A young man named Caden just acquired a 1974 Dirt Squirt today. He did a compression test and is getting readings of like 202 psi!
Not sure what could be calling this. Stock 922501 head, std bore, 2 ring piston, 2 bolt Ace intake manifold. Gets the same reading whether carb is bolted on and throttle is fully open, or if he removes the carb and tests without it!
Any ideas, other than if someone milled the head down? That’s the only thing I can think of. Not a case of having a Super Rat head on an Ace cylinder/piston. Had him check that. Victor
Does it run? check the gage against any air compressor. Is the piston the correct hodaka part? Is the exhaust port pluged? Is it hard to turn over with the kick start? What are your thoughts Victor> ----------- Clarence
Here are the general concepts I would follow to ferret out the problem:
1. Confirm that the compression tester gauge is giving proper readings. The fact that it is giving the same reading over multiple tests is interesting . . . but not dependable. What if the actual compression is 150psi, but the gauge is (inaccurately) presenting that reading as 210psi?
2. The "appearance" of the head being stock may be misleading. It would not be unusual to bump into a "shaved" head. It may require some detailed inquiry about the "squish" dimension of the head to eliminated the possibility of a "shaved" head.
3. Proper head gasket in use. Stock head gasket thickness is on the order of .56mm (.022").
I was shown a pic of the bottom of the head to confirm the part number was a 922501 head & not a 932601 Super Rat head. Is it just me or does it look like the head has file marks on it? Here’s the pic.
It appears that it is not only an all stock configuration, it also has a std bore, 2 ring Ace piston and a 922501 Ace cylinder head with a 2 bolt Ace intake on the cylinder. Stock head gasket as well.
So my guess is someone milled the head down, possibly with a mill bastard file, based on the scratch lines in the photo? What do you think?
Or does it appear that there’s a significant enough carbon build up in the head to reduce the size of the squish area enough to cause the increase in compression? Victor
Well, that certainly isn't the factory finish on the cylinder head sealing face, but it could have been lapped for flatness - and that might not be the issue.
NOTE: The "squish" area is the thin "doughnut ring" of air/fuel volume surrounding the main part of the combustion chamber. Normally, there is a "step" at the outer edge of the "squish band" - commonly .010" or .020" or more. That "thickness" combined with the thickness of the head gasket provides for a ring of compressed fuel/air which is in close proximity to metal (piston on one side and head on the other) which allows for relatively easy escape of compression heat into the metal . . . which helps keep the compressed gases away from detonation temperatures and pressures. If the squish band "thickness" in the head has gone to zero thru head mods - - - that may be a hint compression has been significantly changed by some long lost previous owner.
NOTE: I don't have a loose "92" head to check for squish band at the edge. Sorry.
Victor