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Mitch Ware
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I wonder if anyone knows what carbs were first used by Lotus during the initial developement of the 907. It seems rather unlikely that they were using ZS carbs.

One of my projects this winter is to rebuild my ZS carbs. These are the same ones that came stock on my 71 TR6. A friend of mine has a set of 2" SU carbs lieing around that he said I can put on the JH if I want, we used them when I first got the TR6 running after 30 years of sitting idle and the car ran great with them. It seems to me that since the SU can be bolted right on in place of the ZS carbs on a TR6, perhaps the same thing can be done on a JH.

Anyone tried it before?

Mitch Ware
1974 JH-5 #111119670
1971 TR-6 #CC66950LO

Mark Rosenbaum
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No reason why SUs shouldn't work, and I've seen a photo or two with SUs on a 907 engine, but with 2" carbs, you'd have carb-to-manifold interface problems due to the size differential.  You'd have to gin up throttle and choke linkages, too.

The real problem would be mixture needles.  It's my understanding that the fuel metering characteristics of SUs differ from those of Strombergs, so you can not just use mixture needles with the same profile as the Stromberg B1DK, but would have to experiment, which might take hours or even days of track or dynamometer time.

As for carbs used during engine development, doubtless Lotus would have used Del'lortos for most of it, though they may have used something else (most likely Webers) in the earliest (head design on Vauxhall block) stages.

Jensen Healey
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IIRC the development engines had fuel injection and put out 220 HP. Pretty close to what Jensini gets from a 2 liter with 11/1 compression, Elgin cams and fuel injection. I suspect those HP numbers are achieved somewhat north of 7000 rpm. 

Esprit2
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I don't think there's a significant advantage (if any) to switching from Zenith Stromberg carbs to SU's.   Functionally, the SU and Stromberg carbs are very similar and there's not a significant performance or reliability advantage one way or the other.

The Stromberg's negative reputation is not entirely deserved, and many of the complaints apply equally to SU's.   If it were a Weber or Dellorto guy making the complaints I could understand.   But for an SU guy to complain about Strombergs...  well, a fellow who lives in a glass house shouldn't throw stones.

One of the main reasons for the Stromberg reputation is SU guys who couldn't be bothered to learn the difference.   There are a lot more SU's out there on a lot more cars...  and a lot more SU guys who really don't know the specifics about the relatively rare,  similar but different Strombergs.  SU guys often make the mistage of assuming Strombergs look similar, so they must be the same... right?   Well, no, they're not.

Some over-zealous SU guy would go after a Zenith-Stromberg with a screw driver and totally mess it up out of ignorance in no time flat.   Then in frustration,  declare that those stupid Strombergs are no good!   To the contrary, in some ways, they are actually superior to SU's.

On the other hand,  I don't like any constant depression carb of any brand (SU, Stromberg, Solex, Mikuni... etc) nearly as much as I like more conventional carbs like Weber, or my favorite... Dellorto. With CD carbs, you step on the throttle then wait for the manifold vacuum to work on the diaphragm to move the air valve piston. CD throttle response is very slow compared to a straight, mechanical butterfly throttle.

The flip side is that since CD carbs respond to engine demand/ manifold vacuum, it's easier to start from a stop without stalling the engine and the engine can be lugged down to lower rpm without bucking or stalling.   If you live in San Francisco and live in fear of rolling backwards into the bay after botching an uphill start, then CD's are for you.

If you enjoy spirited driving or use the car in competition where quick throttle response is an asset, then go with Webers or Dellortos.

If you have Strombergs and have money to spend on a carb upgrade,  go for Dellortos.   The performance difference due to switching to SU's doesn't justify the expenditure. Webers or Dellortos will actually make a noticeable difference, and the Dellortos are the better of the two.

Get a book on tuning Zenith Stromberg carbs and put a little off-season time into learning about Strombergs and making the original ones right again.   If nothing else, 30+ years have taken their toll on the rubber diaphragm and some gaskets, so a general rebuild is probably in order.

Happy Holidays,

Tim

Esprit2
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Jensen Healey wrote:   IIRC the development engines had fuel injection and put out 220 HP.

Only the 904 & 906 race engines for the Lotus Mk 62 were fuel injected,  using the Tecalemit-Jackson mechanical system with slide throttles.

When development started in 1968,  emissions wasn't much of an issue,  so the 905 street development engine used Dellortos.    It wasn't until the need to meet mandated emissions requirements arose that Zenith Strombergs were installed.

The 904 & 906 engines were used in the Lotus Mk 62,  a pure race car with Europa-esque profile but no streetable counterpart.  . The Mk 62 car was built as much as a development test bed for the new engine as a race car.   It was felt that racing the engine would accelerate the learning curve.

Regards  --   Tim



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