Big SU on GS850

[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.

 

[robertbarr]

So, you'll be needing a Unisyn to get them balanced?

Anyone else but me remember what those were?

 

[Grimly]

So, you'll be needing a Unisyn to get them balanced?

Only one - doesn't need balanced.

Anyone else but me remember what those were?

I used several types back when. Some better than others. Unisyn sounds familiar, but the trad method of the tube to the ear/carb throat worked well if you were stuck out in the middle of nowhere, but I still preferred to see a figure on it.

 

[Alatheia]

Grimly,
I appreciate this info. Many years ago I had a volvo 122s (Amazon) with a set of SU's. I thought then there ought to be a way to use one profitably on a bike. Not the sexiest looking things ever but I've never been wild about messing with banks of carbs. For now my 850 project will keep it's CV's but this might be a future tinkering idea. Thanks. David

 

[850 Combat]

They were popular with the chopper crowd for a while. A cobby look that people liked. They will probably make a come back. I've never used them, but I've got webers and Dellortos on a couple cars. I like those. The Dellortos, especially. You don't have to run them as rich as Webers.

https://www.google.com/search?q=Har...00&bih=839#q=Harley+davidson+SU+carb&tbm=isch

For Grins:

Webers:

Dellortos:

 

[tirebiter]

Grimly, Have you considered using HS-6 needles and a .090" jet in the HIF44 ? Yes, the needles are shorter but you get so many more to choose from. I've had some luck with oddball carb/engine combinations i.e. Triumph inline 6 with 3 HD-8 SUs by utilizing the "wrong" needles and playing with the fuel level setting to acheive a solid idle and good tractiibility at very slight throttle openings. Plenty of mid and top end also. The ignition timing vacuum control may be what saved that whole secenario.

 

[Grimly]

Grimly, Have you considered using HS-6 needles and a .090" jet in the HIF44 ? Yes, the needles are shorter but you get so many more to choose from. I've had some luck with oddball carb/engine combinations i.e. Triumph inline 6 with 3 HD-8 SUs by utilizing the "wrong" needles and playing with the fuel level setting to acheive a solid idle and good tractiibility at very slight throttle openings. Plenty of mid and top end also. The ignition timing vacuum control may be what saved that whole secenario.

Yep, I thought of that, and given that I have a couple of spare HIF44 bodies I might go down that route for the other bike and ditch the Stromberg on that - many less options for Strombergs anyway. May as well make use of what I've got rather than buy more - otoh, I don't want to get into buying more needles at a tenner a pop, so it's a question of weighing it up.
I'd toyed with the idea of restricting the 44 throat and using a 38 needle/jet, to get it in the same ballpark.
Otoh - I'm taken with the idea of a simple twin SU setup. Thousands were made and are lurking in sheds. Only problem - twice the rebuild cost and every time a change is needed, double the price up.

 

[Grimly]

Grimly,
I appreciate this info. Many years ago I had a volvo 122s (Amazon) with a set of SU's. I thought then there ought to be a way to use one profitably on a bike. Not the sexiest looking things ever but I've never been wild about messing with banks of carbs. For now my 850 project will keep it's CV's but this might be a future tinkering idea. Thanks. David

This is why I've documented this process; for anyone who wants to try it sometime, at least they'll know what to shoot for. If I get this setup as good as it can go it'll save someone else a lot of time.

 

[Grimly]

A slight update.
It's been running through the autumn and winter with the ADK needle, yellow spring and K&N filter, and every time I've been out on it, I've been enjoying it. Not thrashing it, but certainly not driving it economically. As a result, the fuel figures haven't been all that good - 38mpg on average, 34 when I give it a lot of mentalness and even dropping to 24mpg when I was doing nothing but ragging it up and down the local by-pass.
Today I decided to just take it out and drive it like a granny and see what it would do.
Filled up, set off on a long, mostly straight, rural route to the motorway, max of 50mph and accelerating no faster than I would if in my car. On the motorway, I got up to speed relatively slowly (very little traffic around, so no problems with joining or leaving it) and sat at 55 - 60mph, at a fairly steady 4500rpm for about ten miles or so then came the same route back to the filling station.
Filled up, took a note of the figures, and worked them out when I got back home.
After leaving the filling station, I went ragging it up and down the by-pass again. Couldn't resist it, it was like an animal wanting to be free of these damned reins.
Back home, worked out the fuel consumption to be 47mpg. Thought I'd made a mistake, but no, there it was.
There you go; use the power, pay the price; drive like a granny, save money.
I'm now thinking that AAU needle might give me a touch more economy without affecting performance, much. I knew for a fact the original Mikunis can return 60mpg at a steady 60mph, so might aim to get closer to that, but I always suspected the OEM setup was always a bit on the lean side anyway.
Btw, the bike was purring when I got back home - a good Italian tune-up every so often does them good anway.

 

[GS1150Pilot]

Aside from the raw simplicity, is it as rideable, with equal or better performance, as the four carb set up?

I had SUs on all of my old Minis, so they certainly bring back pleasant memories.

 

[Grimly]

Up to the point where it runs out of breath at around 90-100mph the butt dyno tells me it's nearly as pokey as the original. This past year has seen the use of 5W-40 synth oil, which also helped. From the moment that went in, the engine revved more freely, so perhaps an extra HP or two not being wasted in internal drag helped compensate things.
I'd nearly stopped fiddling with it, but I'm still keen to get it working at optimal, within the limitations of the manifold.
I've been reading about the HIF44 conversion kit as was fitted to H-Ds and Nortons and while I'm still of the opinion that a 1.75" is too large for this engine, the undeniable fact was that I could reach 115mph with one fitted, as opposed to 95mph with the current set-up.
The only real problem with the 44 on this is the utter buggeration in trying to achieve a decent tickover and pickup - idle was always like a bag of spanners.