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TR6 TR4 SU carbs for ZS carbs on TR6?

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AZTR4a67

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Hello all,

Background, I bought a 67 4a a few years ago that turned out to be a complete basket case. I had to scrap it, but I still have some bits lying around. In July I bought a 72 TR6 that I am doing a body off resto. It appears to be pretty original but it has lots of rust issues.

i have a buddy with a couple of Triumphs in his stable who hates ZS carbs. I've now owned two Triumps that I have never driven, so I don't know squat about the differences between the ZS and the SU carbs. Any reason I can't put the SU carbs from the TR4 onto the TR6? The bolt patterns all match up and I have all the linkage.

the SU carbs are AUC878
 
If you're doing a "restoration", why not at least try the original carbs? They are more complicated, due to all the emissions features that got added, but can still work well if properly adjusted.

Oh, and swapping non-emissions carbs onto an emission controlled vehicle comes under the heading of "tampering", making it a federal crime. I've never heard of it being enforced against a private individual, but it's still against the law.

But other than that, I don't know of any reason you can't.
 
I've had cars with SUs and cars with ZSs - there are differences of course, but I'd be hard pressed to declare one better than the other.

ZS - Rubber diaphragm makes fitment of the piston to the body less critical, enrichment mechanism (choke) is very effective and not prone to binding, mixture adjustment is different but ends up doing the same thing, spring-loaded needles (don't know if yours has those) need no centering but can eventually wear.

SU - Look better (some would say 'look right' on a British car), the mixture adjustment is simple and intuitive, a lift pin (not all ZSs have one) is a useful feature.
 
I'm no authority on TR4A's but weren't the ZS 150's. So my question is are the SU also 150's? If so they would still be usable but not perform as well I would guess as the stock ZS are 175 on a TR6.
 
I think they would be HS6s -- i.e. 1 & 6/8" in SU-speak so same diameter as a ZS175.

And, of course, some TR4s and 4As had ZS175s.
 
Thanks all for the feedback. I definitely don't want to cross the line and circumvent emissions, my only thought is I only want to rebuild one set of carbs. If the ZS carbs are temperamental, maybe the SU's are better. I honestly have no opinions on the carbs. I am now one my second Triumph and have never driven a mile in one.

I'm having the radiator and gas tank boiled out as we speak. Im gonna drop a battery in and see if it will fire. If it miraculously runs, I'd rather have the least finicky carbs on there as possible.

I'm 90% sure I'll be pulling the motor for a rebuild but there is always a chance it runs.
 
In over 60 years of owning LBCs, I noticed no difference in performance between the SUs and the Zeniths/Strombergs. My 71 TR6 Zeniths do not have a lot of the later emissions crap and have given me absolutely no trouble in 21 years other than a stuck float one time a few years ago, fixed with a smart rap from a wooden-handled screwdriver. As pointed out, there is a rubber diaphragm on the Zeniths that requires a little care in refitting so it doesn't tear. I can't speak for the post-1972 Zeniths but many owners on this board can fill you in. In the final analysis, it's your call.
 
And in some markets (iirc, Australia and New Zealand) weren't there were 6-cylinder Triumphs in the 2500 sedan series (2500S or 2500TC?, something like that) which used SU carbs right from the factory? If so, then there would be factory-correct installations to use as a guide for a stateside TR6.
 
"Some People"..:eagerness:.. often use the term 'emission crap' when referring to post 72 ZS carbs on the TR6, but in fact the difference in the post 72 carbs is the addition of a separate vent for the float chamber when the engine is idling..and the mechanism that switches the vent back to the original location when the engine is turning faster than idle.
You'd have to go back further than the TR250 to have a Triumph sports car with factory installed "pre emission" carbs.

The biggest problem with ZS carbs is that the owner or mechanic is unable to make an adjustment to compensate for a component that simply needs replacement.
 
Couple of things I have observed and done when using the SU on the TR6. Float chambers need to be moved to obtain a straight up and down position and the needles needed changing. Been a long time, but surely someone will chime in with the right one.
If both are adjusted correctly, it's a toss up. Some engines like one better than another.
Wayne
 
I have TR4 or 4A Strombergs on my TR250 (same motor as a TR6). Yhey work fine, had to reverse the bell crank on the throttle linkage to get them to work right. SUs would work fine too. My brother replaced the Strombergs on his TR7 with TR4 SUs and that worked fine too. My relatively rural state does not do inspections, emissions or safety.
 
I also have installed SU HS6's on my 74 Six. I believe you don't need to worry much about emissions as most of these are exempt because of age. I have no emissions on mine at all. I like the SU's as they are simple, easily adjusted, and near bullet proof. Plus 1 on the float bowls. You do need to take them off and fool with the spacer piece to get them rotated for vertical on a TR 6 as opposed as on a TR4. Plus, if you need to with a modified engine, you can readily change needles. The SU competition book has a lot of information on them, and needle profiles.

Perry
 
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