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No vacuum on cylinders 3&4

Neverfinished

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Bike runs, idles steady. Carbs have been pulled and gone through twice. Manifolds on both sides replaced. No indication that the bike is running on only two cylinders and it actually sounds nice at idle. Figured since I got it running fairly well id do a carb sync.

Cyl 1&2 look good - both around 30ng on the gauges. (Not sure what ideal vac level should be). Cyl 3&4 are both at or under 10ng. Any attempt to adjust the screws it dies. Checked for air leaks around all manifolds. Used half a can of starter fluid. No fluctuation. Checked compression on all 4 - between 115 and 125 (120 on both 3 and 4). Valve clearances were checked less than 1k miles ago all .005 +/- .001.

Where should I head next? Timing? I also have a weird suspicion that maybe the throttle linkage between left 1&2 carb bank and right 3&4 carb bank may be compromised / unlinked. But the linkage on that right side rotates with the throttle. And I just put them together and bench synced them. Will pull the bank and check mañana.

There’s nowhere else that carbs 3&4 are married besides the actual carb bank and maybe cam timing?

Why would 3 & 4 have no vac?
 

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What screws are you referring to that you are adjusting?

So syncronizer screws dont have any affect on the vacuum reading? Have you swapped the gauges to different cylinders to make sure your gauges are working correctly? Also make sure the damper valves allow just a bit of needle bounce so you know the needle movement is accurate when engine is running.

Cam timing cant cause this because all 4 cylinders use the same intake cam and all 4 cylinders use the exhaust cam....if cam timing was off it affects all cylinders not left and right halves.
 
Bike runs, idles steady. Carbs have been pulled and gone through twice. Manifolds on both sides replaced. No indication that the bike is running on only two cylinders and it actually sounds nice at idle. Figured since I got it running fairly well id do a carb sync.

Cyl 1&2 look good - both around 30ng on the gauges. (Not sure what ideal vac level should be). Cyl 3&4 are both at or under 10ng. Any attempt to adjust the screws it dies. Checked for air leaks around all manifolds. Used half a can of starter fluid. No fluctuation. Checked compression on all 4 - between 115 and 125 (120 on both 3 and 4). Valve clearances were checked less than 1k miles ago all .005 +/- .001.

Where should I head next? Timing? I also have a weird suspicion that maybe the throttle linkage between left 1&2 carb bank and right 3&4 carb bank may be compromised / unlinked. But the linkage on that right side rotates with the throttle. And I just put them together and bench synced them. Will pull the bank and check mañana.

There’s nowhere else that carbs 3&4 are married besides the actual carb bank and maybe cam timing?

Why would 3 & 4 have no vac?
Check to make sure you don't have a vacuum leak where the gauge vacuum lines connect to the vacuum ports. The ports might be stripped and the gauge nipples might not be sealing.
 
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