SU Problems

[Tri-Star]

Ive spent too many hours with limited success. It has been running rough the last few weeks with extreme loss of power. I tore the carbs apart cleaned them and put them back on and tuned them. Most power has returned but still running rough. The air flow is ballanced, im at 1 and 7/8 turns mixture, both float bowls appear to be working the same. Heres where the issue lies im sure: When i press on the piston lift button on the back carb (making it only run on the front carb) it lowers slighly in rpm and runs a little rough. That means the front carb is tuned correcly from my understanding. BUT when i do the same on the front ( making it run only on the back carb) it stalls out right away. From what i have read this means its lean. Currenly both carbs are at the same mixture but i have taken it all the way to 3.5 turns and it does the same thing. Any ideas?

[spawpoet]

I would guess you have a vacuum leak on the rear carb. could be through the throttle shaft if it has worn excessively, but also check that all vac plugs are in place, and that all gasket surfaces are fully sealing. Also have you checked other potential issues such as plugs and valve lash?? I've had a cracked/non-functioning plug on a couple of occasions.

[zclubhouse]

I am far and away from being a SU guru, but I was under the impression that if a SU is tuned properly you should have a slight rpm increase that settles back down to normal speed? Other than checking for vacuum leaks, I would start with normal tune up procedures so you can isolate the carb variable. A fouled plug or two and you will never be able to tune it proper.