Big SU on GS850

[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=Harley+SU&espv=2&tbm=isch&tbo=u&source=univ&sa=X&ei=PjgBVKrHKYKoigKMrYD4BA&ved=0CC0QsAQ&biw=1600&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.

[tirebiter]

and while I'm still of the opinion that a 1.75" is too large for this engine

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.

I've been around many British cars with SUs and Stombergs. I always figured with the air piston all the way down (like at idle, there is about the same amount of venturie opening on all of them, regardless of carb bore diameter. Almost closed ! I have a feeling you needed a drastically different needle diameter at the extreme top (1 or 2 eighths of an inch) of the profile, than the needles you tried in the 44.

With one carb you COULD cheat and do the sandpaper routine to make different needle profiles for experimentation.

Does the bimetal mainjet holder in the HIF do a good job with the amount of temperature fluctuations encountered with an air-cooled engine ? That function was "iffy" at best on the 1974 (and 1/2) MG-Bs that came into the USA with them as stock.

[Grimly]

I'd decided to try the smaller carb to keep the gas speed up at idle and it had an immediate effect, falling right into a zone of acceptable idle behaviour the 44 never showed. There were variations between needles in the 38, but all of them I've tried so far idle much better and one or two are near enough perfect out of the box, which I found useful, as I could lay that bit to rest for a while and pay attention to mid-range and other fuelling.
The entire thing is a series of compromises and happy accident - the naked manifold might have led to fuel puddling problems when cold, but it didn't, as it heats up quickly from cooling air, and after the first mile is warm enough to keep the fuel vapourised within it. The thermostatic jet control might or might not be working properly, but I won't really know until fit a heat shield to the manifold that will protect the float chamber from engine heat - although that shield was originally designed to shield the carb from exhaust manifold heat on the Mini, it might have a beneficial effect here.
I did an IR scan on the carb and manifold last year, but don't have the figures to hand. I don't recall they were anything to be alarmed about, else I'd have done something by now. There is the normal warm-up period, and once it's warmed up, it runs quite well, but even during the warm-up it behaves like the normally carburetted engine. It doesn't fart or pop, or splutter or die, or do any of the things that a bad carb setup would do. The SU is really quite a well behaved carb once it's in the ball park.
All I really have to chase now is that missing top end, and while the airflow charts say the 38 should comfortably flow enough to meet the demand of the 850, and the manifold has already shown that it can feed the engine in top gear at a higher flow, I want to see what I can get out of the 38, before I move back to the 44.

[tirebiter]

OK, so it's the 38 for now. I undersatnd you wanting to go at this in a logical sequence. You mentioned the "italian tune-up" That usually means somewhat extra rich running at low-moderate RPM and throttle openings.

I have a couple of questions ;

Do you have any evidence of what the mixture is at higher RPMs ? Plug chop or ... ?

Also ... if yo uwere to unscrew the carb damper, does the air piston lift all the way when the bike is stationary and you crack the throttle open fully for a brief moment ?