The carb is dead! Long live the new carb!

[lingeringstin...]

OK, my lengthy abominated SU carb experiment on a miserable eastern block two stroke bike has revealed that SU carbs are compellingly weird, infinitely tweakable, and are now unfortunately becoming "vintage" expensive.


What happened was that about a quarter of a century ago my mate found a 1-1/2" SU carb in a skip (it had been deemed too worn out to rebuild by the mechanic at the time so it was binned) and said to me "I bet you'll never put THIS on your bike", which led to many years of horror, frustration and occasional fun.

For starters the inch and a half SU was really just too big for my bike and there followed many months of swearing and time wasting until eventually I got angry and modified it to be even simpler.

Basically I took the flappy-valve guts out of it and stopped up the shaft holes, glued up the air holes in the slide, drilled some air holes in the dashpot and then connected a throttle cable (later to become strimmer line, the best throttle cable known to man) directly to the slide, Amal stlye, so the whole thing was now basically just an inch and a half tube with a flat bottomed round slide, a needle and a smaller adjustable hole (the jet) where petrol and stuff spewed in. No idle circuit of any kind, one moving part, little to go wrong and a fuel bowl I could get at without having to take the carb off the bike.

Eureka, it worked. And it got about 99mpg! I did have to try about ten different needles before finding one with a taper that sort of worked on my two stroke engine but eventually it actually went like the clappers.

For a long time I was happy enough with it not to bother changing anything much apart from occasionally running alcohol, purple meths, industrial cleaning fluid and a variety of other noxious substances through the carb/intake breaking engines on a regular basis trying to get a hundred miles an hour out of a hardtail MZ with shitty brakes, which to be honest I did finally achieve but the engines didn't tend to last long like that so I eventually just got used to a bike that handled like a tractor with iron wheels that would do around 70 at a pinch but at least the engines lasted well unless you ingested a rock or something through the bellmouth. That's not a euphemism.

In hindsight I should have just bought a tractor with iron wheels. It would have had similar performance and would have been able to carry much more camping gear. But I digress...

The problem was that the inch and a half SU's needle is of a "floating" design which basically means it loosely dangles in the jet hole and wobbles about, and over time this wears the needle and jet in an increasingly rich manner until eventually you run out of adjustment and have to buy a new needle and jet which in my case was about every year or so and it was an annoying expense, but I could forgive that as I was getting about 99 mpg.

The only niggle was that the carb would NOT run properly with any kind of air filter, so I ran it with open bellmouth sucking in all kinds of wonderful things, which isn't great for your barrels and you have to rebore more often but I was getting about 99mpg.

Then one day I got my hands on a very rare, prewar brass SU carb slide (£30, don't ask) with a fixed needle design and discovered that by drilling holes in it and stuff (blasphemy!) I could easily adapt my needle of choice to the antique brass slide.

Eureka, it worked- and with the added advantage of not even needing a spring in it as the brass slide was very heavy indeed, and I was getting like 98mpg.

Then it all went wrong.

Firstly, the brass slide quickly wore out the aluminium carb body causing increased air seepage which I addressed by adjusting the jet tube richer as needed over time and ignoring the clattering noises from the sloppy slide.

Then one day the clattering brass slide managed to finally saw through a small piece of metal screwed into the carb body to fix the slide in place and my engine ate that little metal bit, which unfortunately caused a broken piston and a bad gouge in the barrel. More expense.

By this time the whole carb body was just too wore out to bother with. I realize it was chucked in a bin about 25 years ago for that very reason in the first place, and then I used it after that, so basically now it was REALLY wore out and I decided to just go to an auto jumble and pick another one up for a fiver.

Unfortunately they're not a fiver anymore. I reluctantly paid £25 for a scrubby one that I rebuilt with new (expensive) needle, slide and jet bits that wobbled and wore out as fast as previously but at least I was getting about 99mpg so I didn't care.

That sort of went well until my bike ate a piece of gravel through the bellmouth and broke another piston etc. More expense.

Then I put a different front end on my bike to lower it and improve the handling, which led to scraping my exhaust clamp on the ground at roundabouts so I rejiggered the engine mounts to raise the engine a bit so the exhaust doesn't scrape, which caused the SU dashpot to foul the frame so I had to cut and rejigger the carb inlet which was more expense as I have no way to ally weld, after which the carb ran like a bag of poo and was never the same after that.

Basically that very small difference in the carb inlet completely destroyed the complicated accidental rocket science that had made the abominated carb work so well in the first place.

So the hunt was on for another cheap, simple carb that would fit in the space because I was certainly NEVER going to use an MZ carb on an MZ engine on principle.

Behold the inch and a quarter SU. They're fairly cheap and common and come with a fixed needle as standard!

Being 1-1/4" it's actually about the right size for my engine and I'm not going to cut it, drill holes in it or throw bits of it away this time. I intend to use it as intended intentionally for all intents and porpoises. Now my bike will have a "normal" (car) carb on it WITH AN AIR FILTER OF SOME KIND to keep out rocks, dogs, and small children, which will mean months of swearing and assorted horrors trying different needles and springs and whatnot until I find the combination that works on my engine because I'm not normal and I do silly things to my pistons with a wood file, and that is not a euphemism.

Let the new horrors begin!

[Rogerborg]

But what kind of fuel economy will you get, eh?

Drag yourself into 1967 and fit a Concentric, they're marvellous and some of them don't catch fire.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Ariel Badger]

Did you change the jets and needle to run on dope? My bike runs a 900 main jet on Methanol.
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Bikers make great organ donors, get 115 on your licence today.

[lingeringstin...]

Funnily enough I did dabble in Amal concentrics long ago. I loved the dribbler simplicity. At the time I had a 932 and there were only two problems with it:

Problem number one was that after being on my bike for about a week a thin part of the slide broke off and got sucked into my engine and broke the piston skirt. New piston time.

The older mechanic I worked with said he'd never heard of that happening before so it must have been a fluke. The rest of the carb looked in perfect condition so we replaced the slide with a new one.

The second problem was that about three days later another thin part of the slide (exact same bit) broke off and got sucked into the engine again with similar results as last time.

Much head scratching was done over the matter and no cause could be determined for the broken slides other than my mate thinking that the 932 being a wider bore might not hold the slide as well as a 930 causing the slide to flap about more. Anyway I wasn't about to buy another Amal slide again so I made a heavy duty slide out of a piece of fork leg and that certainly never broke.

Unfortunately I got rid of all my Amal stuff years ago when I started messing about with SU carbs. But you are quite correct- they are pretty good and simple and I often consider doing that again.




I do remember a guy telling me once that you could run alcohol in a 930 concentric by removing the jet altogether but I never tried it.

[stinkwheel]

You clearly need a Wal Phillips fuel injector.

You know the original was made in a racing pit using a straight bit of exhaust pipe and some butterfly valves out of scrap carbs.

Do it!
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Piercee100]

This reminds me of an old BSA I saw at the weekend at the traffic lights. No idea what it was and sadly I was in a lorry or I would have stopped to help, but as he sat at the lights, he had petrol pouring out of his carb as he madly tried to fiddle about covering himself in petrol trying to do something with it... Bike was still running fine though, and he didn't think to stop the engine or get off

I bet his mpg was terrible lol.
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Rides: Honda H100a (Retired), VT500e (Sold), Kawasaki ZX400 (sold), Kawasaki GT550 (Sold), Suzuki Inazuma (Daily), Honda CD185t (Broken), LML Star 2t (Last run edition).

[Teflon-Mike]

And they said Brian Crighton was 'Mad' when he fitted SU's to the Racing Rotaries (Along with twin-shock swing-arms)
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[.Chris.]

SU carbs are great. They work so well and yet are very simple without the multiple jets that you get in most bike carbs. No rubber diaphragms to perish either.
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Chris
1985 Kawasaki Z550F

[temeluchus]

I recall someone making an inlet manifold and converting a Z750 four to run off a single SU carby. I'll try and find a link.
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Some shite cruiser. Now with guns and FREEDOM!

[lingeringstin...]

The only bad thing about SU carbs is that they have expensive bits to replace when they are knackered.

My new obsession with the inch and a quarter carb started out badly by spending a bit of money on new jet and gaskets and a new needle, naturally the wrong one but it's a starting point. Those bits aren't cheap and I'll probably have to buy half a dozen needles before I find the one that works best. Ouch!

It wasn't until after I'd fitted the newly rebuilt carb to the bike and tried for two days unsuccessfully to make it run right that I noticed the play in the throttle valve spindle.

Took the carb off again and removed the spindle and found it to be totally wore out so the carb has been leaking air badly, and like that I could never have sorted out the right needle.

Looking for a new spindle is like looking for a spindle in a haystack as nobody seems to sell just that part. All I can find is costly complete rebuild kits and all I need is a spindle, and the chances are that a spindle from another £25 secondhand carb will also be wore out so I want a new one.

Of course without a new spindle to try I have no way of knowing if the holes it goes through are also worn out too badly, and they probably are in a 40 year old carb, so that would mean reaming and bushing the carb body which is more wasted time and expense if I can never get it to run right on a two stroke anyway.

All this frustration, time and expense is starting to make a cheap Chinese Koso carb look tempting at around £20 all in.

[lingeringstin...]

[Piercee100]

Out of curiosity, do they still make SU carbs or are they just reconditioning them these days? Only reason I ask, is I could do with a pair for my Sceptre rather than the worn out Strombergs I currently have.
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Rides: Honda H100a (Retired), VT500e (Sold), Kawasaki ZX400 (sold), Kawasaki GT550 (Sold), Suzuki Inazuma (Daily), Honda CD185t (Broken), LML Star 2t (Last run edition).

[.Chris.]