Why did taking the YICS bolts out improve my idle?

So...you all have some point have seen my struggles in getting this bike to run properly. I have done everything this site suggests, valves, carbs, jetting... I recently broke my rack and replaced the o rings and shaft seals, cleaned everything (carbs were already very clean), verified my floats. I replaced the throttle cable. No binding. Replaced the intake gaskets. Ditched the pods the PO had on and got the air box filter.
My problem is that as soon as the bike warms up the idle hangs and rises like there is a massive vacuum leak. I've gone over the whole thing with starting fluid and there is no vacuum leak. It idles and runs so poorly that it makes getting a nice vacuum sync almost impossible, but it is synced.
I have it jetted 41/126 with a uni filter and mac 4-1, idle screws out 3.75. I want to knock it down to 40/124 with the OEM paper filter, because it seems to like the #40 more.

So as an experiment last night I took off both bolts to the YICS system to see how it would run. My YICS passageway was very clean. Well, much to my surprise it ran like a champ when I did that. I adjusted the idle down to 1000 rpm and its the first time I've ever heard it idle nicely without sputtering and missing. The revs also fell nice too. I rode it to work today like that and its the first time the bike hasn't tried to drag itself forward with the revs sticking. I thought it would run bad like that but its the complete opposite.
So my question is what could be wrong with the motor or carbs to make this happen? My compression on 1-3 isn't great around 125, #4 is the only strong one at 145. I'm dumbfounded.

 

I can't wait to hear the answer. This one sounds like a stumper. Maybe a one-bike issue of some kind no one's ever had before.

Any YICS experts here? No one talks about it much, since it has no mechanical moving parts, and as such never really has something go wrong with it...

 

YICS interconnects the 4 intake passages.

Running with the YICS plugs out would have meant that the YICS "gallery" is SUCKING WIND, big time. So it would have caused all 4 to run lean, the outer two (#1 and #4) moreso than the inner ones, I suspect. In effect, uncorking the YICS passage creates a giant vacuum leak.

If it suddenly made the bike run better, you're too rich somewhere.

I WOULD NOT ride it that way. The "outlets" of the YICS passages are right behind the heads of the intake valves, if it were to suck in a hunk of debris large enough it could do some nasty damage... not to mention the potentially negative effects of blowing relatively cold outside air right on the head of a nice hot intake valve.

 

sounds like your over jetted or your pilot screws are backed way out. Bottom line is some thing is causing it to run super rich. So when you take the bolts out it leans it up. Check for any thing that will make it run rich.

 

Thanks for the advice.
I've tried searching for a stock exhaust so I could run his thing 100% stock with stock jetting and everything I find is either garbage or expensive garbage. I'll jet down to 40/124 and keep the UNI filter in to lean things out. I need the main bumped up I tried stock 40/120 jetting after I rebuilt the carbs and I lost all mid range as it starved for fuel.
I have a color tune plug but its never idled nicely enough to make sense of it.

 

Honestly?

I'll bet if you went back to a new stock paper filter it would be much easier to get it to run right with the MAC system.

I also think it would be a good idea to revisit your float levels; ensure that the seals between the carb bodies and the float valve seats are intact; and carefully inspect the seals in the bottoms of the enrichment plungers. Make sure all four enrichment plungers are "free" once the choke is in the off position.

 

I had to pull my pilots out to 4-4.5 turns when i put my mac 4-1 on, didn't need to rejet at all.
Probably should shim the needle a little as it's got a little flat spot around 5k, but once she hits 7k goes like stink.

If that helps any. Stock Air box.

 

Mine idles fine with a mac 4 into 1 and standard jets.