Spitfire ZS Stromberg vs HS4 SU on 1300 Spitfire?

[dklawson]

Forgive me if this topic has been discussed recently.

Our '72 Spit is months from being on the road. It has a single <span style="text-decoration: underline">ZS Stromberg</span> carb. I've never worked with ZS carbs, only the the CD carbs on our GT6. What I've noticed in the short time we've had the Spit is that the dashpot oil seems to leak out at an alarming rate considering we're not driving the car. I had an annoying misfire the last time we drove the Spit and it turned out to be a nearly empty dashpot... after only two weeks of sitting after being filled.

I've got several HS4 carbs from other cars. I am considering fitting one of these SUs to the Spitfire sometime in the future. I know the HS4s were used in pairs on some of the 1500s. Google has told me that single HS4s were fit to some Triumph 1300s (specifically the Toledo) in the U.K.

I've got a couple of needles that happen to be the same type (ADD and ACC) as those used on various Toledo models. Has anyone here ever installed a single HS4 in place of a single ZS carb and if so, what needle did you use? Was this swap an improvement or a waste of time?

I'm just kicking around ideas so please feel free to comment either way on whether this carb swap is worthwhile or not. If I didn't already have the SUs I don't think I'd even think about this.

EDIT: Corrected reference to the Spit having an HS4 carb now. It has a ZS carb as stock.

 

[guzzul]

Doug - sorry, I may be confused here. You indicate you have a single HS4 carb, but then go on to say you are thinking of replacing it with an SU. Do you have a ZS carb on the car now or an SU?

If it is indeed a ZS carb, then there is a rubber seal at the bottom of the dashpot that is supposed to stop oil from leaking out of the air valve. That seal may be shot.

Have a look at Nelson Riedel's excellent article(s) on rebuilding ZS's at this link
https://web.archive.org/web/2004032718315...sII/CarbsII.htm

He discusses removing the needle and the adjusting screw, along with this seal. It is a pretty easy job, and the seal usually comes with any of the cab rebuilt kits.

 

[poolboy]

Doug, what oil are you using ? It shouldn't leak no matter what, but a thicker oil might slow it down.

 

[Opa]

Doug,you might want to try a splash of auto.trans. sealer. I use a product made by Wynn.It may soften up the little o ring enough to seal. I use the stuff at about a 50/50 mix. Have used it straight as well in carbs that have sat awhile.

 

[dklawson]

Sorry Ross, my mistake. I typed that the car has an HS4... when what I meant to say is that it has a single ZS carb. I have edited my first post. Thanks for pointing it out.

I know about the o-ring seal and I'm sure I could replace it. However, I have not heard any good things about the ZS carbs anyway. Admittedly this has been largely from the MGB and Midget owners who found these carbs on their late model cars. While I could rebuild the ZS carb, it's way down on my list of things for this project car. My hope was to fit the HS4 since I have some. I am also much more familiar with them and the early Stromberg CD carbs which adjust the jet from "underneath" instead of the needle from "above".

I have always used 20W oil in my SU and Stromberg carbs. My point about the rate at which the oil leaks out is that we aren't even using the car. The oil leaked out without engine heat or vacuum after two weeks of just sitting parked. I'm sure I could fix the ZS, I'm just not sure I want to.

Thanks for the comments and suggestions.

 

[Andrew Mace]

Doug, as you and others might know, The Stromberg carburetor was originally developed by Standard-Triumph at least in part to get around paying then-rival BMC to use "their" SU carburetors. At the time, it was widely claimed by S-T that test drivers couldn't tell whether a TR4 (on which they were first installed) had Strombergs or SUs. Bottom line is that, all else being equal, they're about the same carburetor...meaning that there shouldn't be any problem using an SU HS4 in place of a Stromberg 150, IF you can get the right needle, etc.

I don't have tons of experience with the Strombergs, but I can say that I've seldom had to do much of anything to one beyond occasional cleaning (like an SU), topping up damper oil and replacing of diaphragms. In fact, I did tons of maintenance over about a 15-year period on a friend's bone-stock '72 Spitfire, and I don't think I really ever touched the carburetor!

If you do decide to massage the 1296 a bit at some point to increase from 1972's original 48hp rating, then you'll probably find the HS4 easier to re-adapt. Otherwise, I suspect it's a toss-up as to which is "better"!

 

[IanF]

The oil leaking out is definitely due to the o-ring being shot. Fortunately, it's pretty easy to replace. I did this to the Z-S on Jerseygirl's car prior to swapping on the HS2's.

A local NASS member in SE PA swapped the Z-S for a single HS4 on his '79 1500. Greg.. (I forget his last name). He said the car runs much more reliably now and he uses the car as his DD during the warmer months. I fought so much (actually, still fighting...) with the HS2's, I was tempted to go this route, but the Z-S manifold won't work with the Bell header so my options were limited. Regardless, find him on the NASS list and shoot him an e-mail and he should be able to help you.

For better or worse, I've discovered you'll generally get better/faster answers about Spits on NASS than here (which seem to be more TR-focused).

 

[guzzul]

... However, I have not heard any good things about the ZS carbs anyway. Admittedly this has been largely from the MGB and Midget owners who found these carbs on their late model cars. ...

Just a last thought, and to add to Andy's comments, I also find the Strombergs a pretty good design, and no better or worse than the SU. I guess it does come down to what you are more comfortable with.

Having said that, I am more partial to the earlier carb models, rather than the later-model ZS's (like those that would have been used on the MG 1500 engine). Certainly the ZS water choke can be problematic, as can the temperature compensator. Because of emissions regulations, later model carbs are not as easy to service and adjust. I am not as familiar with SU's, but I assume they somewhat suffered the same fate. What was an elegantly simple carb early on became less elegant in later years.

 

[dklawson]

Thanks again all. The GT6 has "traditional" CD Strombergs and I have no problem working with them. Like others said above, the early Strombergs and SUs seem very similar. My goal here is to put a single carb on the Spit that has a jet that adjusts from below instead of trying to tweak the needle from above. Since I have the HS4s it just seems like a logical fit.

The HS4s I have happen to have two of the needles needles used with the 1300 on the Triumph Toledo. Hopefully that means I've got good starting points for needle selection.

Again, thanks for the comments and observations.

 

[Andrew Mace]

The HS4s I have happen to have two of the needles needles used with the 1300 on the Triumph Toledo. Hopefully that means I've got good starting points for needle selection.

How much difference it makes, I don't know. But the Toledo never suffered from the same emissions regulations that brought a roughly 60 hp engine down to 48 hp, although in the case of the '72, that might be mostly due to lower compression and a different cam profile.

 

[70herald]

Doug
I have a single Stromberg CD150 on my 1300 Herald (same engine) It does not leak oil from the dashpot and runs quite nicely.

As long as the holes for the throttle shaft aren't worn out you can easily rebuild it in an evening. It is much easier replacing an o-ring than trying to figure out what needle is best.

 

[dklawson]

Thanks again to all.

If I try this swap, it won't be until the spring. I don't see a real issue trying the HS4 since I happen to have at least two of the needles used on the Toledo. That implies I should have at least a couple of reasonable needles to start with.