TR4/4A TR4A: Replacing Strom Carbs with SU's?

[karls59tr]

I have a set of H6 SU's that I am currently rebuilding. I realize that some TR4A's had HS6's from the factory and they are supposedly a better carb than the H6 but is there any reason why I couldn't run the H6's on my 4A (same motor)other than needing to alter the linkage? Anyone made the conversion from Strombergs to SU's and know what specific linkage parts would be required? Also...why is the HS6 considered more tunable than the H6?

 

[Geo Hahn]

...any reason why I couldn't run the H6's on my 4A (same motor)other than needing to alter the linkage?...

I think you'll also need to use a TR3/TR3A intake manifold.

 

[TR3driver]

I think you'll also need to use a TR3/TR3A intake manifold.

Why? The H6 and HS6 are the same bolt pattern (lots of folks install HS6 on TR3 manifolds) and AFAIK all TR4A used the same intake even though some had HS6 carbs from the factory. Is it lack of clearance to the inner fender?

I don't know of any reason the HS6 are more "tunable", but they are greatly improved in terms of not leaking fuel at the drop of a hat, and the racers report that they flow slightly better than H6 carbs do. But I doubt the difference in flow makes any measurable difference on an otherwise stock (or nearly so) engine.

 

[Geo Hahn]

Why?... Is it lack of clearance to the inner fender?...

That is what I was thinking. It is close but if too close there are other ways to get around it I suppose, possibly using some alternate air cleaners or massaging the inner wing.

 

[Andrew Mace]

And then there's the age-old question: "What's wrong with the Strombergs?"

 

[HerronScott]

I don't know of any reason the HS6 are more "tunable", but they are greatly improved in terms of not leaking fuel at the drop of a hat

I agree with Randall that the jet design was the primary improvement over the H6 in terms of fuel leakage.

Regarding reasons for moving from Strombergs to SUs, the only other reference I've seen was that SUs have a greater selection of needles for tuning, but I'm not sure that's something most of us need to be concerned with if it's true.

Scott

 

[glemon]

And then there's the age-old question: "What's wrong with the Strombergs?"

My question too, Strombergs get a bad rep, in my opinion it is the later emission strombergs (68 and later) with all the valves and compensators and what not that give them the bad rep. I had pre-emission strombergs on my 4A which I ran as a daily driver for years, and hobby car for years after that. They gove no problems and are pretty much the same as SUs of the same era, the main difference being the rubber diaphragm, which of course messes everything up if it has a tear, but they are cheap and easy to replace, and as long as you are careful with them they seem to last a long time.

They are much more reliable than the H series SUs that as noted are prone to leaking gas.

 

[Bob_Muzio]

There is the length of the respective intake manifold tubes and carb necks to consider. The HS6 has a short neck compared to the H6 and mates on the late TR4 and 4A to an intake manifold with long intake tubes. The H6 carb is long necked and the manifold that you'd find on an early TR4 or TR3 has short intake tubes.
If you just switch one without the other you'll have changed the total length of the intake between the carbs and the head.

 

[Bob_Muzio]

Additionally, I installed HS6s on my '64 TR4 which originally had Strombergs. I did have to source various linkage components, some used as they weren't available at that time. I liked the mod as the rear Strom liked to drip fuel on the starter solenoid and the carbs were, for me, easier to play with.

 

[mtlman8]

I to have strombergs (original 175 cd 1's) everybody seems to want su's...why? what is it about the zs's? not meaning to hijack the post just curious.

 

[poolboy]

Seems to me that when the ZS carbs get to the point of needing a rebuilding or reconditioning, it confounds some folks. And the ZS carbs do have quite a few adjustments (compared to SU's) that may have been messed with in an unsucessful attempt to straighten out a problem.
And let's face it, any carb in good condition will out perform a ZS carb in need of attention.
Having said all that I do like the fact that the mixture can be adjusted on a SU carb while the engine is running.

 

[mtlman8]

can you NOT adjust the mixture on a zs with the engine running?

 

[poolboy]

I was talking about the adjustable needle ZS carbs, sorry.
The engine stalls when you use the Allen wrench type tool because it seems to push the air valve down to the bridge, shutting off the air during the adjustment procedure.
Any "man made" movement of the air valve gives a false indication of the mixture.

 

[mtlman8]

thanks for clarifying, just ordered a colortune and would be pretty upset if I could not adjust the mixture with to engine running.

 

[glemon]

Thats what I was saying the later Strombergs with the adjustable needles and all the other garbage are a pain. The early ones are pretty similar to same era SUs. Some of the parts are a little harder to come by though, the early floats come to mind.

 

[Geo Hahn]

...Some of the parts are a little harder to come by though, the early floats come to mind.

Though Moss now shows them available:

https://www.mossmotors.com/Shop/SearchResults.aspx?SearchText=365-500&WebCatalogID=0

Not cheap, but 'not cheap' is a lot better than 'not available'.

Having one pair of each (SU & ZS) I see no particular advantage in either. I have heard that perhaps the ZS is less prone to leaks though my SUs have not leaked since a rebuild 28 years ago. Honest, no leaks. The ZS's did develop a leak but that O-ring was easy to replace with the carbs in situ. The tuning of both is quite similar and the choke linkage is perhaps a bit fussier on the SUs.

Just to restate -- the comparison is the early ZS with a simple jet adjustment.

Possible tie breakers:

The ZS can be adjusted with a quarter (or 2 shilling coin in my case) no special little wrench needed.

The SUs look cooler.