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Carb Sync - Low Vacuum On Center Carb
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GeorgePBurdell
Posted 2016-06-13 1:12 PM (#151040)
Subject: Carb Sync - Low Vacuum On Center Carb


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Posts: 40
25
Location: Atlanta, GA
I recently had the carbs on my '78 Special vapor blasted and cleaned to solve an erratic running issue. In the reassembly, I installed new butterfly seals. I bench synched the carbs and put them back on the bike - fired up on the 1st kick! However, I have a bit or an erratic idle and some hesitation, so I decided to validate the carb sync with a manifold. I built a U-tube maifold and found that both the #1 and #3 carbs would pull fluid (a mixture of ATF for color and some STP for viscosity) out of the manifold tubes faster than I could attempt any adjustment. Broke down and bought a vacuum gauge amnifold for the job - and similar work on some other bikes.

With the manifold, I can get "good" readings and make adjustments - BUT I can NOT get the outboard carbs down to the vacuum of the center one. At idle - about 1200 rpm, it shows just about 10" vacuum. I can get the outboards to match at about 15", but no closer than that. Bike seems to idle OK - not perfect - and will take throttle. She seems to run OK under load, but I would expect the idle vacuum to be closer. When revved up, the difference gets a bit less, but the center carb remains lower.

I have rechecked the bench sync, verified no leaks at the boots and caps. Any suggestions on what else to look for?
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