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Discussion Starter · #1 ·
Can a perforated ACV render the fuel mixture screw ineffective? My right cylinder is exhibiting some lean condition symptoms - popping on deceleration, valve cover is slightly cooler... when I've tried to richen the mix the adjustment seems to have no effect. I can dial it all the way in without the cylinder missing. i've pulled the screw all the way out to verify that the end hasn't lodged and broken off in the aperture. In order to pull the ACV to inspect it, and perhaps defeat it, I'll have to remove the carbs anyway, at which point I might as well give them a thorough going over, but I just wanted to see if anyone could confirm my suspicion.



Thanks
 

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Discussion Starter · #3 ·
Wow.



in looking at that cutaway it seems more likely that I've got something obstructing one of those tiny supply passages (red is fuel, right?)than just a faulty cutoff valve.
 

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Red is fuel. Blue is air. And yes any one of those little things could be plugged. Only decent way to clean them up is to pull out that center jet and run the carbs through an ultra sonic.
 

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rattliff,can you please spend a few minutes to update your signiture line.include your bike details and your location.it helps others help you.


you never know,there maybe a member just a block away who can help you
 

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Can a perforated ACV render the fuel mixture screw ineffective?

when I've tried to richen the mix the adjustment seems to have no effect. I can dial it all the way in without the cylinder missing.

Thanks


Yes, those are classic symptoms of a perforated air cut-off diaphragm.

When the diaphragm has a hole in it it creates a circuit in the carb between the pilot jet and downstream of the throttle valve via the vacuum drilling that supplies the low pressure to the air cut-off housing, so even when the idle mixture screw is fully screwed in, the hole in the diaphragm can supply enough fuel to keep the engine running at idle speeds (without faltering too, as you have noticed).

The vacuum drilling is obscured in the above pic by the text.

There is also a slight error in that pic as it shows fuel exiting the pilot and off-idle drillings and going back up the venturi towards the airbox instead of the engine....
 

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Discussion Starter · #7 ·
Yes, those are classic symptoms of a perforated air cut-off diaphragm.

When the diaphragm has a hole in it it creates a circuit in the carb between the pilot jet and downstream of the throttle valve via the vacuum drilling that supplies the low pressure to the air cut-off housing, so even when the idle mixture screw is fully screwed in, the hole in the diaphragm can supply enough fuel to keep the engine running at idle speeds (without faltering too, as you have noticed).

The vacuum drilling is obscured in the above pic by the text.

There is also a slight error in that pic as it shows fuel exiting the pilot and off-idle drillings and going back up the venturi towards the airbox instead of the engine....


Thanks for all the info, guys. Pending how the ACV looks when I pull it, I may go ahead and give the carbs a soak in a friends ultrasonic bath. Either way, I'll post how its resolved.
 

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Discussion Starter · #8 ·
Pulled the carbs this weekend and examined ACVs. Both looked totally intact (as they should since I replaced all the rubber about a year ago), but I went ahead and defeated the valves anyway since this seems to be a common enough problem. I also pulled the pilot screws to inspect the unresponsive one and found that there was an old broken gasket crammed above the new one - bingo - this must be the source of my trouble.



not so.



I put everything back together and set the pilots to 3 full turns. bike seemed to start and idle a bit better from the get-go, but when I took it for a spin the popping is still present when I down-shift. And when I say popping, what I actually mean is one singular pop, almost every down-shift. some times pretty loud. throttle response and power seem to be equivalent to my recollection of the bike at its best. The bike does require more choke than I recall at start and is a little sluggish until it reaches operating temp. Oh, and the pilot screw is still unresponsive. My current theories are:

a) piece of old grommet lodged in small circuits leading the pilot/ carbs just need a good ultrasonic soak, in general.

B) cylinders are coked from excess choke and popping is due to predetonation.

c) small hole in lt muffler is the culprit, though I'd expect more small pops if that were the case.



I'm gong to patch the muffler and run some seafoam through the tank before pulling the carbs again. I really can't afford to have the bike down for more than a day or two - weather's in the 60's here this week.
 
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