Tech Question

[Poley4]

This might be a stupid question and I think I might already know the answer, but I'm looking for a little conformation. When a saw has low compression due to a worn or damaged cylinder, piston or rings, it will usually be hard to start, then why when you depress the compression release on a saw in good condition, it does not seem to effect it's ability to start easily? I'm thinking it has something to do with the ability of a worn out engine to produce the proper pressure / vacuum in the crankcase to transfer fuel and air mixture to the combustion chamber at cranking speed. How far off am I?

 

[Lakeside53]

Good question... here's my take.

Decomp valve do make it harder to start a saw - or at least - very easy to flood them... you see this on smaller saws. Even the 361 can be a problem starter with the decomp.Larger saws are less affected by the decomp and can generally start and run at lower compression than smaller saws.

Saws start easier with higher than decomp compression mainly because of the heating of the mixture by the compression.

 

[Uwharrie]

Decompression Valves when put in a saw properly will make it easier to start. Bob McCulloch invented the system while building an electric start chainsaw. The correct way is to put the hole down in the cylinder and not on the top where full compression is. This allows the release to relieve some of the compression but leaving room for the piston to still build up compression for good engine starting. This is not magic, but some manufacturers over the years just didn't get it right.
If you make the hole too large you can relieve too much thereby making them harder to start. I like the Dolmar PS-401 but I think it relieves too much and therfore you have to pull the rope harder to start than is really necessary. The new PS-420 doesn't have a deco but it's easier to start. As Lake said some saws are easier to flood etc.
Another note about re-ringing a chainsaw is that most users only tear a saw down when they need to, this said , I would never just change rings but replace piston and rings. The piston works like a valve and it wears. If you are racing and you tear your saw down weekly I would say yes replace the rings.