SU carbs

[Forumadmin]

People who have had Zeniths refurbished notice a vast improvement. Although on cold starting they should need choke (if not they are too rich!) . Also note that Richard Gane's racing Jup also has Zeniths (although not 30VM) and I defy anyone to beat that car on the circuit with any carb.

However, that said , given the choice, Richard and all the racing boys would put on something other than Zenith, were it not for homologation. Webers and Dellortos are good because they offer a wide range of jets so that you can match your engine and your requirements of power and consumption.

Petrol injection, gas bottle conversion or steam injection are all mods that could be applied. As could larger pistons, super or turbo charging. Most of these have already been tried but I think, without exception, the problems then came with reliability.

For road use, a pair of refurbished Zeniths is fine.

I would be interested in petrol injection as then you can measure on every stroke the amount of fuel required using an engine management system. But why not then just put a Subaru engine in? The joy in the Jup is its 60 year old mechanicals and the satisfaction of using an old car.

[Edmund Nankivell]

I would say the "Joy of the Jup" also includes its soft ride and light, positive steering compared with its contemporaries. Not forgetting the chassis that does not rust plus much of its bodywork being aluminium. I got 22mpg from Zeniths and get 35mpg and better with Webers.

[David Kemp]

Did you need to tinker much with jet settings on the Webers to get them right?

[Forumadmin]

Maintaining correct mixture under all load conditions requires a bit more than tinkering! For racing, which is basically just full on, the requirements are different and petrol consumption is not a consideration. For rallying, the loads and altititudes are also more varied than just normal road going. It has been known for crews to alter the carbs prior to a high altitude hill climb. For modern high speed motorway driving with constant light load , the Jowett set up is not correct.

Most trouble is experienced with the heating and cooling of the carb in its exposed Jowett position. Various electric and water blanket solutions have been tried.

But yes some work was done selecting jets in the 1980's to suit the carbs by Dennis Sparrow (Weber) and Mike Smailes (DelLorto), but of course the real test needs to be done on each engine (which may have different cam and capacity) on a rolling road with the now updated petrol formulation. The first challenge is getting the carb in the right place with respect to the inlet valves. Some have tried extending the inlet tract and of course widening it to suit a larger throat. Some have also tried to separate the siamese allowing twin chokes to be used.

Personally I would spend the time tuning the cam and the timing (both valve and ignition) if you want performance. As for carbs, I would recommend DelLorto as I have had no trouble with them (apart from a loose jet) in 30 years. But Zeniths can be made to work for normal road use.

You might like to read this post from a the alfa boys who also have a boxer engine. It says that Dellortos have better atomisation over the rev range.

[David Kemp]

I notice on the North American Jowett register, blocks spacers are available to adapt webers to the Jupiter heads. Does this mean this is a common conversion in North America? If so what weber is everyone fitting?

[Srenner]

Early Webers are 32 IMPE that were used by Fiat, among others. Modern Webers are 34 ICT or ICH (one has a choke, one does not). These can be mounted on a modified Zenith spacer by chamfering the top from 34mm to the Jowett port size or opening the head up to 34mm.

The Jowett spacer is used in all applications to match the slope of the head. The taller adapter does two things. It gets the throttle rod back to a height that clears the block, but more importantly, it adds volume, like a plenum, so the two adjacent cylinders have a larger volume to draw on.

[David Kemp]

Thanks Scott, what do run on your Jupiter? Also has anyone tried a cross ram manifold like a 60's Mopar? Would good with twin SU's.

[Jack]

The power of JowettTalk (and indeed the internet) on this subject.

Thanks to everyone who continues to contribute to the discussion, just look at the distances involved in being able to contribute like this:

https://maps.google.co.uk/maps?q=brisba ... sic&dg=brw

By my very dodgy reckoning, if this topic was a letter it would have travelled approx 100,000 miles.

Jack.

[PJGD]

If only we had non-siamesed ports, i.e. separate intake ports to each cylinder, we would be a lot better off in my opinion; but we don't, so our options are limited.

If they were separated, we could join cylinders 1 & 2 and 3 & 4 which would give us even-spacing or duration between suction events per manifold. As it stands, due to the 1-4-2-3 firing order, the induction events go 4-2 on one bank and then 3-1 on the other bank resulting in uneven mixture distribution. Instead, imagine two Bradford-like manifolds that spanned the width of the engine that only came together in the middle for a single carburetor (most likely), or a carburetor per manifold. SU's should work well in this scenario. These long runners could be expected to drop the peak torque speed from the present rather high 3000 rev/min to some lower value that might even improve drivability.

Better still would be two runners coming up from each head to a log-style intake manifold running across the top of the engine in which the lengths and volumes would be tuned to behave as a Helmholtz resonator. This might even work with our present siamesed ports. In this arrangement, the peak torque value could likely be enhanced and moved to the speed of choice through judicious tuning of volumes. The calculations for this are readily available on the internet. Of course, such a modification today would take up valuable real estate on top of the engine and change the appearance of the engine significantly.

I seem to recall many years ago that either Gerald Palmer or Charles Grandfield telling us that during the development phase they tried several different carburettors including Amals (!), but Zeniths gave the best results supposedly. On the subject of SU carburetors, I would have thought that the type from the early 750 or 850 cc BMC Mini might be appropriate since each of our banks is 750 cc. A simple short manifold adapter with a 45° bend to match the original Mini attitude could fit in quite neatly. I am not sure how well the SU "constant vacuum" principle would work with two sucks and then two periods of nothing; it might not respond well to that. Most likely it would accentuate the mixture distribution mismatch is my guess, but perhaps the effect would be too small to notice.

Philip

[David Kemp]

Anyone tried a Bradford or VW type manifold with a two barrel progressive carb? Many of my little brothers VW performance books claim this is better for street cars than each head having it's own carb(single port engines)

[Keith Clements]

Mike Allfrey has these carbs fitted . Flip through the album and see the complex heating, throttle and choke arrangements. There are two batches so move forward to this as well.

Once again sorry these are not in the Gallery but well over a 1000 pictures of my travels would have overloaded the system!

More carb mods in this link.

[jowettgeoff]

The Dellorto alternative to the Weber 32IMPEs is the 32FRDA(e). These are still available new from Dellorto. A manifold adapter/spacer has to be made, because the pcd of the flange bolt holes differs from the Zeniths. Also, as the name implies, the choke diameter is 32mm as opposed to 30mm. Talking of twin progressive Webers, I once experimented with a 28/36 Weber mounted centrally on a long twin manifold (Bradford style). The engine was very smooth, but not noticeably any more powerful. I then tried two of these carbs (one per side), mounted on flower-pot style adaptors, cast in aluminium. Petrol consumption was frightening, for no noticeable gain in performance. A long time Jowetteer from York (Tony Fewster) tried SUs, but even with the help of an 'expert' tuner, couldn't get them to perform very well.
A carb which I would have loved to try was the Reece Fish. These didn't have a venturi as such, but fed fuel in through holes in the throttle spindle. Apparently they were quite successful on VW Beetle engines.

GMcA

[aj4cks]

Have fitted Webbers sourced by Dennise Sparrow with adapters . Once you have Webbers you will bin Zeniths

[dougie]

Hi David,
Give Brian Holmes a call re the webers.
I am about to convert my car to weber 30 icf carbs.
We are in your back yard and willing to help.

[aj4cks]

Bought a set of Webbers from Dennis, easy adaption. Car runs sweetly and better than the refurb Zeniths that I had. Tweaked tuning by hand till happy.
Look better than Dellortos