stihl chainsaw

[miller time]

Hello, I am new to the site. I have been working on all kinds of small engine equipment for several years now and am usually able to figure out most problems.
I am working on a project now that has got me stumped. I am working on a Stihl 021 chainsaw for a friend. When he brought it to me it would start and run but would quickly overheat. At that time I checked compression (100psi) and it had good spark. I knew that 100 psi was on the low end so I checked the cylinder walls which were ok.
We decided to do a rebuild, my first try at this. I replaced the following parts: piston rings, oil seals, intake and exhaust gaskets, fuel line, and impulse line.
Now the saw will not start, not even fire And the compression is no better. Also I noticed there is no fuel being drawn from the fuel line to the carb when you disconnect it.
Anybody have any ideas?
Thanks

 

[ILENGINE]

Did you change the fuel filter. Wouldn't be the first stihl fuel filter I have seen clog up.

 

[bertsmobile1]

Very important if you intend staying in the small engine business.
Forget about absolute compression.
Almost impossible to get proper readings unless you have really good kit with multiple ends.
I thought I was having my leg pulled but you can get readings +/- 30% on small engines by using different heads.
Wet & dry compressions and leak down tests are far more important than absolute compression.

Next for the blue smokes you have to do crank case pressures and vacuums . It is vital or you will end up doing a lot of work for no purpose.
Found this out the hard way as well, cost me a bomb and made me look stupid to my customers, not good for business.
Which is the situation you are now in.

Next, check your fuel flow by removing the carb and blowing air through the carb throat.
I use an old vacuum cleaner that has a slot at the back so you can get it to blow.
With air blowing over the venturi you should be able to see & feel the fuel vaporising and being blown out the engine side of the carb.

Running hot is 99% of the time running lean.
Running lean
Blocked fuel filter in tank
blocked fuel screen in carb
cracked fuel line
badly adjusted carb
loose carb
BAD CRANKCASE SEALS

The other 1% is flywheel key sheared which is cast in so often overlooked.

 

[miller time]

To ILEngine, yes I did change the fuel filter.
To bertsmobile 1, do you think I should not have done the rebuild based on low compression? What should I do now?
To both, shouldn't it still at least fire when I put fuel directly into the carb throat? If not, why?

 

[bertsmobile1]

To ILEngine, yes I did change the fuel filter.
To bertsmobile 1, do you think I should not have done the rebuild based on low compression? What should I do now?
To both, shouldn't it still at least fire when I put fuel directly into the carb throat? If not, why?

Nope,
You have to attain a combustable air fuel ratio in there 11:1 to 14:1 ( actually me thinks unleaded is a bit different but near enough )
Then you need enough compression and a spark at the right time.
From the carb throat it goes into the crankcase and gets primary compression, IF THE SEALS ARE GOOD.
If not the air blows out the side and you get almost nothing in the pot and so nothing goes bang.
Then there is the cylinder / crank case seal. The screws get loose & you bleed off the new charge around the transfer ports before finally some thing makes it's way into the cylinder
.
To determine the quality of the rings you need to do a wet / dry compression test and a leak down test.
Then and only if it fails both you go to rings.But the next step is off with the muffler and eyeball the bore.
Any scratches and forget it.

Then you need to be really careful as chrome bores do not get honed but iron bores must get honed.

Problem is your the end of your compression tester must be flush with the inside of your head.
Because both the swept volume and the final squish are so small, you only need a 1cc or 2cc space in the plug hole to give you a reading that is too low and a lot of compression testers can add up to 5cc to the head space so you get readings 1/2 of what the book says with a perfectly good set of rings.

I thought it was all crap till a mate who does very high performance tuning set me strait and showed me the difference you can get on a cylinder of 500cc.
His gear was over a grand, I thought paying $ 50 for mine was over the top.
Bert had an even cruder test.
He held the saw by the pull start handle & gave it a jerk.
If the rope pulled out , new rings if it did not rings were fine.

 

[miller time]

Wow, that is a lot of great info. Thank you. I replaced the oil seals and put a fresh bead of gasket maker on the mating surface before I reinstalled it. What else can I do to make sure the seals are good?

 

[ILENGINE]

Double check the key in the flywheel may have sheared if not positioned correctly when the flywheel was reinstalled.