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Discussion Starter · #1 ·
I have finally, after much trial and error, gotten an SU dialed in correctly, and wanted to write down some things. I read lots of info and did a bunch of research, but precious little of it has to do with one of these little devils bolted to a Harley-Davidson.

Maybe this is good for the tech section, I dunno. I just wanted to get all of this out while it was fresh in my head. Maybe it will help one of y'all or some other chopper pilot who comes about after I've hung up my spurs.

I have an SU carb on one of my bikes. ('37 ULH engine, stroked enough to bring the pistons up to the top of the deck, so 84" or so.) I brought this bike back from mothballs. I also swapped the SU carb that it came with for the one on it now because the one that was on it was pretty tired.

The guy who built my motor said he knew all about SU carbs. He even showed me pictures of his Pan he used to have where he ran one way back when. He was going to dyno the motor, so what the hell, right? Sounds great to me. I spent my time happily knowing nothing at all about that big honkin' thing hanging off my motor.

It's always run OK. Not perfectly, but OK. Good enough. And I was babying it a bit, because I was breaking it in. I didn't want to beat the shit out of a freshly-built engine. Idle was always finicky with the old girl. This is my first BT flattie, so I thought in the back of my head I might have been wailing on it too hard, so I told myself the problem was me, not the bike.

Last year, the fucking thing started breaking up real bad... not just high in the throttle where it used to, but lower in both throttle opening and RPMs. That ain't right. Nothing I did could get it to act right. Most confusingly, the rear plug would come out fluffy black, all rich as shit, and the front would be snow white... dangerously white. No intake leaks, valves adjusted fine, all new mag guts.

After a few hundred miles of break-in, I decided to stick on a set of fancy Flathead Power KR-style heads and have a very, very mild hot rod, knowing I'd need to retune the carby. At this point I knew shit about SU carbs, but given that the mag was passing tests with flying colors and I couldn't seem to find any leaks, this seemed to be the perfect excuse to figure out what in fuck was wrong with the carburetor.

Now, for months, friends had been suggesting I chuck the motherfucker and put a Linkert on it. And they were right. But I'm a stubborn asshole, and my buddy ran it with a fucking SU and said it ran great, and I like a challenge, I guess.

So I started tuning. Writing things down. Testing empirically. I learned a few things.

First, there are two flavors of SU carb. There are the real-deal car carbs, and there are Rivera Primo motorcycle-specific carbs. Know what you have. They are not the same animal. If you have an Eliminator motorcycle carb, just go with their recommendations ... it's way easier than figuring all this shit out like the tuning wizards did in the 60s and 70s.

Second, there are eight thousand variations of the car ones. If you're a hardheaded fuck like me and want the legit SU that guys used before Rivera Primo made it easy, understand these carbs were used on fifty years or better or Triumph, MG, Austin Healey, Jaguar, Nissan, Volvo, and who the fuck else knows what. There are a few basic styles, though, and basically, they all have to do with where the bowl sits. Most of what you see for Harley-Davidsons are HS6, HD6, or HIF6.

Stolen shamelessly from Wikipedia:

The carburettor identification is made by letter prefix which indicates the float type:

"H": introduced in 1937 in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carburetor body just behind the main jet assembly.

"HD": introduced in 1954 with the float bowl mounted with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet.

"HS": introduced in 1958 the float bowl can be rigidly or rubber mounted to the main body, fuel is transferred by an external flexible pipe to the jet. The jet moves down to richen the mixture for cold starting,when the 'choke' linkage is pulled.

"HIF": (1982) the float bowl is horizontal and integral (hence the name) Horizontal Integral Float.

"HV" (1929), "OM" and "KIF" types also exist but were less commonly employed.
The Imperial sizes include 1-1/8", 1-1/4", 1-1/2", 1-3/4", 1-7/8", and 2", although not every type (H, HD, HS, HIF) was offered in every size.

There were also H models made in 2-1/4" and 2-1/2", now obsolete. Special purpose-built carburetors (Norman) were made as large as 3".

To determine the throat size from the serial number: If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8", then add 1". For example, if the serial number is HS6, the final number is 6: 6/8 = 3/4", add 1, total is 1-3/4", etc.

If the final number has 2 digits, it is the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38 mm etc.

The numbers cast into your carburetor, like "AUD 4060" will help you identify what family you have and what needle came in it, but you'll want to use that info to figure out what car the carb came from so you can easily acquire parts.

Now for tuning.

There is really only one thing you can adjust on an SU carb, and that is the idle mix. All you're doing is raising or lowering the part the needle sets in, what SU calls the jet. Unlike the jets you are probably familiar with, though, multiple jet sizes do not exist.

So if you get you a carb and bolt it up to your motorcycle, odds are excellent it will run like shit.

In order to change off-idle, midrange, and high RPM fuel mix, you need a different NEEDLE. And fuck, there are about a million of them. Seriously, there are hundreds. What SU did was to measure the diameter of the needles along ten positions called "stations" along the length of the needle. It allows one to numerically "see" the taper of the needle. A collection of ten measurements is then tossed into a two- or three-letter code. (You should know that code is usually stamped into the needle on the section of the shank that is clamped into the piston.)


Left to right, that's an SC, SM, and RF needle. That's richer (mostly at the top of the throttle) in this pile as you go left to right... but you can see exactly how fucking impossible this is to just hipshoot what you need.

The only recommendation charts that exist are for Brit cars, where a needle code will be given, and then one step lean and one step rich will be listed. Protip: the British are apparently fucking weird and say "strong" and "weak" where we would say "rich" or "lean," respectively.

The shit part is that there is no real way to see what you need. You kind of just have to have a pile of needles and start swapping. For instance, the guy who had this bike before me had an SM needle in there and it was supposed to be the bee's knees. On a Pan forum, another guy suggested a TL needle in a stock 74" engine.

I stuck the SM in there to see what happened. Way better, but not perfect. I kind of reverse-engineered things and ordered needles that seemed to have similar tapers as what I needed, and then used the chart to see what car they came from, then I'd order lean and rich based on the chart.

If you're flying blind, (or just want to know how this thing works) the upper end of the needle (the thick end) controls small throttle openings, and the pointy part (tapered end) large ones. I found conflicting information on the internet about what stations on the needle control what part of the throttle, but my suspicion is none of it matters anyway because these little fucks were never designed to go on a Harley anyway.

When you install these, make sure you bury the shank flush with the piston. That's how they're designed to be installed, and it will remove one more tuning variable from things. Yeah, you can try to dick around with install height, but replicability disappears if you do that unless you mark the needle or something.

Also know that because the diameter of the needles can differ, you may have to reset your idle mix before you go out for a test rip each time you swap one.

I fucked around with that SM needle and had a very high-speed stumble way up high in the rev range. TL was supposed to be leaner, so I never even put it in. I ordered up an RF needle, which was much more slender at the taper (fatter up at the top of the throttle) and then I got the bright idea to look at the chart for the SM, which was running pretty good except for a little stumble at the tippy-top. The chart suggested an RH was one step richer, so I got one of those, too.

One item of note... if you are super-interested in getting the perfect needle (or have needles that are way off), it might make sense to carve your own needle on the drill press. It would be hard to duplicate such a thing on a multi-carb setup like these were originally, which is probably why you rarely see someone polish up their own SU needle, but on a single-carb application like this one, I imagine one could spin one down by hand that would work at least passably well, if the material removal could be controlled. I probably am not up for the task, but some stubborn shithead like me has probably done it before.

The RH wound up being my huckleberry, but I still felt things could be better. One thing I knew was the dashpot oil was important, but I did not know exactly HOW important. Take it from a guy who cruised a LOT of Brit car forums... asking what oil to use in a dashpot was sort of like asking people about abortion, gun control, helmet use, and preferred engine oil all at once.

Here's what I learned: the dashpot oil plays a huge part in what these carbs are doing.
Watch how this piston is supposed to work:

I realized with 20W50 in my dashpot, the slide may as well have been stuck closed. So I tried ATF. Better, but still not perfect. At high RPMs, I still had a teeny stumble when I really screwed on the throttle, and my idle seemed fluctuate a bit up and down... it would rise and fall, almost like it had an intake leak.

So I tried Marvel Mystery Oil, which is really, really light. Finally that slide could snap up and the oil was damping the piston, not preventing it from moving. The bike runs great now. The rear cylinder stopped fouling, presumably because it's not choking for air now. (It turns out that I also just emptied the dashpot of all but a few drops of 20W50 and it damped similarly, so I think there is both a viscosity and volume component, kind of like there is in a hydraulic fork.)

This kind of makes sense that a Harley would be very sensitive to the venturi size and piston damping... an old flathead is probably not producing the same amount of engine vacuum a huge British straight 6 or V8 is making. Rivera Primo always used to warn that the dashpot should be filled with WD-40 on their carbs, which is obviously not thick stuff. Apparently there is a reason for that!

The bike idles so so smoothly now, and there is zero hesitation... throttle response is the best I've felt on a Harley; I'd call this carb equal to a Keihin CV in terms of responsiveness. It took me literally months to get the bike to this spot, but it was worth it. I've heard people praise this carb and curse it, and I understand both sides of the argument now. It's a bundle of work to get one of these dialed in, but once it happens, look out!

So, long story short: if you want to tune an SU, have a handful of needles and a few different oils on your shelf.

Hopefully some of this helps someone who's struggling.

1,181 Posts
Some more musings for you as I procrastinate on studying for a job interview. . .

There is another tuning tool for SU carbs. They make at least two different weight springs (between slide and dome) that I've seen. The lighter one can allow the slide to flutter in two-cylinder applications making damping oil choice important. The heavier one buffers this out but has the slide in a lower position (leaner) in any given rpm. You jet accordingly.

Light spring:

Heavy spring:

I took some pics to illustrate some of things that Lame described for the visually biased among us. . .

The jet in the carb body that surrounds the needle is known as a Grose-type jet and moves up and down to change the mixture. It's in the top right of this pic (red arrow). The needle profile is shown here too. I think this one is a BBX type.

Like many automotive carbs, the SU has a fast idle adjustment that accompanies the choke. There is a detent on the shaft that holds the carb in starting/fast idle position until the first time the throttle is opened, then it snaps back.

Choked/fast (when throttle opened, shaft rotates along blue arrow):

Not choked / normal (to start, rotate shaft along blue arrow)l:

The cylinder with the nub sticking out (blue arrow) is known as the tickler and pumps fuel into the venturi for starting. You squeeze it by hand a few times before starting. Not all SU carbs have this. I can't tell you which do or don't. The magenta arrow is the mixture adjustment (there's only one).

The float bowl contains a bracket which moves the Grose jet up and down when the mixture screw (red arrow) is turned. Yellow arrow tries to show the direction. Under that screw is a spring which keeps the jet from wagging around and losing adjustment.

There is a great book explaining these carbs in more detail. I found it very handy. Weber/SU/Stromberg manual

If you hadn't guessed, my parts stash is not wide, but pretty deep . . . here's the carb drawer.

Have fun.


4,050 Posts
Discussion Starter · #3 ·
Prettiest hair in town. I did not know about that larger spring; that's good to be aware of. In fact, being familiar with Japanese CV carbs like the Keihin on late-model HD's, I actually wondered why SU didn't implement this method of damping; seems simpler. As you've pointed out, Jason, they did and I just didn't know as much as their engineers did way back when.

I appreciate the addition of photos; I probably should have taken more.

This is interesting stuff.

Jason has just shown what I believe to be an HIF carb, a later style one. My silly little carburetor has no choke mechanism on it. On the car mine came from, the choke actually bolted to the mouth of the carb. I don't have it, but even if I did, it would stick out too far to be usable on this bike without reworking something on here. In fact, on the throttle shaft, I simply have a clamp-on cable holder. Only way to make it all fit on my bike.

Incidentally, the carb on this previously had a choke on the bottom, inline with the jet. You'd basically actuate a lever, and the carb would be silly-rich. The replacement carb I sourced did nto have this. In order to start this motorcycle, I need to remove the air cleaner, seal off the mouth with my hand, and kick. (It's a very silly-looking ballet to watch this all happen!) From there, fuel is drawn into the carb and jugs, and then I can light it off. This is also part of the reason tuning this was so important to me... no one wants to go through that struggle and then kick eight jillion times.

I also have no tickler. That is, I believe, a later addition to the SU carb. (There's also a slide-lifting pin for tuning which most people seem to agree doesn't work at all.) Here, look. You can see the slide lifting pin, but that's it. And, you can also see (much more clearly!) the fuel bowl itself. Offset bowl is an HD6, and Jason's, being integral, would be an HIF.

Further... my old Jag carb doesn't have a Grose jet, either. All I have is a "jet bearing." The Grose jet (if it's like some of the other SUs I have fooled with) is sort of like a check valve. There's a little glass or steel ball held captive in the bottom of the jet, and it normally closes off the fuel supply, like a needle and seat would do. When engine vacuum "picks up" the ball, fuel is permitted to flow. Ones with an offset bowl have a normal needle and float, though, like this.

If you need to adjust your idle mix, you'll either have a little screw on an offset adjuster like I do, a screw in the body like Jason's, or an external jet nut and lockring that all accomplish the same functions. Here's that same photo, with the idle mix adjuster circled:

Much of the info I found was based on HIF carbs, which are the most common (and latest and greatest) of the series. That said, the older models are a bit simpler (though they can be harder to tune) and are still very serviceable. While we're on the topic of later HIF carbs, you should be aware that there are bowl extensions available, kind of a wafer like an Evo rocker box uses that extends the fuel bowl. I can't speak to how necessary this is on a Harley, but I've now seen a few of them, which leads me to believe it may be helpful. I think this image came from sixball, a member on here I think is sort of SU-crazy. You can see the extension on the carby, though.

Jason, when might I have the opportunity to buy you more beer? I'm not going to California any time soon...

1,181 Posts
As I understand it, the Rivera Eliminator versions are based on HIF models. I know at the time I bought my first one (Elim II) in '98, they were still being made.

I think this one is an earlier Eliminator I. Date stamp shows '93 which sounds about right (but I'm not an expert in these).

Getting any details beyond marketing hype from Rivera about how exactly theirs differ from original is impossible. Want a good laugh? Read this Benefits and Features

They basically take credit for the design and don't point out anything that they've done.

I tried to get some tuning help from their tech years ago and they were pretty useless. Basically "keep fucking with it until it's good". A far cry from the "best carb every built in the history of automotive design!!! 5 min install! Needs no tuning!"

So now they live in my stash ;-0

Ran pretty well on my 74" shovel for a couple years.


p.s. - send real beer to San Jose. All we have is bitter hipster shit that gets you drunk off a short glass
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