P-V Clearance on Cylinder 4

[renders]

I am in the process of doing the P-V clearance check. I completed Cylinder 1 both Intake and Exhaust. Now I am turning to Cylinder 4 and there I observe the following strange behavior that I don't understand. On compression stroke (when piston is at TDC again after 1 full 360 degree turn) I am unable to open the valve more then 1.25 mm on the Intake valve. I am pretty certain it does not hit the piston because when I advance the cycle and the piston travels down I still can't push the valve open with the rocker screw.
At 0 degree - right before Intake operation on cylinder 4 starts and piston is at TDC - I have the correct rocker clearance. I open up the retaining nut and make the clearance 0 so that the screw touches the valve. Then when I open up the valve in that position I get more then 3 mm clearance. But when I rotate the crank a full cycle my clearance is fairly large and when I try to push the valve down using the rocker screw I hit resistance at 1.25 turns.
Is this even a valid measurement? However when I do the same on cylinder 1 I get the same clearance at 0 and 360 degrees as expected. What is wrong with this picture?

[stownsen914]

Can you tell if it's a hard stop vs. resistance? Maybe you have a sticky valve guide (especially if it's new) or a nick on the valve stem causing the valve to not slide properly in the guide.

Scott

[renders]

The strange thing is that the valve seems to open fine on Intake cycle (0 degree +) and I can push it down at 0 degree 3 mm+ no problem. The issue is only after one crank rotation (360 deg). I tried to apply light force but it would not let me go further and I am hesitant to apply brute force even though the valve should open especially when the piston has moved away sufficiently. Another thing I noticed was that when I back out and then advance the screw to the point where it touches the valve with 0 clearance the position of the screw (identified by how the slot on the screw head aligns) changes in the exact same cycle position. But I noticed this on the other valve as well. I would expect this to be the same all the time if the mechanism is precise? Should I just ignore this and move on?
It bothers me that this only occurs after a full rotation cycle. I hope there is no tension somehow building up somewhere? I think I will go and try to open the Exhaust valve and see what happens.

[Eagledriver]

I think you have a couple of issues going on here. First of all when you are at overlap between the exhaust and intake stroke, the valve should be open slightly. If you are backing off the screw and closing it to zero clearance you are doing it wrong. It should not be at zero clearance it should be open! It should be open by the amount you set it to when you timed the cams. When you are at TDC of the compression stroke then you should have your normal clearance. If you want to go to zero clearance than that is the stroke to do it on.

Now on to why the valve won't open when you turn the screw. The screws on these engines get stretched all the time by people over torquing the lock nut. When that happens you can't screw the adjuster in very far because the threads bind.

-Andy

[Flat6pac]

Do you have the piston in upside down? Experience tells me this is a difficult adjustment....

Bruce

[renders]

I think the issue was a tight valve guide. I forced the screw in and pushed the valve open that way and later around it was easier. The good news is I have sufficient clearance. I also replaced the old screw with a new one and all is working fine now.
On the zero clearance issue I am following Wayne's book and in section Clearance Check the instruction is to reduce the valve adjustment clearance to zero (at TDC). This is what I meant. I think the instructions in the book are clear and hopefully precise
Anyways all seems ok now. It just appears that some valves are a bit more tight then the others.