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Carl Heideman
Carl Heideman
8/11/08 11:01 a.m.

No matter what the name on the valve cover, so many British classics rely on the ubiquitous SU carburetor: Jaguar, Triumph, MG, Rover, Rolls-Royce, Bentley, Morris, Austin, Sunbeam and so many more. And not only did almost every British manufacturer specify SU carburetors, but so did other companie…

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7/9/15 1:59 p.m.

Great article Carl. I thing I was hoping you'd address the effects of ethanol laced fuel on both old, rebuilt new carbs and replacement parts. What gives and what can I do to prevent potential engine fires?

Gary Dork
7/9/15 3:43 p.m.

Yes, very good article Carl. Thanks. I've been a big fan of SU carbs since I had them on my P1800 years ago. For my current Spitfire project I could have replaced the original tired HS2 carbs with a single 2BBL Weber DGV, but chose to have the SU's rebuilt by Joe Curto for about the same money. My preference. I like the performance, they're simple to tune, and I think they look great on a vintage sports car.

Basil Exposition
Basil Exposition Dork
7/9/15 5:12 p.m.

This was a good article in 2006 and is still good.

Chasmo, I've heard horror stories about ethanol eating the aluminum in carbs, but I've never witnessed that and I think it is overblown. Generally, the fuel in the carbs evaporates fairly quickly if the car just sits, so the worst you get is gunk clogging up stuff. I have found that the ethanol will attack rubber stuff over time, though, causing leaks in carbs with rubber diaphragms and the like. Had to do the triple carbs on my E-Type because of this. New diaphragms were sourced from Joe Curto and he assured me they would be unaffected by ethanol.

Gary Dork
7/9/15 7:05 p.m.

In reply to Basil Exposition:

Yes, I agree. I think the ethanol issue is greatly overblown.

VClassics Reader
7/9/15 7:30 p.m.

I've worked on countless cars with SUs since gas has had ethanol in it, and I have seen zero problems I'd attribute to it. I have seen it decay old fuel lines in a hurry, but that's been it.

RossD PowerDork
7/10/15 8:54 a.m.

Neat article. I wondered why those 'little british' carbs looked so different from 'normal' carbs. Neat to know they are actually quite elegantly designed.

Basil Exposition
Basil Exposition Dork
7/10/15 10:11 a.m.

I've never used Webers. I have 4 British cars that all have SUs, including the vintage racer. Virtually every British car I race against uses SUs, as well, so I don't know about the Webers being better for racing. We do some modifications and often go up a size on the SUs to get them to flow more, but that's about it. The guys that I have talked to that have tried Webers tell me they are much more difficult to tune and are much more complicated with lots of fiddly bits.

7/10/15 2:46 p.m.

The design theory is brilliant.

Where people become frustrated with them is because:

1-They are not aware of the design theory of a constant velocity carburetor and/or are not wanting to learn it.

2-The throttle shafts tend to be universally worn out by 30,000, miles making it impossible to tune the carburetor. I think the later HIF carbs were better in this respect.

3-The floats are harder to adjust than you think: I find that the only reliable method is to adjust the floats to whatever it takes to just cover the jet when it is fully retracted. Ignore all methods described in the book telling you to use drop test or 1/8" rods or whatever. Does not seem to relate to the proper level in the jet where it matters.

4-Very few people know about the drop test that needs to be done to ensure that the "Bell" and piston are a proper match. Not only do you need to be aware of the test, but with all of the mixing and matching of parts over 50 years, there are a lot of mismatched bells and pistons that can drive the unknowing mechanic crazy.

Once you can get past the learning curve, the behave and tune pretty much as described in the manual and do a great job.

wspohn SuperDork
7/12/15 3:35 p.m.

You can have SUs and one throat per cylinder. I used to run these (very rare indeed) 


frenchyd New Reader
7/30/15 1:43 a.m.

In reply to VClassics: I agree with you. Fear of Ethanol is way overblown.. Once you start to understand it there is potential for power increases at far less than racing fuel costs.. Alcohol has added oxygen molecules in it. That means you can richen the mixture up (something very easy to do on SU's)add a little timing and gain power.. Or you can use it's higher octane to increase compression without the need for expensive race gas..

Careful though, methanol is corrosive and right now it's cheaper than ethanol..

TR8owner HalfDork
7/30/15 2:59 p.m.

In reply to Basil Exposition:

I like SU's also but if you really want to make an uncool British car like our TR7 vintage racer cool then its best to do this.

Gary Dork
7/30/15 4:17 p.m.

In reply to TR8owner:

Yes indeed. That's cool!

DeadSkunk UltraDork
8/1/15 5:09 p.m.

Once upon a time I tried running twin SUs on a VW Beetle engine.I had a terrible time tuning them until I spent a couple of hours with Olie Clubine. He was a distributor for SU bits at the time. When I told him what I was trying to do he gave me some needles and springs and suggested a combination that worked like a charm.

frenchyd New Reader
8/7/15 12:30 a.m.

In reply to Carl Heideman: The cost of tuning SU carbs is a tiny fraction of the cost of tuning Webers

With SU's I often will ream out the Jet or sandpaper the needle on the dyno* until I wind up close to my goal.. Then I carefully measure and check my inventory. Over the decades I've saved hundreds of needles, jets etc.. However if you don't have that contact Joe Curto

My dyno is a long hill where can time events from marker to marker.. It helps that I have air density meter, accurate air temp gauge, exhaust gas analyzer etc.. so I can be consistent.

I bought one set of Webers for a V12 Jaguar and by the time I had the carbs even close I'd spent over a grand not to mention the dyno time..

Air density changes constantly and with fixed Venturi Carbs like Webers it gets much more critical to adjust for those changes.. Jets and venturi's can easily run over a $100 per carb per adjustment. On a V12 Jaguar depending on the altitude of the track and the weather changes you might need $3-5000 to cover all your bases.. SU's on the other hand often require nothing more than one or two flats adjusted on the mixture..

redvalkyrie New Reader
8/23/15 10:52 a.m.

SUs look physically shorter than Weber DCOEs even when the DCOEs are fitted with short air horns...the actual bodies of a SU appear more compact. Is this true? I'm thinking SUs could be a good choice for slant four engines where a normal DCOE carb and manifold would be a real tight fit between the head, brake master cylinder, and strut tower.

Carl Heideman
Carl Heideman
8/28/15 7:52 a.m.

Guys, sorry I'm a little late to chime in. I'm glad you liked the story. I'd almost forgotten about it--2008 was a while ago.

Frenchyd's got a very good point about Webers. I bet I've got about $5000 in tuning parts at Eclectic Motorworks (jets, emulsion tubes, air correctors, venturies, aux venturies, etc.). When you own a shop, that's fine as you'll got to your parts bin all the time for customer work. But when it's your only car, it's probably cheaper to take it to a shop than build your own assortment. I'll say that to me it's an urban myth that you've got to constantly change jetting for weather etc. on a street car. Sure, if you're a racer and you're wanting that last bit of HP for the long straight, you may change something for a temp or humidity change, but on the street it's not an issue. Just like properly setup SUs, our customers hardly ever need their tune up touched once we've got their ignition and carbs dialed in.

Regarding ethanol, we seem to be seeing these issues:

  1. The carbs do need to run richer and we're not seeing more power on the dyno (less BTUs in ethanol). Actually a little less HP. If could build an engine from scratch for ethanol, that would be a different story--we would add CR and some other things to take advantage of it. Cars especially need more idle mixture and tend to produce more smell and black smoke as a result. We'll usually run the hottest thermostat possible to help keep the combustion temps up for better idling (it takes more heat to burn an ethanol mixture).

  2. We see more trouble with fuel boiling after the engine is shut down and heat soaks the carbs. Many people are calling this vapor lock, but what really is happening is fuel boiling out of the float bowls and flooding the engine. Holding the throttle all the way to the floor on hot start usually starts the car, but heat shields really help here. MGs had heat shields from the factory, many TRs didn't. TR6's especially have been problematic until we fit heat shields. (We also see trouble with American engines and have fitted heat shields under Holleys with good success for hot start issues.)

  3. As mentioned, we're seeing fuel lines not lasting as long. We feel like we're replacing more floats than in the past. Internal corrosion is also an issue.

  4. We've also seen ethanol fuel attack fuel tank liners so we tend to replace tanks rather than coat used tanks with a liner.

I'm not a scientist, but I have enough repeated anecdotal evidence to say these issues seem to be consistent with ethanol laced fuel. We can get ethanol-free fuel and seem to see less of these issues in customers and cars that use it.


Gary Dork
8/28/15 11:45 a.m.

Excellent follow up to an excellent original story. Very beneficial. Thanks again.

Ian F
Ian F MegaDork
8/28/15 12:52 p.m.

I'm definitely having a hot-starting problem with the single Z-S on my Spitfire. There is a sort of home made heat shield, but it doesn't seem to be doing much. I recently acquired a factory shield that I plan to install soon. Hopefully that and a tune up will help. The car ran great for a few months and then sort of went wonky.

Apexcarver PowerDork
9/3/15 9:20 a.m.

I am working on a 1275 equipped bugeye with later HS2's and am in the process of rebuilding the carbs.

A few quick questions.

Regarding throttle shaft wear, what amount of play would you feel necessitates reaming/bushing? One of my two carbs had a worn shaft, but the body had minimal wear and very little play on a non-worn segment of the shaft. I am trying to get a feel for the threshold of need for this service.

Would you suggest greasing or anything (graphite/other) to avert the throttle shaft wear post-rebuild?

How much importance is there really with what oil is used in the dashpot? Some sources seem to indicate that specialized oil ($$) should be used and others indicate to just put your motor oil in.

Carl Heideman
Carl Heideman
9/3/15 11:08 a.m.

Regarding throttle shaft wear, my test if the car is running is to spray carb cleaner at the shafts and if the idle changes, they're worn (i.e., there is a vacuum leak). If the carbs are off, a visual test is usually enough. If you can see (or feel) a wear line, the shafts are worn out. If it's a small nick, you can often just replace the shafts and be okay. If it's a big chunk of the shaft worn out, you can assume the bodies/bushings are worn too and will want to address them.

I oil throttle shafts when I put them in and then drool some oil over them from time to time and hope that it wicks in. I don't know if it helps a lot, but it can't hurt.

Regarding dashpot oil, everyone has an opinion as to what is best. My opinion for street cars is that any oil is better than no oil. The purpose of the oil and damper is to keep the piston from rising too fast so there is enough vacuum to suck more fuel into the mixture. The oil/damper is basically the accelerator pump. Racers have sworn by ATF because it's thinner, but remember they're at WOT much of the time and want that piston up fast (in fact, serious race engines with SUs don't even run dampers). I prefer a thicker oil than ATF, so I run engine oil or 90 wt in my own and customer cars. Years ago, we put every type of oil in some HS4s on an MGB and ran them on a dyno. No difference in power. There is always a lean spike at initial tip-in and it was slightly shorter with thicker oil, so that's what I recommend for street cars. For racers, whatever gets you the fastest lap time or highest RPM on the long straight is the answer.

frenchyd Reader
10/4/15 10:52 a.m.

In reply to Carl Heideman: Carl you are spot on. I should have said something about racing when I made my comments.. For the street there is plenty of tolerance in carbs to adjust for weather changes etc.. Same thing with regard alcohol, the power comes from the ability to use higher compression without detonation. If the engine is stock the slight richening effect that can be tuned into gas with alcohol won't yield any measurable power increase.

erohslc Dork
10/5/15 4:40 p.m.
wspohn wrote: You can have SUs and one throat per cylinder. I used to run these (very rare indeed)

Burlen is reintroducing the dual choke SU carb, the DU.
Prototypes were shown at the Goodwood Festival.
Sizes will be 1-1/2 and 1-3/4
Available '.. sometime in the new year.', no mention of prices.


erohslc Dork
10/5/15 4:48 p.m.
Carl Heideman wrote: ... Years ago, we put every type of oil in some HS4s on an MGB and ran them on a dyno. No difference in power. ... For racers, whatever gets you the fastest lap time or highest RPM on the long straight is the answer.

I would not expect any power difference.
Would be interested in quickest response time to redline when you crack open to WOT.
On the dyno, run at 3000 RPM, go WOT, and capture time needed to get to redline.
Even better, do the test in 1,000 RPM increments.
As you say, shortest laptimes win!

wspohn HalfDork
10/6/15 11:16 a.m.

"Burlen is reintroducing the dual choke SU carb, the DU. Prototypes were shown at the Goodwood Festival. Sizes will be 1-1/2 and 1-3/4"

That link doesn't work - do you have an URL for this? Very interesting. I know some guys that would run these instead of Webers, just to be different!

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