SU Carburettor Tuning - Part 1 | Roundtail Restoration

In this first part of a (probably) two part series, I check the timing of the piston's fall, check the float bowl level, and center the jets. More to follow soon!
If you are interested in more pictures of the restoration and detailed write-ups of what I'm doing, please visit my website www.roundtailrestoration.com
Want to see all of the body restoration? This playlist starts shortly after engine rebuild and goes through the present: • Triumph Spitfire Resto...
Want to see my engine rebuilt? This playlist covers most of it: • Triumph Spitfire Engin...
Want to see other random technical videos, not exclusive to my Spitfire (like generator testing)? • Technical Videos

[@hecklejeckle1333]

Hi Chris. I've searched KZbin (and other sites) for days and there is no better video (especially quality) than this one for demonstrating the correct jet centering process. Many thanks

[@roundtailrestoration]

Wow, thanks a lot. It kicked my butt for a long time getting those right so hopefully I'm it go smoothly for others.

[@cabdriveruk]

Great video Chris. Seems like the job is going well up to now. Look forward to part 2. Cheers, Jeff.

[@roundtailrestoration]

Thanks, Jeff. Magic of editing!

[@GooseDave]

Chris. So many great tips here. This vid is a keeper. I didn't know about the tape trick. Thanks for sharing - now on to part 2.

[@roundtailrestoration]

Thank you! Definitely some learning as I went there, but you eventually get the hang of them.

[@Zincaloom]

What an interesting carburetor. Had them on a MGC I had years ago. I remembered checking oil in the engine and the carburetors. Never had any problems with them.

[@roundtailrestoration]

You're lucky! I figure once I get them properly set up, it'll just be a matter of maintaining them. But, right now, they are all out of whack!

[@dbarr15]

Very nice video!

[@roundtailrestoration]

Thanks!

[@dbarr15]

So, "getting close to the end" "running out of things to do" What are you plans for Dorothy when you are "finished"? Move on to a new project? Add her to a collection? Sell her? I have enjoyed you videos!

[@roundtailrestoration]

Thank you. For now, my only plans are to drive her. She will be a collection of one and there are no plans to sell her. I'm always exploring new project options, but I don't really have any room to take on another. We'll see what the future holds!

[@isaacburlo9547]

Great video, used this to help me tune my twin HS2's.

[@roundtailrestoration]

Good, I'm glad it helped! Definitely a skill to learn, if not an art.

[@isaacburlo9547]

@@roundtailrestoration Agreed. I spent the last part of an hour trying to centre my jets and make both pistons have the same 'clunk' sound. Their not 100% the same but close enough.. their 60 yrs old now so I guess thats the best I'm going to get.

[@roundtailrestoration]

@@isaacburlo9547 Yup, I think that's the best you could hope for!

[@grahamjohn331]

Great info Chris, thanks. I imagine that you can use the piston lift button to lift the piston to fill the dash pots. Fill the pots to just below the top edge, any extra will lube the carb and burn off. I have not yet been successful in keeping oil in my ZS carbs, even after replacing the o-rings twice. Oh well, winter project.

[@roundtailrestoration]

Hmm, I hadn't thought about doing it that way, Graham. Might be a good way to cheat. I got loads of 3-in-1, so I might just give that a shot! Thanks!

[@ericpfiffner5615]

Geez I wasn't able to faithfully watch your video right away like usual, we were at the beach. I'll get back on track.

[@roundtailrestoration]

I allow it this time! Beach as in ocean beach or lake beach? I'm not very familiar with any oceans near where you are! In any case, I hope you enjoyed it!!

[@ericpfiffner5615]

Phew, good I'll be ready for part 2! No beaches here that's for sure. We went to Myrtle to play in the Atlantic, it was fantastic. Needed a break from playing covid.

[@roundtailrestoration]

Nice. I lived in Charleston for a while when I was in the Navy. Nice down there. Too damn hot, but nice.

[@drmoss_ca]

About forty years ago I was messing with a pair of SU carbs in a TR7, with a Haynes manual as a guide. It didn't seem to go well, but at the time I did not know the car was probably a Friday Afternoon Special, and the timing marks on the flywheel had no relation at all to TDC. That made getting it to idle and run smoothly just a little bit difficult!

[@roundtailrestoration]

Oh, man. Yeah, that would be a challenge! Sorta like when I got this car and they had put the distributor in 180 out. New cap, rotor, plugs, wires...and not running! Took me a while to figure that one out. Fixed it on the rebuild, though!

[@martynhalsey1859]

Hey Chris, what a good and useful video. I must thank you as I have a 1981 Spitfire, which I have fitted the waxstat conversion kit to. It starts and idles ok but misfires when revved so will be trying exactly what your video explains. Regards Martyn

[@roundtailrestoration]

Thanks, Martyn. Good luck!

[@frankstrobel4350]

thanks for the video. I have HS4's on my car. and always wondered what centering the jets was all about. (my carbs were rebuilt by someone else)

[@roundtailrestoration]

Black magic, Frank. And luck!

[@JG-eh1qq]

Nice start Chris, balancing will take a bit of time. I would suggest buying a synchrometer off Amazon for balance air it will help.

[@roundtailrestoration]

Funny you mention that, John. Somewhere on this planet is a synchrometer that I purchased for just this purpose. Could have sworn I brought it over to the garage...damned if I can find it. I'm going to resort to the hose listening method for now.

[@WegsGarage]

Yikes! Us midwesterners don't know what safety inspections are...not sure if that is good or bad! Carbs are sounding good. Definitely an art to getting that right. We have come to the conclusion that practice makes perfect. Tools help, but experience is king.

[@roundtailrestoration]

Lol, yeah, they're a bit strict here. I'll get some more practice because there's more running to do!

[@karljay7473]

I'm surprised the fuel line runs in front of the carbs like that. Cool to see the old sync tools again. I used to rebuild these carbs back in college days. They're excellent carbs when you get them running right, I'm surprised more cars didn't use them. Even ended up with a Predator carb on a V8 and dual Asians on a 2L Toyota Celica. I have the old 8RB 2L Toyota that can with dual Asian SU carb.

[@roundtailrestoration]

Thanks, Karl. Yeah, I was kinda surprised, too, but that's about the only place when the fuel line comes up the other side from the carbs. Getting them set up right seems to be my biggest hurdle right now. I'll keep trying!

[@karljay7473]

@@roundtailrestoration Getting everything at "zero" is a key thing from the starting point. I see you did the butterflies, it's very rare for those to need replacement, remember they have a direction, the edge has a bevel such that it only goes in one way. That means that the edge is NOT a 90 deg cut, it's off a bit so that the butterfly sits at a slight angle (not perpendicular to the throat), but the butterfly edge should be parallel. If you look at used US carbs, you'll see a "seal line" that is the thickness of the butterfly. You might have progression ports and they can be different sizes/shapes. It's important that the butterfly seals because too much flow and it'll draw thru the progression ports or pull too much from the main jet instead of pulling from the other side of the butterfly. One other thing is that you need to adjust the valves and timing using a vacuum gauge. IDK if you have a tuning port on the intake, but if you adjust the valves and set the timing for max vacuum, that should give you a good starting point. It's kind of a "wack a mole" game because you need a well tuned engine for proper vacuum in order to properly adjust the carbs, but you need well adjusted carbs to know if the timing is right. IIRC (and it's been a while) you remove all vacuum to the timing and set the time at it's base (no advance from mechanical or vacuum) then start setting the idle sync. Once your timing is off it's base, you change things and you have to get it down. A slight opening in the butterflies or butterflies in wrong (no parallel seal edge, TWO actual touching edges of the butterfly touching) . Remember that once you are off idle, the linkage will change the sync. So you probably have an idle set screw on each carb, then once you remove the tension from the linkage, those screws have no input, they only effect the idle sync. Once you have tension on the linkage, it becomes the point of sync and you should have an adjustment for that. Off idle is where you spend most of your time, so it's important. Don't think that just because they flow equal at idle, that they are synced. So sync at idle with the least throttle you can and no timing advance. Then kick it up past idle, maybe 1500 and sync there. There should be online guides for tuning based on a vacuum gauge, but it should be near 18 and steady (IIRC). I thought 20W was thin, IIRC, we used to use jack oil at about 10W, but again, that was years ago. The thicker the oil, the slower it rises and IIRC, that gives a bit more torque. I live in California and had mine well tuned, took it down to San Francisco and it just ripped up those hills. The SUs can pull hard all thru the RPM range. One other thing you should do is add a heat shield. I made a "waffle plate" from alum/steel and washers that kept the heat from the header off the intake. IIRC, old fuel boils at 140 and new fuel even less. I actually have a custom made asbestos shield on mine, made a HUGE difference in the performance when you get cold air in there. Someone else posted about doing a "quick lift" on the pistons to determine if the mix is right. I don't remember if it's right when it doesn't change much, but I think that was the key. You can also use the "quick lift" (a few mm) to see if they both act the same. Past that, you can read the plugs. Reading the plugs is a pain because in order to do it right you have to stop the engine, pull over and read them. A better way is to put an O2 sensor in the header. I bought a "spark plug foul protector" (threaded tube) and had it welded into the header. I attach a voltmeter to the O2 sensor and it tells me the tune of the carbs. You can also get a Air/Fuel gauge and do the same. It's good for telling you your A/F mix during accell/decell/climb/down hill/ WOT etc... Comes in handy when tuning. One last note: don't always blame the carbs. I worked in the industry, people LOVE to blame the carbs. The carbs depend on signals from the engine, mainly vacuum, air speed and throttle. Poorly adjusted valves and/or timing makes a difference.

[@roundtailrestoration]

Thanks, Karl. Lots of great info to absorb. I've considered getting a Gunson Colourtune - supposed to make setting the mixture about brainless, but I'm not sure. I'll be back on it this weekend!

[@karljay7473]

@@roundtailrestoration I've never used a Colourtune, but I've heard good things about them. The concern I have is that you might not know what the problem is. So if the Colourtune is all about mix ratio, you might be good on that and bad on valve adjustment, timing, butterfly setting, idle sync or off idle sync. It is a good idea to go thru a list and remove things from doubt.

[@roundtailrestoration]

Yup, good point. And, besides, I wouldn't learn the skill of setting mixture by "hand".

[@mattfelmlee2102]

Hi, did you drive the car with air filters off? My engine won't run smooth when I get on it. Back off the pedal and it runs smooth. Giving it some choke really helps.

[@roundtailrestoration]

I did not drive with the filters off. Makes me too nervous I'll suck something in. I would say you've got a mixture issue, or the floats aren't moving smoothly. Sorry, but I'm not good with troubleshooting problems with these carbs yet.

[@redbarchetta8782]

Wrong 3 in 1 oil. You need the Blue label which says SAE20 on it. You have their penetrating oil version. It is WD40 like.

[@roundtailrestoration]

No kidding? I was this many days old before I knew there were different kinds of 3 in 1. I'll look that up. Thanks!