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MG MGB Technical - replace Z cab aut. choke or go for SU



Just had an engine rebuit and installed on my 78B.
I was about to install the caburettor (Zenith 175 cd 5t) when I observed that the fast idle (spring loaded) plastics switch was missing. This switch is activated by the automatic choke operating liver unit.

My options are;
- shoul I just replace the automatic choke assembly
plus rebuild cab.kit for about $200. or
- look for a use choke assembly or
- Buy a webber cab

PS. i am geting it ready for smog.

looking forward to driving it and will appreciate your comments.
nat (2)

Since you're in California, the smog check will
include a visual inspection. The guaranteed way
to pass is to keep the Zenith, and as you suggest,
buy the new autochoke unit for about $150.00.

On the midget BBS, someone was able to pass
smog by replacing their Zenith 150CD with an
SU carb. Apparently the smog check requires
a sidedraft carburetor, but they don't specify
which manufacturer. Hence a Weber won't work on
your MGB for smog purposes, being downdraft, but
a single (not double) SU would. Victoria British
sells a SU carb conversion kit for $500 or so.
Ronald

How about a single SU hif6? You'd have a real carb plus a single sidedraft.
gerry masterman

I don't think I would put that crappy autochoke back on I would go for the manual choke conversion kit. Mine has been a dream ever since I did it.
Josh

Geez Gerry...I thought I put that "real" carb thing to rest....a properly functioning ZS (which some may claim is an oxymoron of sorts) should flow just as well as an HIF6 (I have some good theories, which seem to check out, as to why most ZSs require the awkward air cleaner to work with the late model B setup but that's outside the scope of this inquiry). Having said that, the SU HIF6 is a simpler device and you'd probably not have any problems with emissions testing (as long as they don't look for the heat flap on the air cleaner...which WAS, last I checked, on the CA emissions checklist). It all sort of depends upon where your emissions test takes place, as to the extent they'll beat up on your visual inspection. A Weber will not work in CA for the emissions test. Now, there's always the guy who encountered the tester that passed the visual without having a proper, single carb sidedraft configuration, however the chances of encountering such fortune are typically slim. I did know someone who passed the visual with an SU HS6 and a pancake style air filter but he knew the tester and his car did actually meet the tailpipe requirements (which, as you know, are sent directly from the test site to Sacremento, obviating any fudging of recorded numbers on the part of the technician).

You'd probably get by with a manual choke conversion (I passed smog in CA with one), however in your case, the plunger associated with the fast idle cam is still needed for the manual choke conversion to work properly (meaning that you do need to either have your auto-choke mechanism rebuilt or replace it with a new one). On the other hand, if you car is warmed up when you take it in for testing (which is a MUST anyhow), this mechanism will never be engaged and its absence will probably not be noted by the inspector. The only question is whether or not you can get by without a fast idle on cold start. If you can, it won't effect your emissions testing (however it is inconvenient), which gives you time to purchase a new unit or have yours worked at your leisure. If and when you do get another auto-choke, the manual conversion is really a nice addition (puts you back in charge).

Hope this helps.

Barry

Barry Kindig

I tend to agree with Barry. A properly maintained ZS isn't that bad. Given a choice, however, I would take the SUs any day. But with smog testing it doesn't sound like you have a choice. (I've never had to deal with that).
Considering the disgust most people have for the ZS and how they were swapped for other carbs, I would think there are buckets of used parts to be had for very little. I'm actually surprised no one has offered you a fast idle plunger or a whole autochoke. If nothing comes up here try posting your needs in the "Parts Wanted" section and see what happens.
Rick
Rick Jaskowiak

Barry, I supposed that you noticed that I'm biased . Of the hundreds of comments I have heard of ZS carbs, the number of favorable ones I can likely count on a single hand. SU's on the other hand, rarely get a deserving bad comment. If you are willing to take just a bit of time to understand an SU, you may never have any problems with one. Perhaps you can say the same for ZS's. There was no offence intended to you or any other ZS fans
gerry masterman

Gerry,

As you'll recall from one of my previous posts, I now run a twin HIF4 setup (albeit a somewhat modified one). I like it but also have to acknowledge the results of my work with the ZS to date (using an extensively modified environment and a different metering needle as well...it's documented to death in the archives). My HIF4s are actually modified to flow a bit better and accept the early HS4 non-biased needles (I'm running #006's now). I like their simplicity, however, I also did find some advantages to tuning the single ZS (hard to believe, I know). On the other hand, I sort of like tuning the twin SUs...it's just kind of fun and a bit more challenging (although now that they're set, I doubt I'll be having to perform much regular maintenance in that area). Anyhow, as I already expressed, the twin HIF4s flow better at high revs and provide a bit more apparent low-end torque (even over the very modified ZS setup I had previously described). And believe me, the HIF4s now sit in a very optimal environment for street use. However, all other things being equal (going back to the original discussion), a single ZS CD175 would perform just as well as a single HIF6...sort of comes down to the math at some point, which was the very reason I found the exercise so interesting at the time. I wanted to perform my own objective analysis of performance options in the area of induction, rather than following the herd (if you look in the archives, you'll find that carbs are the most discussed performance option, even though they do not provide the greatest return over other mods...made it all very interesting).

Again, the reason the late model B single ZS setup is so limited has much more to do with the integral manifold (and very lean metering needle required to complement that poorly breathing system) than it does with the carb per se. The choke mechanism on the ZS version implemented on the late model B was more complex than the HS4 or HIF4 setup for the earlier cars. However, different implementations of SUs have their own external enrichment setups as well...just never for the MGB.

Anyhow, I don't take any of this personally. It's just interesting to see subjective statements thrown around so freely in this environment, after so much has now been documented to dispell such notions. I always found it interesting that folks like John Twist and his crew couldn't figure out how to make the ZS setup work with anything other than the original type air cleaner (at least that's what he documented in his own Q&A book he used to post...before he started selling all of his tech advice). On the other hand, I found someone to work with who would actually design and fabricate something that would overcome such limitations and viola...proof once and for all that the problems were always elsewhere. The fact that I could find no objective reason for a performance difference (aside from various opinions), just drove me nuts, so I had to know. No rocket science...just simple math. And I think we all know that I'm certainly not the resident expert here...just someone with sufficient curiousity to look a bit further.

Have fun,

Barry
Barry Kindig

This thread was discussed between 10/05/2002 and 12/05/2002

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