Topic: Carb Sync/Air Cut-off valve question

[CBJoe]

I found the info below pasted into an old forum talking about the air cut-off valves on carbs.  I have a 1980 CB650C with the mechanical type carbs and the diaphrams need replaced.  I'm running mac 4-1's and i've just installed POD filters with 100 mains (upgraded from 90 stock).  I was having trouble sync'ing the carbs (i believe due to these worn diaphrams) and also with adjusting the mixture.  Ok, Here's my question:

Is there any DOWNSIDE to blocking off the access port in the intake side of the carbs to disable this system?? 
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The air-cut valve interrupts the idle circuit's air bleed, to richen the mixture available from the idle circuit, on trailing thottle. The benefit is that an otherwise ultra-lean mixture is not sent to the exhaust to mix with its constituents and then afterburn (not "backfire"), causing the familiar "pop" sound. The air-cut valve began appearing on Hondas during the 70s, when idle mixtures began to be leaned (I think the XL250 was first). When the diaphragm fails, manifold vacuum can access the idle circuit directly, resulting in a leaner than normal mixture at idle and a richer than normal mixture at high rpm (difference is manifold vacuum can access both air and fuel sides of the circuit, but one is stronger at different rpm due to the circuit's small size). These symptoms of course make the problem very hard to troubleshoot, so write this down somewhere. :-) Happily, the air-cut valve is redundant on properly adjusted carburetors, that is, when the mixture screw is richened up to about 3% CO (carbon monoxide). My advice is to disable the system, thereby permanently eliminating it as a possible problem. This is easily done by simply blocking off the hole in the carb casting that comes from the intake manifold, with a suitably sized piece of rubber inner tube or something similar. Incidentally, the air-cut valve is not an emissions device, but rather a consumer confidence device.

-- Mike Nixon
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Thanks for all advice.
Regards
Joe

[MRieck]

 I would disable it and check for vacuum leaks at the carb boots too.

[CBJoe]

I finally broke down and purchased new rubber manifolds ($80 freaking bucks).  I had to cut the old ones off.  I think this is the first time that I haven't had air leaks there.

Anyone else see a problem disabling the air cutoff valves?

Thanks

[CBJoe]

I was looking at the exploded parts manual and I noticed that it lists the Main Jet as 95.  My carbs had 90's and when I put the POD filters on I up'd the Jets to 100.  Is this big enough to be running PODS?

[scott_cb650]

So, how's your progress going?

I also have a ’79 CB650 and have an inconsistent idle, so I was wondering if you determined the usefulness of the air cut off vales on the carbs.

So did you end up using new air cut off vales or by-pass them?