Stihl 021 Craps Out On Idle

[hotajax]

Starts up OK, runs like a crazed woman at high speed, but when I put her down to go do something else, it dies immediately when it's hot. Is this fuel starvation or rich fuel.?

Which screws are which for high speed / low speed?
Thanks gents.

 

[066 redeye]

Should be marked.
Usually L closest to cylinder.

 

[EFSM]

Starts up OK, runs like a crazed woman at high speed, but when I put her down to go do something else, it dies immediately when it's hot. Is this fuel starvation or rich fuel?

I would be concerned about an air leak. Sucking maverick air will make it run like crazy wide open but has a drastic affect on idle mixture, especially when hot. A carburetor adjustment problem doesn't change the running behavior as much from cold to hot.

 

[hacskaroly]

it dies immediately when it's hot

Like EFSM stated, possible air leak, when it is warm/hot and at idle, it might be pulling more air past say the oil seals and then skewing up the air/fuel mixture with too much air causing it to die. You can check for air leaks at the intake (between the carb and the cylinder) and at the oil seals. Depending on usage and age, its possible you have an oil seal that is hardening up or warping enough to let extra air in when it is hot.

But to answer your question, my guess is neither fuel starvation or running rich, but flooding with air (from the wrong spot).

 

[Wood Doctor]

You may also be running low on compression. That 021 Stihl saw could be over 25 years old, made from 1990 to 2002.

 

[hotajax]

Like EFSM stated, possible air leak, when it is warm/hot and at idle, it might be pulling more air past say the oil seals and then skewing up the air/fuel mixture with too much air causing it to die. You can check for air leaks at the intake (between the carb and the cylinder) and at the oil seals. Depending on usage and age, its possible you have an oil seal that is hardening up or warping enough to let extra air in when it is hot.

But to answer your question, my guess is neither fuel starvation or running rich, but flooding with air (from the wrong spot).

How do you check for air leaks? Seems like a good idea to give it a shot. Thanks As an aside, this saw ( given to me ) has become a real PITA. I spend more time on this constipated piece of crap than I do on all my other pieces of equipment combined - 3 more saws, 2 weed whackers, a pushmower, and a hedge clipper. Most of them are Echoes, and they love to run, not sit on the shelf like a privileged garage queen. So if you live near SE Pennsylvania, and want to come get it, let me know.

 

[hacskaroly]

How do you check for air leaks? Seems like a good idea to give it a shot. Thanks

Best bet is to use non-flammable brake cleaner. While the saw is running, you can spray it around the intake boot or around the flywheel side and clutch side oil seals. If there is a leak, it will suck in the break cleaner and the saw will stop running (does not harm the saw). Other people will use ether or flammable break cleaner and that will cause the saw to race, but at the risk of damaging it (engine races with less oil mix and potential to score the piston/cylinder).

If you use the non-flammable spray and say on the clutch side and the saw shuts down, then you will need to replace that side oil seal. Since you are replacing that one, you might as well replace the other side too, usually the two oil seals come together in a pack.

 

[hotajax]

Best bet is to use non-flammable brake cleaner. While the saw is running, you can spray it around the intake boot or around the flywheel side and clutch side oil seals. If there is a leak, it will suck in the break cleaner and the saw will stop running (does not harm the saw). Other people will use ether or flammable break cleaner and that will cause the saw to race, but at the risk of damaging it (engine races with less oil mix and potential to score the piston/cylinder).

If you use the non-flammable spray and say on the clutch side and the saw shuts down, then you will need to replace that side oil seal. Since you are replacing that one, you might as well replace the other side too, usually the two oil seals come together in a pack.

Except now, it won't run at all

 

[hacskaroly]

Except now, it won't run at all

No, it will run after that, the cleaner will cycle through and the fire/air/lightening mixture will be good enough to start it again.

 

[hacskaroly]

I suppose you could use WD-40, it will cause the engine to surge but will at least have lube with it. I am not a fan of causing those engines to surge more than they should that is why I use the non-flammable break cleaner.

 

[hotajax]

So continuing the story, I got the saw to start after about 30 pulls. The more I pulled on it while it was cold, the more it burped, until it finally stayed running for a few minutes. That eliminates lack of spark as the problem. Soon as it got hot, it died. I do have a brand new carb that I can put in, probably in less time than taking it to someone who knows more than me. Any ideas, you have me undivided attention.

 

[EFSM]

Are you sure the compression is good? What you're describing sounds like low crankcase compression, due either to bad rings or a worn piston skirt on the intake side.

 

[hacskaroly]

So continuing the story, I got the saw to start after about 30 pulls. The more I pulled on it while it was cold, the more it burped, until it finally stayed running for a few minutes. That eliminates lack of spark as the problem. Soon as it got hot, it died.

Sounds a lot like when a saw if flooded, pull about 25-30 times to thin down the fuel mixture in the cylinder until it burps a few times, belches smoke and then starts, though with it dying when it gets hot it sounds like another problem altogether...much like what EFSM stated.

 

[Outback]

When it gets hot and dies, pull the plug. If its wet, its flooding. If its dry, ground the plug to the cylinder and see if you have spark with your eye's. If you have spark, its lean, not enough fuel. If you don't see spark then you have a bad coil that quits when it gets hot..... If its lean there are several reasons why that could be.