66 timing issue on a current gen carbine.

[Fast Enuff]

Got a 66 19" round barrel carbine that's been a real adventure trying to get everything timed perfectly.  Trying to learn the ropes of dealing with these guns myself but could use a little brainstorming.  She's at 95% but has one last slight timing problem that's keeping the gun from running flat out.  Here we go:

The parts:
5th gen C&I short stroke.  Whisper spring on the left, Slix on the right.  It has had a whisper on the right also during all this.

The relevant backstory:
As you shut the lever the bolt passes through the carrier, and the carrier then begins to fall, barely missing the rear of the bolt head.  This part is well timed down to the last couple thousandths and there's no extra room as they pass each other.  But...

The problem: 
The timing on all my other rifles allows the carrier to give up and fully fall when the lever is still maybe 1/2" from contacting the stock and being fully closed.  This one is dead nuts on... when the lever touches the stock the carrier completes it's fall.  The problem is that with it set that critically the carrier doesn't always fall down out of the way in time when running the gun fast and you can catch it when opening the bolt.  This is generally when the gun is being run very fast and is not simply the shooter short stroking it.  It can't be left like this as it does need to run very fast.  We've done everything we can think of to the lobe on the lifter arm, replaced it a couple times also hoping that might help, tried a couple types of springs, checked spring tension but It's just not the spring even though everyone wants it to be.  The lifter arm just seems like it needs to get started bringing the carrier down a tiny bit earlier.  But when you do that, it contacts the bolt on the way by.

Solutions?:
So... first tell me if you think I'm on the right track with the problem... If so then which part should I gently clearance:  The carrier, the back of the bolt head, or both?

Or is there another option for this stubborn mule of a rifle?

[Pettifogger]

On 4th and 5th gens everything is moving much quicker in the cycle.   Sounds like the carrier is rubbing the back of the bolt head on its way down.  This is a very common problem.  Did you put in an aluminum carrier that is milled for the 5th gen?  If it is only rubbing a little file some material off the back of the bolt head.  Springs shouldn't make that much difference as the C&I's have positive slam down.  Again, sounds like the bolt binding on the carrier.

[Fox Creek Kid]

...Or is there another option for this stubborn mule of a rifle?

Yes, there is: put it back to stock. 

[Fast Enuff]

Pettifogger,

Thank you for the quick reply.

Sorry forgot to mention that it does have the extra clearance aluminum carrier in it.  For grins tried an extra milled new brass carrier that was lying around and it contacts bad enough the action won't cycle without a little help.  That seemed interesting so I looked at the brass carrier on my main 66 rifle that's been working great with a 5th gen and saw the carrier had been clearanced just enough on the edges to let the bolt go by.  Another mystery explained.

So, that is precisely what's going on with the problem rifle.  If the lifter is caused to move down sooner, it does catch the back of the bolt head.  I thought maybe I had heard something before about needing to take a little off the back of the bolt head but filing on the bolt isn't something that I wanted to rush into without some confirmation first.  Thanks!

Out of curiosity, why take material off the bolt instead of the inside edges of the carrier?

[Coffinmaker]

I ain't Pettifogger, and I didn't stay in a Holliday Inn Express last night, but I do build competition Rifles.  You do have two choices.  One is to remove some metal from the back of the front bolt head lug, the other is to remove a little material from the Carrier Block. 
The Carrier Block is a little more difficult to get at and keep the angles correct for appearance.  The angle doesn't really matter for function.  The back of the bolt is easier to get at once removed from the Rifle.  Either method will work.
Positive Slam Down causes the carrier block to start movement down much sooner than stock interfering with the bolt.
You also want to observe the Carrier Block Rise, with a dummy round in the carrier to insure the the bolt retracts into the receiver so the cartridge rim does NOT contact the cartridge guide lug at the bottom of the bolt.

Coffinmaker