Stihl 029 Compression?

90 is lower than I see on my saws, but I wouldn't think compression running 10% lower than ideal would be a "no start" issue. Since it starts fine cold, I'd be checking the following:

1. Bad tank breather. Open and re-close gas cap, to normalize tank pressure, then try to start.
2. Check summer/winter switch. Leaving in winter mode can cause gas to boil in carb.
3. Check gas. Using "winter gas" can boil in carb in summer, causing same problem.
4. Check carb mounting and gasket (although you'd probably notice run problems if it were this).

Some experts will tell you that 90 psi compression reading is right at the border of pass-fail. This presumes that the reading was taken accurately- it's easy to mess up things and get erroneously low reading. With luck a new ring would get it done, but good luck getting to it on those stihls.
According to some experts, best compression reading for a small 2-stroke is gotten with a cold engine- it drops for hot engine.
Besides the obvious piston function compressing/expanding gases, it has to function as valves. Lack of ring sealing interferes with that.

I'm skeptical that compression is your problem. How confident are you in your gauge? Most Stihls I've worked on are typically in the 150-180# range, the only exceptions being fried motors or one really worn-out commercially abused 044. Compression readings are prone to vary widely from gauge to gauge, so I'd reserve judgement at this point.

If you lift the saw off the ground by the starter handle, how long does it take to drop to the end of its rope? Is there some reason compression should be so low on these saws? Have you pulled the muffler and checked for a scored piston? Do they have cheap aftermarket cylinders that might've suffered plating failures? Have they been subject to extremely heavy use for many years?

NCFord said:

Even if it's been run for 5-10 mins, and stops it cannot be restart.

Click to expand...

Is this true even if you try to restart it immediately, within a few seconds of shutting it off, or only if it sits for a few minutes?

When trying to restart hot and you've been pulling and pulling the rope with no joy, sniff at the exhaust outlet. Do you smell raw gasoline? If you smell gas then either it's flooding or you have an ignition problem. I doubt it's the tank vent or any other problem (filter, carb, impulse, tank vent, etc.) that would starve the motor for gas, because those would tend to cause problems while running, not just during a hot restart.