how can sitting cause temorary low comp?

I am sure I will be told I am wrong on this but it happens because a piston ring has some "gunk" on it or some fuel leaks down from the carb into the cylinder when the car first sits, either the gunk gets sticky and gooey or the gas turns to varnish (I am sure there are other reasons) and causes the piston rings to get sticky so it no longer seals properly.

When you start driving the car, fresh fuel and fresh oil gets on the rings and breaks it loose and it starts having compression again. I suppose it could also damage cylinder walls and hurt the motor but the theory is that it will loosen up and be fine.

When I purchased my Dino, it had 150-155 lbs. compression in 5 cylinders and 90 lbs. in the number 6 cylinder and smoked and burned oil. I followed several recommendations from here and elsewhere and used some Marvel Mistery Oil both in the fuel and directly injected into the proper carb while running (Of course AFTER changing all fuel hoses, cleaning tank and doing brakes, etc. since the car had sat 12 years) and over time and miles of Italian tune-ups, I have 145 lbs. in number 6 cylinder and it no longer smokes and runs great, so I am living proof that it can happen and can be fixed.

Of course bad compression could be broken rings, damaged cylinders, etc. and in this case I don't know of any way to tell except driving it hard for a while.

Oh yeah, one more item, I suppose some valves could gummed up and not fully seating causing low compression. I would be worried that you might burn a valve before it "cleaned" itself if that is the case since without fully seating it will get hotter. But the driving it hard is fun so its a great way to try and fix it

 

Also if a car sits for a long time the valve seats can get some corrosive build up on them as one cyclinder will be "open" while the others may be closed. When you test the compression, the valve will not seat properly and there's your leak. As you run it more, the corrossion will losen up and the valve will seat better (although it's always going to be a little off).

 

Small flakes of carbon from the top of the piston or the head can get squished onto the valve seat & will reduce compression until they are either burned off, or pounded off by the valve seating.

If a car sits a long time, the carbon will tend to dry out & flake off, ending up lying on the piston until the car is started. Most gets blown out, but it's not uncomon for some to get stuck on a valve sealing area.

On rotary engines, especially on the carb'd ones, so much carbon can come off that it literally would lock up the engine. If you were lucky you could work it loose/dissolve it out, but if not, you were in for an engine rebuild!

 

The moral of this story is DRIVE THEM, don't just let them sit!

 

In the context of the dino discussion in Vintage... the car had only one cylinder that had markedly low compression, the rest were okay. How can the mechanisms proposed in this thread accommodate that observation?

Is it likely that a single chunk of carbon would block just one valve? Would only one ring get stuck?

 

If it sat for a long time and that one valve was open it could cause low comp. on that one cylinder.

 

Don't know but that is the way it was on my Dino, only 1 cylinder was low and I hoped it was just a stuck ring or valve since I have seen it before on other cars. My other cars are old, as in 70 year old toys so I have seen this before. Can't explain it but it happens. I have also seen it where it was broken rings that tore up the cylinder walls....ooops

 

Just hope it's not a bent valve; as original valves were sodium filled which have an unfortunate tendency to bend. Would not want to be trying to fix that by driving the car very long or very hard.