Stihl 021 bog down

[Vesa Julin]

Any ideas where to look for the fault? I can saw a couple of cuts and after the saw warms up it will do this (link to youtube).

I have tried rebuilding the carburetor, checked the fuel line, sealed the intake and tried tuning the carboretor. Same result with or without tank vent. The spark plug is new. I don't have the equipment for a pressure test, only for a vacuum test.

 

[stihltech]

Get an inline ignition tester to verify spark is not shutting down. Really need to do a pressure test.
First guess is that you are running out of fuel due to lack of feed. Open the fuel cap when it starts to happen to see if there is a difference. Just because the vent is removed does not prove the hole is open into the tank. A vacuum test of the tank would verify.
Sound crazy? It has happened more than once.
Take a real good look at the carb again. No corrosion. Verify you have the CORRECT diaphragms and gasket ,and in the correct order. Common issue. And how is the piston? No scoring in the exhaust and look for machine marks ACROSS the piston (those are a good thing). Does it stay up when held by the starter handle? And please tell us if you used aftermarket (Chinesium) parts. I am throwing those fuel filters and lines away right and left to cure running issues, Fuel should come up and out the fuel line with the cap and vent on.
More to come. We are waiting (not so ) patiently.

 

[Vesa Julin]

Thanks. There is no difference without the fuel cap. The vent hole also is open. I used an Oregon rebuild kit. I checked the carburetor again, nothing there. The piston also looks good and the saw has really good compression.

I will first rule out the faults that I have the tools to check. I suspect the crankshaft seals. I'll do a vacuum test on them when I have time again.

 

[Gord404]

spark aresster screen sometime gets so carboned up the will run like crap

 

[lone wolf]

Vac leak or bad carb IDK without being there pressure test it then buy a carb if it passes! Look for ripped intake boot or fuel lines etc.,

 

[ZeroJunk]

We stock at least 50 carburetors for the different versions of that series of saws. I would replace that. Takes maybe 3 minutes.

Difference being I don't have to pay for it, and if that doesn't correct the problem I put the old one back on.

But, That would be my suspect.

Usually if you have an air leak you will have a high idle and have to chase it by increasing the fuel with your idle adjustment needle.

Did you change the fuel filter ??

 

[ZeroJunk]

Also, make sure you don't have any leaks in the line going to the primer bulb or the bulb itself.

It's a smaller diameter line.

 

[SteveSr]

Get an inline ignition tester to verify spark is not shutting down. Really need to do a pressure test.
First guess is that you are running out of fuel due to lack of feed. Open the fuel cap when it starts to happen to see if there is a difference. Just because the vent is removed does not prove the hole is open into the tank. A vacuum test of the tank would verify.
Sound crazy? It has happened more than once.
Take a real good look at the carb again. No corrosion. Verify you have the CORRECT diaphragms and gasket ,and in the correct order. Common issue. And how is the piston? No scoring in the exhaust and look for machine marks ACROSS the piston (those are a good thing). Does it stay up when held by the starter handle? And please tell us if you used aftermarket (Chinesium) parts. I am throwing those fuel filters and lines away right and left to cure running issues, Fuel should come up and out the fuel line with the cap and vent on.
More to come. We are waiting (not so ) patiently.

I vote ignition coil. Electronics like to fail hot. If you have a neon bulb or a small fluorescent you can hold one end near the coil wire and see if the light goes out when the engine stalls out. A neon pocket AC tester will also work. Tape one lead to the spark plug lead and the other can be connected to cylinder ground or just hang on to it. You won't get shocked.

 

[Pioneer]

+1 on the ignition coil. Have an 025 that will go until the coil is heat soaked and then it stops dead, zero spark. Once cool it runs fine again. A bad coil can also mimic a tuning problem and have you chasing your tail trying to adjust it out.

 

[Vesa Julin]

Thank you for the good comments and suggestions. It's nice to notice that we are not alone with our saws and problems.

There might be multiple problems working together. I don't know the history of the saw since I bought it from a garage sale for a few bucks.

I figured out how to make a vacuum and pressure test, and it failed both. It held pressure only about 1 to1.5 bars. I thought that since I'm going to do a small porting on the saw, I might as well disassemble it. The seals were oily, felt hard, and came off the shaft far too easily. Now that I've started doing a full rebuild, I am going to replace all seals and buy a new carburetor if the saw doesn't run after the rebuild. (And test the spark... The tester cost only a few euros.)

 

[DND 9000]

1 to 1.5 bar pressure is much to much, if you put that pressure on the saw. Normally 0,5 bar is used for the pressure test.

 

[ZeroJunk]

7.25 PSI for us Neanderthals

 

[Okie]

How long does the saw have to rest before it will re-start and take throttle?
Does it start good when cold?

Have you checked the spark arrestor?

 

[Vesa Julin]

1 to 1.5 bar pressure is much to much, if you put that pressure on the saw. Normally 0,5 bar is used for the pressure test.

Seems like I got mixed with bars and pounds...

How long does the saw have to rest before it will re-start and take throttle?
Does it start good when cold?

Have you checked the spark arrestor?

It runs only on cold engine. Spark arrestor are not required here and there is none.

 

[SteveSr]

Thank you for the good comments and suggestions. It's nice to notice that we are not alone with our saws and problems.

There might be multiple problems working together. I don't know the history of the saw since I bought it from a garage sale for a few bucks.

I figured out how to make a vacuum and pressure test, and it failed both. It held pressure only about 1 to1.5 bars. I thought that since I'm going to do a small porting on the saw, I might as well disassemble it. The seals were oily, felt hard, and came off the shaft far too easily. Now that I've started doing a full rebuild, I am going to replace all seals and buy a new carburetor if the saw doesn't run after the rebuild. (And test the spark... The tester cost only a few euros.)

These Stihl clam shells are known to leak at the clam shell joint. Be sure to use a sealant that is rated for gap filling like Dirko HT.

 

[Vesa Julin]

These Stihl clam shells are known to leak at the clam shell joint. Be sure to use a sealant that is rated for gap filling like Dirko HT.

This one had some grey sealant on it, so it had been already opened. Sealant was loose, therefore clam was easy to open. The sealant might not even been rated for fuel contact.