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Big SU on GS850

Grimly

Grimly

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Just a teaser - I'll take some pics shortly and bung them up.
 
Sorry, but I have to ask, ... who or what is "big SU"? :confused:
icon_shrug.gif


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Had them on my 67 Rover 2000TC. I think they were a common upgrade for 240Z owners......

 
SU's where great carbs. New a guy who had a set of twin units with remote fuel bowls on a CB750. Nice and simply butterfly valves controlling air flow, so fuel / air mix was controlled purely by the pressure difference lifting the piston/needle (if memory serves correct). MOD seemed quite easy. Units came of a mini, rejetted, he made up a manifold out of copper pipe.

image.axd

2cbe209e63389434ffff800dffffd502.jpg


http://en.m.wikipedia.org/wiki/SU_Carburettor

Same bloke had a Z9 supercharged methanol burning drag bike being feed through a single SU.

Lots of filter choice to boot.
 
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You could change the mixture by changing the weight and volume of the oil in the vacuum chamber, complicated stuff to figure out on your own.

MAybe the Big SU is really Big Sue.
 
tkent02 said:
You could change the mixture by changing the weight and volume of the oil in the vacuum chamber, complicated stuff to figure out on your own.

MAybe the Big SU is really Big Sue.
Click to expand...

No need to **** around with the dashpot dampner. Just play with the needles. The carb did the rest. No vacuum chamber, pressure dif, was transmitted to top of piston, bottom was at atmosphere, so piston rises and falls depending on the demands of the engine. No direct throttle link. Throttle open/closed butterfly which controlled air flow through venturi.

"Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. ,The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions."
 
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Had a pair on a 63 Volvo- ok, but it took Keihin (and old man honda) to improve the CV scheme.
 
As promised, here are some pics...

Technical details...
S.U. Carburettor, c. 1980
Model HIF 38 (38mm throat).
Yellow spring
AAZ needle at the moment.
The first one I fitted was an HIF44 (44mm throat) and while it was ok, it never idled properly. The better gas speed over the jet on the smaller carb leads to a better idle and pickup, and for local running around the loss of top end isn't an issue. The 44 ran up to 115mph, this one tops out at 95mph or so.
Fast enough for around here, at any rate.
When I did this conversion I was looking at the thick end of ?600 (would have been around $1000 at the time) to rebuild the original carbs, so stupidly expensive were genuine Suzuki carb parts at the time, and since I had a deal of experience with S.U. carbs I decided to go with this. I did this on the other GS too. So, as you can see, I saved a thousand pounds by doing this. Both bikes at the time were just used as work hacks, spending their days running up and down the motorways at steady speeds, so a bank of multiple carbs was definitely over-kill anyway.
It works, and for my needs, works fine.

DSC_0001pancake.jpg
 
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No need to **** around with the dashpot dampner. Just play with the needles. The carb did the rest. No vacuum chamber, pressure dif, was transmitted to top of piston, bottom was at atmosphere, so piston rises and falls depending on the demands of the engine. No direct throttle link. Throttle open/closed butterfly which controlled air flow through venturi.

"Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. ,The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions."
Click to expand...

What do you know, just like the Mikuni CV carbs as fitted to the GS by the factory. :rolleyes:
 
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