50+ PSI loss from tight valves?

I went and saw a Seca 750 for sale yesterday. It was very nice, with only 11K on it! However, a compression test only gave 100 psi for each cylinder. Oil added to one cylinder did not do much in changing the value, so I thus assume it's the lack of the valve shims being adjusted. However, I'm just surprised that tight valves would result in a 50+ psi decrease across the board.

Does anyone have direct experience in seeing that significant of a drop due to tight valves?

 

Sure, in extreme cases a valve (or valves) never close, always remaining open just a bit....you can end up with zero compression.

Also, think about what happens if YOU were a valve.......you get pushed around by the cam bucket/shim assembly, which gets pushed around by the CAM LOBE. The lobe is tapered so that it starts pushing at you at a certain point in the cam (engine) rotational cycle, and doesn't STOP pushing on you until much later in the rotation, at which time DUE TO SHIM CLEARANCE, you get a little break before the whole cycle starts anew.

If there is little to no clearance, then the cam lobe STARTS its pushing onto you sooner, and ends its pushing LATER in each rotational cycle.....you (the valve) spend more time being lifted off your seat, due to the lack of clearance. Not only do you remain open longer during the compression stroke, you also don't get as much "face time"(ha-ha, a pun.....) with your valve seat, and that face time is when you get to cool down, and the lack of it really burns you up.......literally. You don't get to release your heat to the valve seat during that fully-closed time, and it's the equivalent of being cast into that ever-burning lake of fire and brimstone......

Not only that, but remember that there is a moment when both valves are open (called overlap" in camshaft-speak), and if the intake valve is OPENING SOONER, and the exhaust valve is CLOSING LATER (both due to being too tight, i.e. not enough clearance), then this overlap time (measured in degrees of cam rotation) is going to increase = lost compression.

If that's not enough, the timing of valve opening and closure is what is known as cam timing, and tight valves will ADVANCE the cam timing (when the valve opening first begins in the rotational cycle), and this effects engine power in very real ways. While slightly advanced cam timing can increase engine performance, too much (or, even worse, a variable amount for each cylinder, based on differing clearances and thus different cam advance profiles for each cylinder!) makes a real mess of things. It's certainly not what the design engineers had in mind when they were tuning the engine for certain output at certain rpms......i.e. throttle responsiveness.

And finally, these engines are what are known as "interference" engines, meaning that within the cylinder, there are certain points in time when the piston and the valves occupy the same physical X, Y, Z coordinates within the cylinder (the piston at and around the TDC, the valve at and around full opening). At higher rpm's there is always going to be a little bit of valve "float", where the valve movement "lags" behind the cam lobe profile, in essence "floating" above the cam lobe profile for an instant or two, until the valve springs can overcome the momentum of the valve being rocketed off the top of the cam lobe profile. Although not an issue during the compression stroke, during the exhaust stroke if the valve is held open too long (lack of any shim clearance, or a "negative clearance" situation), the piston and valve may "interfere" with each other in decidedly unfriendly ways.

I keep referring to the LACK of shim clearance as being the major issue of concern, since that is the most common scenario......as time goes by, valve clearances are REDUCED due to the valve pounding on its seat in the head (due to spring closing pressure banging the valve closed), thus forcing the seat further up into the head, thus bringing the valve stem/bucket/shim CLOSER to the cam lobe. This is the reason that periodic shim adjustment are needed, and, they are most needed on newer engines, as that is when the most valve seat recession occurs (first 10K miles or so). After that point, most of the valve seat compression is done, and valve clearances become more stable for longer amounts of time.

Of course, the opposite effects occur if shim-to-cam lobe clearances are too LARGE, the valve never opens fully, and it remains open for less time (the cam timing is retarded rather than advanced); you'll build full compression pressure, but won't be able to take advantage of it (not as much fuel entering, not enough time for exhaust gases to be pushed out). You won't be risking burning valves or having piston/valve interference issues, though.

Wow, just writing this out puts me into a cold sweat. I'm going to go check my clearances right now.

 

I'm going to go hug my feeler gauges.

 

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