The TS400 is an excellent saw.
My favorite to run.
The Cylinder design reminds me of an 036 cylinder, but 49mm.
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The stop switch problem is very common on that saw, and is usually either because the wire is broken off at the connector contact on the bulkhead mounted switch, the short yellow jumper wire has lost it's ground, or the stop switch actuator rod is bent and is not pushing the contact spring at the switch.
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There are two different kill wires, to correspond to either the early 3 bolt coil, or the later 2 bolt coil. The wire for the 2 bolt coil from Stihl is supplied as an ignition conversion kit for about $11 - $15
and includes a new spark plug wire, and kill wire in a sheathed covering to keep it from rubbing a bare spot in the kill wire , and a new air guide with wire retainers.
If you don't want to buy the kit, and you are going to solder it on, make sure you triple cover
it, and the lower wire section that is close to the metal crankcase with heat shrink.
They rub through quickly If allowed to vibrate.
Then here comes a gremlin...
The 3 bolt coils are notorious for the kill function failing at the terminal.
This failure can be tested by using a self powered and lighted continuity tester.
If you ground one end to the iron frame of the coil, then touch the other end to the kill switch leg on the coil, it should light up, but about 50% dimmer than when you touch the two ends of the tester together. If it doesn't light up at all, then the kill function is bad. Viola.
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On the rewind spring;
make sure the pull rope is #4.5 or #5, and no more than 40 or 42" long.
Unload the spring tension and unclip the rope rotor pulley.
Then after you've pulled the rope rotor pulley out, pry out the spring carrier carefully, and check the condition of the spring. If someone put a rope in that was too long it will break the spring.
If concrete is built up on the rewind spring, from somebody oiling it, ( this makes dust stick to it), you should unwind it and use a knife, or paper towel and clean it off. Then use something like brake cleaner to end up with a dry spring.
* Skip the next mundane paragraph if you already know how to wind a starter correctly. LOL....
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Then wind it back up, reinstall it, then install the rope on the rope rotor pulley.
Then drop it into place while lightly pushing down and rotating counterclockwise until it drops in and engages the spring. Then install the washer, pawl and clip.
Next, pull the rope all the way inward through the eyelet. The rope is already tied to the rope rotor pulley, so rotate the rotor counterclockwise until the rope slot it is at it's closest point by the eyelet. Then engage the rope in the slot, pull it upward, and wind it 6 times, or until the spring binds.
Then carefully rotate the rotor pulley clockwise until the slot is back at the eyelet. Then hold the rope rotor and pull the handle to feed the rope back out though the eyelet.
The rope should now retract correctly. Give the rope some super hard yanks all the way to its end to see if the knot slips. The rope cannot stick up from it's slot in the rope rotor pulley, or it will interfere with the pawl clip, causing the pawl not to fly out when the rope is pulled.
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A pressure & vacuum test is jumping the gun, IMO.
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I recommend you pull the fan cover, and starter off.
See if there is any up/ down play in the crankshaft (this is aka radial play).
There should be zero. Period.
If there is, then crank bearings are needed. I can offer advice if needed after you check them... This is already a Novel.
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To check the piston, remove the starter, then back the 3/4" white plastic nut of the belt tensioner (on the cut arm) counterclockwise 90 degrees until the cam holds it there. Next, remove the three 3/4" belt cover bolts. From there you can remove the guard assembly, then the muffler.
*Note* If the T27 screws are rusted and you strip them, you'll need to use a cobalt 3/8" drill bit to drill the head completely off the screw. WD40 helps. You'll be left with a long enough stub to use vice grips to unscrew it easily.
Another thing notorious to cause problems on this saw is a metal baffle flap that vibrates and breaks off inside the muffler after a few thousand hours.
It can get back inside the exhaust port if the saw gets rolled around when transporting. Which is bad.
If you look in the muffler inlet and you can see if the flap is still on yours.
I recommend bending it back and forth to force it to break off and fish it out as a preventative measure.
The exhaust spark arrestor needs cleaned every 100 or so hours. It pulls out after you remove the clip, then break it loose from the muffler rust with pliers by rotating it. They also keep sand, debris, and mud daubers out. They are a good thing on these saws.
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Inspect the piston next. Grab the starter hub and rotate the crank until the rings are visible.
Then, very slightly wiggle the crank back and forth.
If there is a noticeable play in the ring groove up and down, or the machining manufacturing texture marks are completely gone, then at least a new piston kit is needed.
If you pull the cylinder off and there is chrome loss under the intake port, above the exhaust, or at the very top of the cylinder.
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The crank lower rod can be a bit loose side to side, but zero up/down movement.
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They quit making that saw about 2006, so those carbs had that old welch plug glue that lifts, and starts floating around causing problems. Usually ends up under the needle lever.
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The fuel hoses were updated to a two piece hose and an elbow connector. if yours has a ribbed one I'd replace it.
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Next the 180° bend "U" shaped hose left of the fuel hose should be pulled and the metal grub screw cleaned out so it leaks slowly for the tank vent. They get plugged up over time.
If this hose is loose, fuel will seap out. I use a viton o-ring next to the shoulder of the hose that is outside of the tank. This will tighten it up. A trick I use on the hard to install vent hose is to squirt a bit of laquer thinner on it just prior to installing. It makes rubber slick for a few seconds until it evaporates. This works for intake boots, and AV buffer plugs too!
Also put an O-ring on the tank vent check valve to the right of the fuel hose if it is loose.
The tank vent should hold 4psi, then bypass if it's working correctly.
I think that about covers the most common issues.
Hope this helps.
By the way, the Osha stickers are free from your local Stihl dealer.
Alex
I'm gonna just quickly mention @Dan Forsh since he's got a few TS400s in his signature, and if he's still about he might be able to give me a couple of tips.
