Nearly every classic British sports car that shows up in our driveway has some form of run quality issue. As part of our never-ending quest to improve the fleet, a large part of our work is focused on worn SU carburetors.
A nice new butterfly is present on the HS2 carb below, but the clean edge of that disk cannot seal against the irregular surface of the throat of this badly pitted carburetor carburetor body. New carbs were needed to make this Bugeye run well.
This particular project started when I detected a somewhat subtle misfire on a very nicely prepared car we worked on this week. We had already tuned the car and it ran well, but I knew we could do better.
We re-checked all the basics and found nothing out of sorts, until Terry noticed the intake throat on one of the carbs was quite pitted. He removed it, closed the butterfly and the back light reveals how poorly this carb butterfly was seating in the body.
Here, air flows right around the closed butterfly and feeds the engine air when it is unwanted. The carb bodies shown here are shot. We removed and replaced these carbs. Now, the misfire is gone. The idle is smoother, the engine is more responsive. Problem solved. You can tune almost any worn set of SU carbs and compensate for their worn components, but new carbs make life so much better.
We are often working with unevenly worn carb pairs, which makes engines run unevenly. Most of the world’s SU carbs are worn, and so there is a limit to what you can do with your old British sports car, until you sort out the carbs.
One of the most common causes is throttle shaft wear. The bushing on which the throttle shaft rides wears over time, and unmetered air leaks around the bushing. Thus when the butterfly is closed, air is still running through the shaft bushings. This makes idle adjustment and synchronization very difficult. Idle issues will often manifest as run quality issues at higher RPMs because tuning done at idle is based on flawed measurements and worn parts. Look at the throttle shaft wear in the video above, and you can see how air would leak past the shaft without any difficulty.
The solution is to rebush your throttle shaft bushings. They need to be properly reamed so that they fit just right. This can be done if you have the correct reamer, but more and more, we are replacing the entire carb set with a new pair, (since everything else is also worn). This option immediately gets rid of the multitude of other worn parts that we often find on these old carbs.
Click here to see a new set of HS2 carbs and to learn more about the new carb set we use on many of our restorations. This HS2 set is also a great replacement for any engines still running the older and smaller H1 carbs. Feel free to call our parts department at 203 208 0980 if you have questions about how to best address any SU carburetor woes.