SU Vacuum Leaks. It’s not always the shaft bushings

[zKars]

Some things just become matra’s. 

With SU’s, people ask you how to know if the ones they have or are about to buy, are worth anything, and the “Standard Response” is “wiggle the shafts and see if there’s play” as they cause a vacuum leak. Then we typically add to that “get Z-Therapy to rebuild with their cool sealed shaft bearings”. End of discussion

Well as I’ve come to discover there is another nasty vacuum leak area of concern on SU’s that I think is JUST as prevalent as shaft wear. The nozzle fit in its sleeve is often very worn.

[zKars]

One of the cars I worked on this winter have an older set of Z-Therapy rebuilds on it, so when I got to the point of putting the carbs back on, I didn’t look too closely for problems. 

But as I was syncing and setting up mixture etc, the engine would randomly change idle, always upward and sit there unstable as hell. Blip the throttle, fool with the choke and it would settle down. Then off it would go again with very little provocation.

Finally realized I was just grazing the little fuel hose on the bottom with a tool or finger while working on them, it would set the idle off on one of its fits. What I was doing was shifting the nozzle around in its tube, creating clearance and a vacuum leak when it sat just so.

I’m waiting for new nozzles from Z-therapy now, should solve the issue. The back carb is much better and doesn’t exhibit this behavior. 

Other carb sets I have laying around, from which I was going to snitch parts, are all worse. The nozzles literally rattle around in the tube. 

So expand your mantra when you help yourself or others decide on SU quality and vacuum leaks. It’s not just them shafts that get worn.

[zKars]

I will add that in speaking with Bruce at Z-Therapy about this, that a worn jet tube or the nozzle is actually very un-common, at least to the point of being a vacuum leak.