SU Carburettor Explained - Everything you need to know (and possibly more)
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- Опубліковано 30 жов 2019
- An overview of everything you need to know about the SU carb.
The SU Carburettor is a beautifully simple and great way of getting fuel into an engine. Used in cars all the way up until 1993, these carbs are simple, reliable yet often misunderstood.
How to Tune the SU: • How to Tune SU Carbure...
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Music used: Dancing Hearts - Anders Enger Jensen • Dancing Hearts
#su #carb #carburetor - Авто та транспорт
I went from not knowing enough to feeling like I could teach someone else about these carburettors. Thank you.
One thing I was always unsure about is exactly where to add damper oil. After watching your video I realised I should be directing the fluid into the tube where the plunger goes only. Is this correct? Before I would just pour oil in and I suspect it was just filling up around the tube in the piston recess where it can actually do anything.
Yes you are correct, it is just the tube where the damper piston goes that you fill with oil.
Glad you enjoyed the video!
exactly what I was thinking...I'm English and grew up in the 70s and 80s and I realised I didn't know how SU carbs worked...now it's such a relief I have filled that gap! 😀
@@PompeyMatt17 they are so simple
The 2 holes in bottom of the piston are on the butterfly side they make suction in the dash pots and more the throttle opens the more it makes vacuum to the piston to suck it up the dash pot take one apart and and have a look maybe you could add that to a future video ,just a tip from the uk here at (ultimate jaguar tuning) I get customers from all round the uk with transport there cars to me that so called specialists garages can’t fix we have a tuning side and a restoration side from welding to axles rebuilds gearbox’s plus we build many different classes of race cars, we are not cheap but like most jobs we can take a few hours where others take a week then the car still ends up coming to us plus we call at garages that can’t fix jaguars and Daimlers we don’t have to advertise for work and we work 7days a week apart from the tuning side where I have odd weekends off to race my self like I said in another comment I have parts if needed or any advice good luck on your channel I have just recently found it I like it 👍🏻
Similar design to a motorcycle carburetor, but more complex
Thanks, really, really appreciate it. I'm bringing a 1970 Datsun 240z back to life after a 30+ year hybernation and this has really been helpful.
The purpose of the damper is to slow the rise of the piston on sudden acceleration, to give an initially richer mixture, like the throttle pump on fixed jet carbs. It does NOT restrict the fall of the piston, if held up then manually then released it should drop with a sharp click.
The piston rises when the throttle is opened because the large vacuum above the piston skirt is offset by atmospheric pressure under the skirt, which enters the carb through the additional holes in the air filter flange above the fixing holes.
The float and valve do not go up and down all the time. The valve is not an on/off valve but a continuously variable one, so as soon as the float takes any pressure off the float valve fuel starts trickling in to keep the float and hence the fuel level in the jet at a constant level.
Overfilling the damper with oil does not fill the upper part of the piston, any that does get there runs straight out into the carb throat through holes at the bottom of the piston.
The piston is not a 'tight' fit in the cover, it is a 'close' fit. If the cover and piston are touching anywhere the carb will not work correctly.
Buddy, you are killing me here! Every time I turn on my computer there you are with another lesson.
Thank you and as always, very well done.
Great video! Very clear explanation and demonstration of how these carbs work. Thank you!
Great job! I am having some carb issues with a vintage motorcycle (not SU carbs) and this explanation is helping me to know where to look. Thank you!
Very very useful. I've got a '91 Mini that I'm in the process of preparing to take the engine out, and figured it was about time I understood exactly how the carb worked... And now I know...
Thanks very much for taking the time to explain how they work. Much appreciated.
Great information. After years of puzzlement I know understand how the SUs on my Triumph work. Thank-you.
Getting ready to rebuild the Su carbs on 62 spitfire.. THANK YOU.. You are a great instructor. Ill be watching these when I get to the tuning portion!!
Best explanation of how an SU carb. works I 've seen so far. A lot of general information on carbs tends to be about the US designed Holley and Carter designs rather than these British designed carbs. Thanks for sharing here, regards from Australia.
Your channel is hands down my new favorite. Thank you, and keep it coming!
I'm glad you enjoy it!
Man you have saved me. I just bought my first Rover, a 3500S and these carbs completely baffled me. I spent hours looking at them yesterday trying to figure it out. I finally get it because of your video. Thank you so much!!!
What a great video ! These carbs have been a mystery to most people and your video proves how simple and sensible they are. Thank you!
Thanks Rob! They are beautifully simple in how they operate 🙂
You have NOOOO idea how badly I needed to see this video! Have a '71 RR Silver Shadow that wouldn't start. This video explains everything! Thanks!
I'm glad I could help. Thanks for watching!
Super helpful... working on my mini's twin SU carbs now!
THANK YOU, THANK YOU! I'm a "newbe" with a 1970 MG Midget 1275 MKIII. This short video is PRICELESS and will be a great help to me because my front carb just started spurting fuel from the overflow port on the float bowl! Did I say THANK YOU?!!
One detail he might have mentioned, although I guess that really goes under 'tuning', is that the viscosity of the oil in the dashpot determines how rich the fuel mixture is when you accelerate. Most of the time 15 wt oil is what you want. "Marvel Mystery Oil" is perfect.
Excellent video! One very minor correction; the last production car to use an SU carb was the Reliant Robin, which was made until 2001! There were even a few produced in 2002 by BN plastics.
Great video. I now understand how my carbs work.
Down to the point, precise, no superfluous yada-yada, will watch more of your vids!
Great video thanks. Nice simple carbs, always liked and trusted them, very reliable.
Well explained, thanks.. look forward to how to set the mixtures and get them running smoothy 😊
Thank you Ian!
I just watched this video, great info. Makes my wish I had SU's instead of the Zenith Stromberg carbs the US market got stuck with.
I just started my friends 1950 mg after 6 years sitting and after replacing the fuel pump and changing the old gas out it started and ran after tapping on the carburetors a couple of times and runs amazing
Came here looking for the SU carb, and instantly recognized Anders' music for the intro. That was enough to ensure I'd watch all the way through!
I’m glad you enjoyed it! He makes great music
Thanks - a really useful video. I have a 61 Mk2 and on a run recently fuel was leaking out of the float chamber. Stopped leak by tightening everything up but need to do some investigation over the winter when she’s laid up. I shall be consulting this video again then! Thanks again!
I'm glad it was helpful! I had some similar leaks on my old XJ6, it stopped when I took the lids of and retightened everything. It gave me time to order the gaskets.
Excited about the new series! A+ Adam..as usual!
Thank you Richard!
Thanks very much. I've just got my first Morris Minor so learning about the carb is most helpful. Colin UK 🇬🇧
Good luck with your Morris! They’re great cars
Very helpful , thanks and well done !!!
Professional explanation, much appreciated. 👍
Great video man. Long time subscriber. Love it. Going to start getting into my barnfind XJ6, thank you for all your videos.
Thank you! good luck with your XJ6
Very clear demonstration ... thank you.
Moving the jet is how the choke works as well, and what you 'tune'. On other models its a large nut underneath and the connection to the float chamber is via a flexible hose (which does sometimes break)
Awesome video! Very informative.
There is another often overlooked little item on those carbs: it is the damper piston cap. There are 2 versions available, with a little hole on the top and without. Now there are also 2 versions of the „bell housing“, with a little „fin“ on the top and without. Now these items are very important to vent the piston when moving up BUT the 2 versions mentioned above must not be mixed, that means if you have the version with the fin on the bell housing check inside the top throat and see that there is a hole drilled and it should be clean and with this version bell housing you must use a damper piston cap WITHOUT the hole!!! Otherwise if there is no fin with that venting „fin“ than you must use a cap WITH the hole. Those 2 Versions get mixed up a lot since people replace those dampers over the years and use the wrong one thereafter, sometimes they just replace them to get a brass top because it looks nice and shiny but get the wrong type.
So recap: it works fine as long as you don’t use 2 vents or no vents that must be avoided
Also as you mentioned that the floats will start to leak and sink down, consequence is overflowing fuel out the vent pipe!
PLEASE DO NOT TIGHTEN THOSE SCREWS INTO THE ALUMINIUM HOUSING YOU WILL SOON BREAK IT!
Solution: get new floats (plastic ones are available from Burlen) that don’t leak AND check the fluid level adjustment (the funny looking lever that closes the vent) or check that the valve closes tightly else replace the valve. At that time also renew those gaskets. Your problem will be solved and the gas milage will drastically improve. 😁😁😁
Yep, I've been running SU's on Harley's for years and I've run into that issue several times when guys who didn't know any better would fix them or would try to assemble one from parts from different junked carbs and would get all that mixed up.
Those things run fantastic on Harley's, there's not another carb out there that has the throttle response on one like those things do, especially on the older engines, they had intake ports that were too big and port velocity suffered as a result, the constant velocity principle of the SU's really peps up the older one's.
Thanks a lot for that information Bernhard. Much appreciated.
Nicely done. Clear explanation. I'm just starting to learn my XK120.
Glad it was helpful!
Very good explanation. Thank you!
No mention of how vacuum in the bell chamber rises the piston to increase the size of the venturi allowing more air/fuel into the motor while maintaining high air velocity and atomization at the discharge jet, there is a small orifice in the bottom of the round slide that is the top of the venturi and this is how low pressure is created in the bell ....
Great video. Just to let you know the SU carb was used up until 2001 in the british Reliant Robin 3 wheeler 😊
Excellent description 👍
Great video, thank you.
Great explanation, thanks for taking the time.
Glad you enjoyed it!
Very good explanation !!
ThankS !
Thanks for the heads up,
Very clear, thank you for that!
You're welcome!
Thank you. Very useful and clear.
Thank you, now i can understand how they work👍
thank you. had a rolls royce silver shadow, These carburetors were an anomaly to me, didn’t dare touch them and couldn’t figure out what they were called. pissed petrol all over the ground. Eventually sold it, now i learn its easier then regular carbs.
Awesome video 🤘
Awesome video and very educational.
Own 67 XKE 4.2
Glad you liked it!
Still the best SU Carb episode on you tube !
Glad you like it!
We were tuning SU H8 carburettors on a 3.4L Jaguar C Type today. All about the air intake in the end.
I had to figure out how these carbs work, because my MG TC has a pair on it, and I don't know how these work, so thank you for all of this information!
Glad o could help. Thanks for watching 😊
Perfect videos, watch every vid you release. I would be looking forward to see adjustment of 4 carb setup on XJ12 Series 1
Thank you! I would love and early XJ12. I do have a friend who just got an early XJ12 Coupe on carbs. Might be able to do a video on that car
@@LivingWithAClassic That would be great Adam!
Great video. Thank you
That was a very good demo of how the carb works and it is very simple. However, I am not quite sure how the piston goes up and down. There is that spring of course to help it go up but what exactly draws it back down when you let off the throttle? Is there an internal vacuum line to the top of the piston so that when you step on the throttle it draws the piston up and then reduces the vacuum as you let off?
I first saw one on a 65 panhead. you can always tell who's rolling up, with a su carb on their bike, whistlin.
They do make a great noise
Hi Good video I may have got the dash pots mixed up on the Mk 11 How can I check if the right ones are on the carbs Mike
Good video . My '75 R-R has 2 - S,U carbs .
I’m glad you liked it!
Hi Adam, very good. I have a problem with engine on idle and found that on one of the carbs at the bottom of the piston there is a small (5 mm) plastic stopper that was damaged, thus there where no clearance between piston and "nozzlebridge", resulting in no fuel delivery by this carb. I heard that in later SU HS8 production, this plastic piece was replaced by a brass stopper. My question is what should the clearance between piston and bridge be. Im looking at putting a brass screw there and file it down to dimension.
Best regards
Nils
Very well done. Thanks a lot
Glad you enjoyed it!
Can you explain the poppet valve spring and how it effect throttle response
Thank you so much. I just bought a 1970 Rolls-Royce and I was having nightmares about the carbs. I thought they were unique.
Glad the video helped!
nice explanation, thanks.
I'm glad you liked it!
MG Midget owner and I love these caburettors. I am not a mechanic but I can fool people when I fix these things. English cars are easy to work on once you forget all your habits you learned working on American cars. Very simple but very different.
Good video! 🙏🏽 thank you….
As the fella said to me when he sold me my first LBC [''61 Sprite] 'The closest thing to fuel injection'
On a Harley Davidson these carburetor's are absolutely without peer, no other carburetor will give the throttle response on one like an SU will, especially on the older Harley's, the ports were too big for the engines needs and as a result port velocity was too low, the constant velocity principle of their operation really helps the older one's with throttle response.
Back in the day we used to scrounge around junk yards looking for them on wrecked European cars with the prize being a 2" version from off of an E type Jaguar, there is however an aftermarket supplier of performance parts for Harley's called Rivera Engineering that offers them in kits for different models of Harley's, they take brand new ones and do a few simple modifications required to make them motorcycle friendly like provide the correct type of throttle cable brackets and an intake manifold so you don't have to modify one to work, in '97 they started offering the improved version that doesn't have the troublesome bushings in the dash pot and instead has a caged ball bearing assembly and they also don't require an oil level in them like the earlier ones.
In the late 80's Keihin designed a carburetor for Harley that's a sort of hybrid between a regular butterfly type carburetor and an SU, the idle and intermediate fuel circuits work like a conventional carburetor with the slide stayed closed but once it transitions through them then the vacuum operated slide with it's same type of needle configuration as an SU takes over.
There's even a guy in Daytona Beach named Carl Marrow who owns Carl's Speed Shop who makes performance Harley parts that sells his own version of an SU called a Typhoon.
And the fact that they just look cool as hell on a Harley with that big dash pot sticking up on them doesn't hurt one bit either.
Just trying to get a Midget running after 15 years. Both needles were dirty and float bowls. Would you expect fuel to flow up through the jets with the needle's and casing off? The pump is on and filling the float bowls.
su carbs was used all the way to 2002 as the su hs2 was used in the reliant robins till 2002 just so you know but great video
I am working on a project of mine (1970 mgb gt) and I just had to throw a rebuild kit in to fix a fuel leak
Reliant continued to use the SU carb right through to 2002. Much, much longer that the Maestro or Mini.
Hi, I have a 1979 Spitfire with twin SU Carbs and I am wondering if the metering needles are correct. They seem to angle very slightly to one side. I have unscrewed the grub screw and tried to adjust them but that hasn't done anything. I wonder if the guide/sleeves are too loose or the little springs that give the needles tension are too worn? Or is a slight angle to the needles normal? I thought they should go directly down into the jet orifice. Any suggestions would be great. Thanks. Rob
very well explained. the needle can also be worn i am told.
Thank you! Yes they can wear. When rebuilding I recommend checking the tolerances or simply replacing the needles
Hi my twin su carbs on my mini is iddle-ing very high. I tried all setting on absolute minimum and still its high. what could be the problem. After my cylinder head came back I had this issue.
Hi, what is the correct fuel level at the float bowl? Or how can I regulate the fuel level at the mid section?
Mi jaguar mk2 1967 3.4 has fuel leak bottom plenum thru the zirk..can you give me some orientation??
I have a SU H2 (I think) off a 19630MG midget. It has 3 -4 brass lines out of the bowl. I assume 2 are for fuel, what are the others?
Hello how much oil should I put in the carb and how often do I need to put oil in it.
Where can I get that electronic valve on the SU cars..mine is dead ...what is it called even
I've seen some SU's with a jet lever, what are they for if someone can help me
I need a rebuild kit...where can I get it
Thanks again.
You’re welcome
Good video. Didn't see how the dashpot piston moves up when throttle is opened. Is there some hidden passage that pulls vacuum above the piston to cause it to move upwards?
Thank you! Yes there are holes in the piston that causes the vacuum to let it rise up
The cable connections are the worst thing, the aligning the needle and jet is also a pain... but overall they just work and they are a dream to tune
Thanks
Well put bud
Thanks!
Great video!
Thank you! Looking forward to checking out some classic minis now
@@LivingWithAClassic It seems you and I have some shared interests in British cars! Hahaha
I do love them! Have been thinking of getting something with an A series in it. They are such sweet engines
They modified the needle and seat to Viton tip, you never showed the diaphragm and spring, a common fault was rust or sand getting in and rotting the spring collapsing it causing bad mixture /running, also blocking the hole from float chamber to carb.
Datsun was licensed to build SU type card but with fuel pump injection.
So would the Constant Velocity piston make the throttle feel less responsive because of the time it takes for the piston to go up, or would it run better or feel more responsive because it keeps optimal air velocity going into the engine at all times? Or does it not matter and I still don’t understand the pros and cons of it. Also, at what point/rpm are the standard SU carbs the bottleneck for making more power. I know it would depend on rpm as well as engine size, but how much air and fuel do the carbs allow through under full throttle. Only so much air can go through before the vacuum from the engine is too much for the little British carbs that could. Thank you, this is an amazing explanation!
It would feel more responsive because the proper air fuel ratio is being maintained at all times and the motor does not suffer a power loss due to lean condition, a non variable venturi carb compensates for this with a accelerator circuit fuel shot, the debate comes in when one asks which carburetor can respond to rapid throttle opening the best.
Some would argue that firing raw gas out of a squirter into the venturi at rapid throttle opening is a less than idea method as this is not finely tunable and wasting fuel.
SU's can flow a lot of air but making it a multi barrel carb on a heavy breathing engine is not doable so high air flow demands are met with multi carb installations .
@@luckyPiston Thank You! Luckily I figured it out, but this is a very good explanation.
There is a great deal this doesn't cover. There are also significant differences between the earlier H Type and an automatic choke version.
And significantly there is another issue which will have a profound impact.
1. Never assume that the parts in the full kit are correct or good quality
2. Never assume that the parts on your existing SU are correct. Even when they have been rebuilt by some mechanic
On my twin H manual Sus the choke bottom nut couldn't be adjusted properly. Or fuel leaked despite new seals jet and jet body.
I found that the brass mixture nut on the jet was incorrect with much too much clearance around the standard jet. This meant that the bottom of the jet could move sideways hence leaks. I also found that the new jet bodies while they looked the same actually were not drilled the same. So you could not set the top of the jet flush with the body. And kit parts were not the same or as good as the original. E.g. jet seals and brass compression ferruls. Some are not suitable.
I highly recommend getting a vacuum guage and a glass spark plug so you can actually see the colour of the combustion flame and adjust while running.
The new jets all had sharp drilled holes that required beveling and the tubes fine polishing. Never sand!
SU carbies will not leak when everything is correct. Sadly many English cars have had fires because Sus have either been not maintained or set up incorrectly.
Cork Gaskets must be oil soaked at least 24hrs and I use assembly lube.
Do not over tighten and absolute clean is required
Cheers all
i put up my 1st Jag XJ6 video..finally...
That's great! I will check it out
Don't forget the Japanese, sports from the 240z early 69 into the 70s that had them as well.
Skinners Union 'knockoffs'
They are not really "SU's", they are Hitachi, a copy of the SU.
AISIN.
I had the same type as they call it "piston" carb in a Mitsubishi lancer Ck2A
Yet another of life’s mysteries solved !
Now on to the next one
My car wouldnt start, turned out the valve letting the petrol in had blocked... simply blown through - not that the petrol residue tasted nice
Hey man I’m having trouble checking the mixtures. If checking with the lifting pin, do I need to fully depress the pin?
You probably have info by now, but with the engine set at a fast idle, find the lifting pin, push it in until it contacts the piston, then use it to raise the piston by about 1/8 of an inch and listen to the engine revs, if they go up the mixture is too rich, so you need to raise the jet ring, if the engine stalls, it is too lean and so you wind the jet down. When the mixture is right the revs will raise slightly and then settle back. Easy as .....
@@chrisknight6884 I did but thanks for reaching out. After looking a little more closely I realised the pin travels a bit before contacting the piston. After that I could lift it the right amount. Thanks
So, why is the piston movement dampened by the oil cylinder?
Bra cause if the piston rises too quickly the engine would bog down. It would get flooded
@@LivingWithAClassic
I thought you might think that. No, it's quite the opposite. Check it out.
What's "Bra"?
Same carb in my datsun roadster
They came with 2 different carbs, the SU's and a similar designed Stromberg, most the guys who race the 240Z's will scrap the Stromberg's and replace them with SU's, they say you just can't get the Stromberg's to run like them.
There's a guy in the Pacific Northwest, I believe in the state of Washington, that races 240Z's that modifies and reconditions SU's for racing, his business is called Z Therapy, one of the improvements he makes to them is to machine the bodies where the throttle shaft passes through to accept tiny little sealed ball bearing assembly's and makes new throttle shafts and butterfly plates, that forever eliminates the issue of the hole's in the body egging out and leaking air past them, you can find his sight at ZTherapy.com.