Is 120PSI vs 100PSI a big deal?

[acme0007]

I have a Stihl 021 that won't run properly, sounds like it's starved for fuel. I try to rev it and it won't climb then trigger the throttle and I can get it to climb but it soon fails and bogs and dies.
Well I rebuilt the carb and put on a new fuel line, same thing ! Well then I checked the compression warm if that matters, bearly 100 more like 97-98 PSI. So I checked my other 021 that fired right up and runs great it reads 120PSi after pullins several times.
My question is not only what do ya think but does that much compression only 20Psi difference matter or is there probably another freaking problem :confused2:
I know it's just a small little baby saw but it's soooo cute sitting up there with the 088 and 660 flexing it's badges.

 

[3000 FPS]

Yes 20 psi is a big deal. But I would take the muffler off and check the screen for carbon. If it is plugged up at all it be causing some of your problems.

 

[Boleclimber]

Generally saws near or below 125PSI have low power and trouble starting.


I am surprised your 120PSI saw starts and runs good.

 

[Duke Thieroff]

Pull the muffler.

It has a scored piston.

Chris

 

[acme0007]

Yes 20 psi is a big deal. But I would take the muffler off and check the screen for carbon. If it is plugged up at all it be causing some of your problems.

It's clean but I did not check out the piston for scoring ?

 

[fearofpavement]

It's clean but I did not check out the piston for scoring ?

Just pull the entire muffler off the cylinder and you will see the side of the piston. It should be smooth and not have a bunch of up and down lines on it. If you have the spark plug out, you can pull the rope slowly and see into the cylinder including the opposite side of the cylinder to check it all out. If it looks scraped up in there, that would explain your low compression. 100psi is not enough.

 

[mowoodchopper]

120 and 100 is a pretty big diff in a saw, 130 psi will run good while 100 psi runs like crap

 

[stihlrookie]

I don't know alot but neither number sounds good.

 

[blsnelling]

Nope, not a big difference. They both need rebuilt

 

[FATGUY]

you guys might be surprised as to how many good running saws only have 120 PSI compression.

 

[cpr]

you guys might be surprised as to how many good running saws only have 120 PSI compression.

Power Products.

 

[nmurph]

The OP doesn't give me the impression that he is pulling until it stops climbing...and yes, I agree with Brad that 125PSI is too low to run well enough; time for a rebuild.

 

[LaurenceCly]

it is plugged up at all it be causing some of your problems

 

[mowoodchopper]

you guys might be surprised as to how many good running saws only have 120 PSI compression.

agree, I have heard many times that a saw with under 140 is worn out, but its just not true, I have tested a few new huskys that were at 130-135. 120 psi is not a deal breaker for sure, from my exp though when you go under 100 psi you have probs, I have had saws start and run that low but they sure didnt have much for power

 

[bcorradi]

If your under 120 psi you'll have starting, power, and idling issues imo. I think a lot of the PSI readings discrepency is what we all use for testing compression and our methods.

 

[russhd1997]

Could be the compression tester itself. It may be giving readings that are lower than what is actually there. If it is off by 20psi the good running saw would be at 140 which is good and the poor running one would be at 120 which is not.

 

[Fifelaker]

Pull it over when cold until it stops climbing. Then post those numbers.

 

[mowoodchopper]

If your under 120 psi you'll have starting, power, and idling issues imo. I think a lot of the PSI readings discrepency is what we all use for testing compression and our methods.

agree with that too, I would be willing to bet if we all went and checked the same saw with our gauge and way of doing things , there would be at least a 20 psi diff in our readings in alot of cases, not all but alot imo

 

[acme0007]

I tested the saw several times and never got the comp above 105. Pulled it a bunch of times trying to increase the reading, it would start low and climb slowly until max out. Readings right there around 100, it's a dial reading so it isn't exact but it is accurate enough. The saw that runs good always maxed out at around 120-125, granted it is used and probably at least ten years old.
The good one starts and idles good, 2-3 pulls and go. The sick one starts hard 5-6 pulls and won't really idle worth a turd.
I will pull the whole muffler and if needed pop the cylinder and let ya know.
BTW thanks for the replies it really is helpful to have a bunch of saw nerds to ask for advice! ...... I kinda like that"SAW NERD", my kids agree.
I do include myself in that description so don't feel bad.:msp_w00t:

 

[3000 FPS]

I tested the saw several times and never got the comp above 105. Pulled it a bunch of times trying to increase the reading, it would start low and climb slowly until max out. Readings right there around 100, it's a dial reading so it isn't exact but it is accurate enough. The saw that runs good always maxed out at around 120-125, granted it is used and probably at least ten years old.
The good one starts and idles good, 2-3 pulls and go. The sick one starts hard 5-6 pulls and won't really idle worth a turd.
I will pull the whole muffler and if needed pop the cylinder and let ya know.
BTW thanks for the replies it really is helpful to have a bunch of saw nerds to ask for advice! ...... I kinda like that"SAW NERD", my kids agree.
I do include myself in that description so don't feel bad.:msp_w00t:

NO were not saw nerds we are saw servicing engineers. :hmm3grin2orange: