but only intake is sucking at any given point (pretty much... yes there's some overlap - just bear with me here)

so when one intake opens and sucks from one carb the velocity of the fuel is determined by the diameter of the opening... for the one carb/intake combo that is using it at that instant. Put a bigger opening and you've got lower velocity.

The only way your figures would make a difference would be if all 6 intakes opened at the same time.

Clear as mud????

[/ QUOTE ]
You can't "throw out the mixing factor". It is not "slight". There is considerable overlap. Think about it. On a six cylinder engine, there are three cylinders intaking & firing on each revolution. With a firing order of 1-5-3-6-2-4, & a cam duration of 240 degrees, there are at least two cylinders "intaking" quite heavily from the front or rear carb for a period of around 70 crankshaft degrees, throughout each revolution. ie. cylinders 1 & 3, on one revolution & cylinders 6 & 4 on the next revolution.

05-27-2006, 07:23 AM
Hello all,

I don't think that the size of the HD8's are the problem as they are variable choke or venturi so the air flow velocity will be much the same whether they are 13\4" 0r 2". Agreed that they may not fully open at full power but too large is preferable than too small, from a power point of view.
What sort of throttle linkage is fitted, as I have experienced surging before with a rod rather than cable throttle due to engine movement opening and shutting the carburettors' throttles.
Incidentally, my P.I. Triumph has 6 by about 11\4" chokes.


05-27-2006, 02:29 PM
Hi again,

Yes, it could be a mechanical issue such as linkage or cable and that would be a good thing to check. But, I'm sorry Alec, I have to respectfully disagree. I think the oversize HD8s are at the heart of the problem.

Oversize carbs will boost performance almost exclusively in the upper reaches of the power range (5000-5500+) - where a stock engine starts to run out of oomph - and then *only* if matched up with a compatible high performance cam, oversize valves, porting/gas flowing, headers, a freer flowing exhaust system, increased engine capacity, etc., etc., etc. The improved car's power curve might be 2500-5500, or 3000-6000, or 3500-6500 rpm, depending largely upon how hot a cam is being used, but also on the other performance-oriented modifications. Increased top-end power (i.e., peak horsepower) achieved by going to larger carburetors and all these other modifications comes at the expense of idle smoothness, low-end torque and general street-worthiness, not to mention worse gas mileage. Very careful jetting might bring back some of the car's good manners. But - very likely - not all.

The right size carbs, properly jetted, will help move the power curve back down to a comfortable street-usable range, say around 1500 to 4500 rpm, performance will taper off gradually above that and peak HP will be reduced a little.

Undersized carbs would keep the low-end performance intact, might even make the car more docile at idle and just off idle, could give better fuel economy, and might even improve initial acceleration. But, they would gut performance significantly above 2500-4500 rpm. Slightly smaller carbs *might* be a viable consideration if driving the car at altitude, such as in Colorado where most everything is at least a mile above sea-level. I successfully ran 30mm primary venturi (or chokes) in my TR4's Weber DCOE when I lived there. Weber specifies 32-33mm for the car and I've now got 34mm fitted, am living near sea level and have a number of other performance modifications along with increased engine capacity (however can't report actually running experiences... the car isn't yet complete and the Webers are yet to be dyno tuned ).

Speaking of Webers, because they allow such fine tuning of the primary venturi, installation specifications provided for Triumph can be a very good point for an apples-to-apples comparison.

For example, on TR2/3/3A with 2 liter, 4-cylinder engine (83mm bore), four 32mm venturi are specified. This works out to 402mm/square total venturi bore, or 201mm square per liter.

For 2.5L 6 cyl. TR6, Weber specifies six 27mm venturi. Multiply it out and you'll find this gives 509mm/square total venturi bore, or 203.5mm/square per liter.

In other words, the ratio of venturi size to engine capacity is almost exactly the same!

Now, this comparison is rather simplistic and doesn't consider a lot of other tuning factors. And, I'd only compare Weber DCOE with Weber DCOE this way, as an illustration. It wouldn't be useful to try to directly apply these DCOE numbers to SU/ZS, just because the carbs' functions are quite different.

In his book, "How to Improve TR5, 250, 6" Roger Williams notes: "It's important to not over-provide choke or carburetor bore size. It is tempting to think your standard 2500cc engine will go better with 2 inch SUs. This is unlikely to be the case for it is absoutely essential to generate sufficient vacuum within the carb to draw fuel into the carburetor's venturi. If you over-provide choke size, your engine may not generate enough air speed to draw fuel evenly [note: uneven draw at a constant throttle position, such as cruising on the highway = surging]. So, in general, standard or fast road [moderately improved] 2500cc six-pots may not move sufficient air volume to generate low-down torque from twin 2 in. SUs, although a highly tuned 2500cc engine might be worth a try. A 2700cc [significantly over-bored] six-cylinder engine is the most likely entry point for 2 in. SUs - but do not dispose of your 1.75" too quickly!"

Where he didn't have direct personal experience, and sometimes even when he did, Williams gathered info for his books from various tuners/vendors in England and a few in the U.S. or elsewhere, many of whom have vintage racing programs and have tried out their mods on the track and on dynos.

A couple other things occured to me....

Some SUs were fitted with an "anti-runon" valve, which can be seen as a round "doohicky" mounted in the butterfly. These really mess with air flow and mixture, and I cna see where they might cause surging problems. Since we know for certain that those 2" SUs were taken from an unknown donor, might be worth peeking inside to see if they are fitted with that anti-runon valve. The AR valve can be seen from the outside when the piston is lifted, but is really obvious from behind when the carb is removed from the manifold. Alternatively, if you have the carb's model number, you can look it up on Burlen's website and see its specs and what it was originally fitted to.

Another thing to look for, some SU have "wax stats" on the bottom to control cold start enrichment. This is essentially an automatic choke, but can malfunction when used on certain cars where the exhaust system is near the carbs and intake. I've heard of these but don't know if I've ever seen SUs with wax stats, so I can't describe what to look for.

In the end, the best test Paul could do would be to just swap the carbs out for a pair of HS6 and see if the car doesn't run a lot better. Personally, I bet properly jetted 1-3/4" SUs would solve the problems.


05-29-2006, 02:06 AM
Hello Alan,

the point about S.U.'s is that the choke area varies to the engine air demand. This gives quite a bit of tolerance regarding sizes. Think that Jaguar used three 2" units on their approximately 1\3 larger capacity engine. Certainly a more 'sporty' engine than the Triumph, but even so.
I would certainly go through other things first before discarding the 2" units. I don't think that HD8's ever used the run on poppets or the waxstat jets so are unlikely to be an issue.


05-29-2006, 09:53 AM
/ubbthreads/images/graemlins/iagree.gif with Alec. It's more a matter of matching the needle to the area of the carb that is used. There is a TR6 here in Fort Collins (5,000 feet) that uses two HD8's. The intake manifold was port matched to the carbs, the engine is 8.5:1 compression and it has a very mild cam.

I have triple 175's on my TR6 and after finding a more appropriate needle they work very well at altitude. But there has been more done to my engine than the other TR6.

The TR6 carb linkage is the weak spot that I would investigate. Converting any carbs on a TR6 to a throttle cable would absolutely be an improvement.


05-29-2006, 05:08 PM
Hi Alec and Shawn,

Please understand, I don't doubt that HD8 can be made to work with some fiddling, tuning and needle swaps. The variable venturi created by the moving piston does give all SU carbs a lot of flexibility. I just think a street car will likely perform noticibly better across its normal rpm range with 1.75" carbs. A lot of other serious engine improvements would be needed to really take advantage of the HD8s, and most of the benefit would be in the upper reaches of the rpm range.

Even tuned to their best with a relatively stock or modified TR6 engine, I think the larger carbs will significantly reduce fuel economy, make for a rougher idle, reduce torque and low to mid range acceleration.

But, don't take my word for it, someone asked the same of Kas Kastner, who has experimented with, tuned and modified more than his share of TRs over the years. Here's his reply:

"The most common problem with bigger carbs on an engine, particularly a street engine is the loss of low speed velocity through the inlet tract and a resulting poor low rpm performance. Race cams and higher compression, generally will give a need for the bigger carbs.

On a street GT-6 the 1.75" carbs are a definite improvement over the 1.5" Strombergs and easily installed and worth the trouble as that engine [2 liter] is far far under carburetted. I think that a 2.5 [TR6] with three 1.75" would be a terror and probably okay at lower revs on the street also. The S.U.'s are so forgiving they can be used in places other carbs wouldn't stand a chance. The Weber requires the proper size venturi to make it work for the cam and flow charastics of the engine but an S.U. just adapts to the requirement as it has the variable venturi as part of the design. Anyway, I have a second motto, 'One valid test is worth 10,000 expert opinions'."

Alec, your point about the Jaguar motor is a good one, but I think each engine's specific efficiencies need to be considered, too. The Jag 3.8 liter has a dual overhead cam and likely a better flowing cyl. head than the TR. The same engine in earlier cars such as Mk II, Mk III and XK150 was fitted with two or three 1.75" SUs. I'm not all that knowledgeable about Jags, so am guessing this might have had to do with an earlier, less capable head design, or perhaps those cars were simply under-carbureted.

And, I'm glad to hear the anti-runon poppet and waxstats are unlikely. Looking at some reference books regarding SUs, I see that at least some HD8 might be a little unique among SUs, by using a plastic throttle shaft bushing without a proper seal. To me that sounds like a potential air leak - which would almost certainly upset mixture and inlet air flow - and might be another thing worth checking.

As Shawn points out a larger set of carbs would need the manifold modification to match. It occurs to me, if that were left unchanged, there would be a step-down in size, which would make for a nasty air flow stumbling point. Yet another possible cause of problems and thing to check.

I find Kas' point about TR6 with triple carbs intriquing. He's saying essentially the same thing others here have noted. The cars are likely to see a nice improvment, on the street or on a race track - if triple carbs are each feeding a pair of cylinders, rather than a pair of carbs each supplying three cylinders.

I also like Kas' motto #2 (His #1 motto is "Never be beaten by equipment"). It's pretty much how I tried to sum up earlier... the best way to find out if 1.75" carbs can solve the problem would be to install them and see how the car drives.

I'm mentally stashing away Kas' comments about GT6, for the day when I finally get one!

05-30-2006, 08:40 AM
Hello Alan,

if the question had been that I want to fit 2" S.U.'s then I would have said no, that 1.75" or 1.5" would be the choice but the engine alreadty has 2" which I believe will work if set up correctly and any faults remedied. (Incidentally, I do not consider leaking throttle shafts a big problem as it is at idle and low speed that they affect running. Drive with a wide throttle gets rid of that particular problem :-))

I also believe that the 2 x 1.75" used even on the 3.8 Mk2 Jaguar Sedan was a cost thing (William Lyons was cost conscious)and gave a quoted 220 BHP (Gross I think) while the 3.8 E Type had 3 x 2" and with a slightly better head and higher compression gave 265 BHP. In this case I think the extra cost was an image factor rather than true engineering need.


05-30-2006, 09:21 AM
Very long story short (e-mail me if you want more):

1) Two 175 carbs are too small for the TR6,
2) HD8s work just fine once you have the correct needles.
3) I don't recall the needles I used and my car was tuned to 5000 feet so what worked for me won't work for you.
4) The best way of determining the best needle size is using an Exhaust Gas Oxygen sensor.

Photos of my TR6 with HD8s are at:


I've since converted to a turbocharged computer controlled spark and fuel management system. More information at:


Hope this helps

05-30-2006, 11:09 AM
I run triple HS6 carbs on my MGC, but of course it has a larger displacement than the TR6.

If you don't want to switch to a pair of HS6, you should be able to make the HD8s work, but it may be a long and laborious job. There aren't a lot of needles available for the .125" series. You might get lucky - try for instance the Rover 2000 TC needles.

Otherwise, another option would be to switch to .100" jets which would give you a wider range of available needles. These should be readily available.

The HD8s ARE more than you NEED, but nonetheless should run alright with the TR6. A mod for all-out racing MGBs was a pair of HD8s. Many people have fitted them to engines much less highly tuned and wondered why they don't make any difference....If you aren't running an 8000 redline you don't need them on a B....

05-30-2006, 06:40 PM
Until Bill mentioned it I had forgotten about the 2000TC's. The TR6 in Fort Collins with the HD8's got them from a 2000TC.


Andy Blackley
05-31-2006, 12:04 PM
Another tuning option not often used would be to change the air piston spring, say from blue to red. I beleive the red is "weaker" and would allow the piston to rise a bit higher, faster.