I wish I had the internet 30 years ago 🤣

Hi Adam, your video is the first to pop up on my search for SU carb adjustment!

The number 1 issue with old SUs are throttle shafts that leak air.  Spray carb cleaner around both sides of the throttle shafts in each carb and if the idle changes you need to replace the shaft bearings.

The very first step is to set the float level.  If it's too high the carb floods and nothing else you do will work.

Thanks!

I have not had a carbureted classic car for 15 years, but I am getting a TRIUMPH TR3 this week. It has H6 SUs so this was a great reminder tutorial. Thanks. I am now subscribed.

I'm not sure if you have an earlier video but the first step is to make the main jets are serviceable and set to the correct initial heights and that the throttle spindles have no lift. If any of those three are not right it's a waste of time carrying on.  Jet heights have often been messed around with by owners in the past.


You also miss an essential step while tuning and that's to clear the inlet manifold of excess fuel regularly. This was a standard procedure and every few minutes entailed raising the RPM to at least 2000 RPM and holding it there for 30 seconds. Then you can use the lifting pins and make adjustments however the lifting pin method works well on some engine and less so on others so the best method is to check spark plug colour after a run.On this age of car the lifting pins were never intended to be used. They were just a hangover from earlier times.


You also seem to raising the RPM intermittently using the cable quadrant however you still have the two carbs disconnected to reset air flow.

You should emphasize the importance of the throttle plates (butterflies) closing completely at idle. You did mention the throttle stops but if they are not in the correct position on the linkage the plates could then be slightly open. Also a timing light is useful for reading rpms while  making adjustments

Thanks for the video. Tuned a set by ear. Buying a tool for air flow to fine tune them. Also learned how to properly use the dashpot lifter, thanks again.

Thanks for the video. It has been a great reference for me to keep my customers 240z running as it should. Hadn't touched a set of su's since the 80's, now in the past 3 years I've had 2 customers show up with 71 240z's both with carb issues that the local z specialist would not touch. Thanks to your video I can tune them peffectly.

Thankyou Adam, that is so helpful. I'm gonna give it a whirl tomorrow 🙂

Thanks for the video.  I'm refamiliarising myself with tuning multiple SUs after buying a Rover 3500 P6.  I've had many cars with twin SUs over the years.  It's been a few years now though since the last one (Jag 3.4S) and I've never had HIFs before so that's new.

When I bought the car it was running and driving nicely but obviously running rich as it smelt fuelly and also started without choke no matter what the temp was.  Running very sweet now though and starting easy too(with choke when needed).

New to side draft,I'm not even certain my single is a SU.Working on my MG B.Great video

Excellent Tutorial - Just what I was looking for!

With the SUs on Datsun 240Zs, I think the single hardest part of tuning them is getting the float level right. There is a lot of books that differ, including actual service manuals, as to what the best procedure is to get the float correct. There was a great tool made for it, but apparently the guy who created it doesn't make/sell them anymore. The float needle valves have a spring in them and the neoprene needle tip make it difficult to feel exactly when the float touches. 14mm isn't easy to measure precisely...

Super helpful for tuning my XK120- thank you!

Restoring my 240z and I have 2 SU carbs!! Your videos will help a lot! Cheers man and happy new year :)

Great video on the basics of working on SU Carbs. I worked as an MG, Austin Healey mechanic in the 1960s and early 70s and although the SU Carbs used in them are similar to the ones on this Jag there are some slight differences in design. To adjust the jets for rich/lean mixture on MGs you use a wrench to move the jet up or down by changing the height of the seat. They used to sell a special tool for this but I always just cut most of the handle off a suitably sized open end wrench which gives you a stubby sized wrench which is much easier to use underneath the carbs. For synchronizing the carbs I just removed the air filters and held the end of a small rubber hose on the edge of the carb throat and placed the other end in my ear. The sound of different air velocities is quite distinctive and with a little care very precise adjustments can me made with this method. One point not made in this video is that it is absolutely key to do a thorough job of cleaning the inside of the dashpots as well as the skirt of the piston that runs up and down inside it. Many times varnish that collects on these surfaces can cause the piston to hang up, or lag behind in responding to throttle changes. As mentioned in this video these pistons and dashpots are a matched set. The reason for this is that fairly tight tolerances are required for them to work properly. Unfortunately this eventually became a problem with MGs (at least in Calif) as EPA requirements forced MG to tighten up these already tight clearances. I owned a small sports car repair shop in Calif in the early 70s and MGs sold in Calif at that time were very difficult to set idle speed on as the dashpot pistons were constantly hanging up. It would have been illegal to hone out the dashpot chambers a tiny bit but doing so may well have solved this problem.

Great lesson- thank you.

These videos are completely enlightening and motivating. Going to refurb some roundtops now to replace the flat tops in my 510. Thanks!