SU Mod (lifted from the net)

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source: http://www.mgexperience.net/archive/su_improvments/207908

"On HS2s and HS4s for racing we thru-bore them, meaning

no bridge what so ever

, on HS2 you will actually run out of room, meaning a hole in the bottom of the carb you have to epoxy up. However for street cars this would ruin driveabilty, but with race carb nothing but wide open throttle matters. On a street car I would not worry about bridge mods, if I made any mods to a stock carb for the street I would think about beveling the slide so the square leading and trailing edge is nicely radiused towards the opening front and rear. There are a good many books on SU carbs that was mentioned above, also consider getting on chassis dyno with a air/fuel ratio meter, you might be pleasantly surprised with the gains you can make with stock carbs with the proper needles and jet adjustments."

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note the epoxied gentle ramp leading to the bridge that still keeps the "step" for turbulence.

Edited September 5, 2011 by Blue

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Norm's famous "tosser" mod

 

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Draw-through with extra fuel bowl.

 

 

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Triple SU's

 

 

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Bridge removed to enlarge opening and brass "wing" positioned as a substitute.

 

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Source: Col. Joe Hauser http://www.datsun.org/fairlady/JoeCarbs.htm

CARBURETORS

Under SCCA rules for G Production, the Datsun 1600 was required to run a pair of Hitachi carbs. These were very similar to British SUs. In fact the needle valve from one would fit in the other. The SCCA rules allowed substantial modifications to the carbs as long as the bore was not enlarged at the butterfly valve. So over the years we made the following modifications:-

Rounded the front of the piston

Removed the step at the jet

Angled the rear of the jet to help draw fuel mixture

Increased the diameter of the bore to a point just in front of the throttle plate (3/32")

Thinned the throttle shaft and butterflies

Used very light springs on top of the pistons, or no springs at all

Tuned the intakes with velocity stacks to suit the RPM range of the engine

Set carb float levels slightly higher than stock so no starvation at high RPM. I used brass floats. The most important and difficult part was finding the right needle end jet combination.

My final solution was boring the stock jet larger to .095 and finding a needle that would provide a good fuel air mixture over the rev range I was using. My final solution was to customize the needle on the dyno and at the track by chucking the needle in an electric drill and dressing it down to suit. Not very scientific but it worked.

I also matched the carburetor ports to the intake manifold ports. After some trial and error I found that by leaving the carb ports slightly smaller than the intake manifold that it was as good or better than matching and took less work. (This is the reversion factor). The intake manifold was matched to the head in the same way.

The intake manifold was enlarged to properly match the carburetor and the head ports. The hole in the balance tube was enlarged to 9/16". This smoothed the power out put and improved the idle.

HEADERS

The opposite of intake is exhaust, if you get it in efficiently you must get it out efficiently. There were no good headers available for the 1600 when I started racing so I was forced to make one. Out came all the books I could find on building a good header. So I made two headers, both 4 in to one. One was made from 1½" exhaust tubing and one from 1 3/4" tubing. Both were made so that I could extend or shorten the tubes to find the best length of the pipes. Tubes were made as close as possible to the same length and after much dyno and track testing we found that the 1½" tubing about 29½" long, dumped into a 2½" pipe, gave us the best all around power output. The 2½" pipe was extended to the rear of the car with a megaphone on the end. However when SCCA decreed the 104db sound requirement the megaphone had to be removed. The power loss was minimal if any.

I loved the sound of that megaphone.

Col. Joe Hauser

[Bonzi Lon]

Always great information, Blue.

I like my Roadster's megaphone.

Bonzi Lon

[John Coffey]

Again, all the SU mods listed above are for track use where you're at full throttle or nothing. On a 200hp track 240Z running SUs the throttle is a light switch: brake, turn in to the corner, slap the throttle to the floor. In some cases the mods above will not allow the car to idle and you will have to keep it running by blipping the throttle.

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TWM-Throttle Bodies-3004 Series DCNF

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su tapered piston