Round Tops Fuel Systems

[NVZEE]

Not sure if this falls under the “better mouse trap” or “new can of worms” heading but it seems worth knowing about ...

https://roundtopsfuelsystems.com/

 

Edited May 28, 2018 by NVZEE

[Freez74]

They look very nice, and only 1000 dollars a pair.

[240260280]

I see brass throttle valve rods running through the centre rather than steel.  Do they still use bushings or have they upgraded to bearings?

[siteunseen]

Are these the ones zcardepot.com sales?  Look the same.

https://zcardepot.com/all-products/fuel/carburetor/su-round-top-carburetor-kit-new-240z

[NVZEE]

They look like what ZCD is selling. There’s facebook discussion that this is basically the later flat top SU with a round dome that gets away from the stigma associated with it.

Edited May 28, 2018 by NVZEE

[zKars]

While there may be similarities to some of the updated functions that the old flat tops provided, these brand new carbs have nothing in common with them.

These look like an excellent alternative to our old SU's. Seems to have the best of all the features you could ask for in a constant velocity carb. Brand new, modern support and parts availability being right up there with the design improvements. 

[NVZEE]

Aha — looks like the “real” British SU, model HIF44 for Jag, Mini, MG, etc.:

https://www.scparts.co.uk/sc_en/british-cars/jaguar/jaguar-xj6-series-i-iii-and-daimler-sovereign-series-i-iii-1968-1987/carburettor-fuel-injection-and-air-filter-competition/carburettor-and-fuel-injection/carburettors-51063.html

Edited May 28, 2018 by NVZEE

[w3wilkes]

Slightly unrelated... Last year when I did a full fuel system cleaning, tank lining and hose replacement with E85 rated lines I also asked ZTherapy about E85 fuel with their rebuilds and they told me that they ran a soak test a few years ago and saw no ill effects. I have no personal experience since I've run nothing but non-ethanol fuel since the fuel system refresh.

[Captain Obvious]

Yeah, those are the HIF series. Basically round tops with integral float chamber, and some other improvements.

From http://sucarb.co.uk/technical-carburetter-maintenance-settings-hif:

The type HIF (Horizontal Integral Float Chamber) carburetter has been designed primarily to meet the requirements of exhaust emission control carburation systems. Functionally similar to previous S.U carburetters, the HIF uses the variable choke/constant depression principle to achieve the precise induction of mixture required to control the toxic elements of exhaust emission to within statutory requirements.

I wonder what needles they come with. Was that mentioned somewhere and I missed it?

[NVZEE]

It’s a little vague:

The pre-installed needle is perfect for street and light track applications on a stock L24. 

[Captain Obvious]

Haha!  It's a LOT vague!

There was a guy on zcar.com who was recently trying to adapt a pair of HIF6 carbs off an old Volvo onto his 260. He was using BCL needles. There are also recommendations out there for BCA, BCE, and BBZ.

Long story short there was that he eventually gave up and went with a pair of standard HS round tops although he never really got to the root of why he couldn't the HIF's to work properly. He cut his losses and bailed.

Fish or cut bait...

[tdoakley]

On 5/28/2018 at 9:01 AM, NVZEE said:

They look like what ZCD is selling. There’s facebook discussion that this is basically the later flat top SU with a round dome that gets away from the stigma associated with it.

They were designed and built specifically for the the 240, 260, and 280 Z cars.  The HIF design was chosen to fight the vapor lock issue.  The carbs use a dual pass through fuel feed to insure that the floats are loaded equally when subjected to cornering loads.  These carbs utilize a temperature compensating device to make them more friendly in hot and cold weather. The use the same fuel pressure as the original Hitachi versions.  They bolt to the stock manifolds and use the much of the stock linkage.  Adapters are available to fit stock air cleaners.  Screen fitted velocity stacks are also available.

Work has continued on the dyno, and the latest versions are configured more toward performance (although the 2018 version meets or exceeds the orginal Hitachi roundtop performance in our tests). The bits reacquired for this are easily retrofitted if desired. All of the modern needles will work if your custom cam and headers require it.