Big SU on GS850

[Grimly]

Number three of four

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[Grimly]

Number four of four

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[mvalenti]

Where did you find that 1 into 4 intake?

[GelandeStrasse]

Cool idea. How does it run?

[Flyboy]

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."

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

[Grimly]

Where did you find that 1 into 4 intake?

Got a local alloy welder to make it after I'd cut the sections out as necessary. It was originally an intake manifold from a Hillman / Chrysler Avenger of 70s vintage, on which the Stromberg carb shared the same flange dimensions as the SU. Come to think of it, it might even have been from a later Sunbeam Talbot, same engine/carb setup as the Avenger.
The flanges to head are simply the alloy inserts from the original Suzuki carb rubbers.

Cool idea. How does it run?

Getting better all the time. When I did it (to both bikes) the useage of them was long distance motorway work and it was pretty much something/anything would work at steady state continuous speed, with the odd plug chop to keep things right. I did try a few needles back then and settled on one for the next 20 years. As I said above though, it never idled all that well but that didn't really matter too much. When I put this one back on the road I decided to sort it out properly and that's why I went down a carb size - the idling is much better and tractability isn't really too much different from stock. The main difference is running out of top-end breath, but <shrug> it's not something that bothers me.

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

Which I would still have on the bike if they weren't so rip-off expensive to rebuild. I never really had any objections to the Mikunis at all as carbs - in fact, with my previous history of SUs I was quite at home with them. In reality, the Mikuni is closer to the Stromberg CD series (which the other bike has on it).
Of course, the Stromberg has only one diaphragm to replace (at a cost of about a buck) and not a bank of four at a time.



Still to do;
Make a larger pancake intake filter - the one that's on it is just to get something on there to filter crud, better than nothing. I had a K&N classic car one for the HIF44, but the carb mating plate is totally the wrong size/fitting for the 38. Not that big a deal to molish something up that will work properly.
Inside the filter is a stub-stack, which probably helps a bit.

[Flyboy]

Which I would still have on the bike if they weren't so rip-off expensive to rebuild. I never really had any objections to the Mikunis at all as carbs - in fact, with my previous history of SUs I was quite at home with them. In reality, the Mikuni is closer to the Stromberg CD series (which the other bike has on it).
Of course, the Stromberg has only one diaphragm to replace (at a cost of about a buck) and not a bank of four at a time.

Dave, not knocking it, we know about prices, just responding to Jag's copy and paste of how versatile the SU is, and its ingeniously simple operation, just pointed out that the Mikuni works on exactly the same principle.
for commuting, anything that mixes air and fuel at around 15:1 is good.
Now a nice 38 DCOE Weber should liven it up.

[Grimly]

Now a nice 38 DCOE Weber should liven it up.

That had been given serious consideration at the time and they were cheap back then, especially in the South of England where lots of tuning boys lived.
Space considerations ruled it out, though. The only way of doing it would have been side-on, and the mixture distribution would have been horrendous.

[Grimly]

Carried out what will likely be the final needle change, to an AAF, with yellow spring. Testing continues, but it's running well.
This was prompted by the fitting of the K&N element from the other filter to this one. Turns out the skinny pancake filter was being more restrictive than I thought and the K&N, being deeper and better at its job was allowing much more air to pass, bumping the mixture onto the weak side at 70mph - it stopped accelerating totally in top gear. I sussed out what was happening, richened up the mixture a bit and avoided running it in the weak zone for a couple of days until I fitted the AAF, which is richer from the mid-range upwards, by just the right amount, I believe.
I've still got a couple of needles to try, but I suspect I won't actually need them now, as the configuration is pretty much where I want it.

[GregT]

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

Well yes, but as he's pointed out, much more adjustable. The biggest point with SU's is the spring selection available IMO.

[Chris G]

My brother and I have a 1980 Range Rover fully modded for off road comp, shortened the chassis by 10" radiator in the back too much to remember, spent months working on it, twin SU's sounded great, not heard it running in many a year,now tucked away at the farm in the back of an old horse box

[Grimly]

Seem to be on the final straight with it now. I junked the crappy ebay pancake filter - filter is too kind a word for it - and used the front and rear pans of to retain a K&N element that was on the HIF44 until recently.
This had the unexpected bonus of pointing up the shortfall of the original pancake - it ran out of breathing room before the manifold did and with the AAZ in, it was like hitting a brick wall at 70mph. It just wouldn't go any faster. Immediately realising it was doing this because it was now weaker than my grannie's tea, I tootled around at 50mph for a day or two to avoid damage. I then fitted an AAF - an improvement, but still not willing to go much above 90mph.
So, realising all that lovely air the K&N has allowed in has changed the game, I decided to throw the ADK back in and see what happened. On the mintylamb comparison, it looks as though it will be somewhere close to right, when its idle is adjusted for.
It seems ok with the ADK - the idle isn't quite as it should be but it pulls well from 2K upwards and happily tops out (9K) in gears 1 to 4. Top road speed isn't known but it never went faster than an indicated 115mph with the HIF44 on it.
It's weird, this carb fiddling lark - with the combination of inlet doohickeries; K&N, stub stack, HIF38, yellow spring, ADK, plus the characteristics of the manifold itself (very random) there's now an interesting surge of torque in the lower rev band that wasn't there before. Things just happened to come together to cause that. It's actually very useful - very useful, indeed. Especially at lights...
Looking at the filter now you can see the crapness of the ebay pancake - the K&N probably has 4x the surface area.




Here's a comparison of needles mentioned in this later part of the thread...


From this very useful site: http://www.mintylamb.co.uk/suneedle/

[850 Combat]

They are something that you remove to install Webers.

[850 Combat]

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

Those are interesting cars. Highly unusual front suspension. Probably the best thing Rover ever made, and certainly better than what replaced it. My brother test drove one and took me along. How did yours treat you?

[Grimly]

They are something that you remove to install Webers.

If there was space, a Weber would have been tried, believe you me. I don't have an axe to grind either way, having worked on both, professionally, many decades ago. I like Webers, but the setup costs can rapidly run away, and if doing this on a small budget it makes more sense to use what's available cheaply and easily.
I'd also considerd a downdraft Weber, like the 28/36, but really didn't think it would have rewarded the effort, even if it fitted.