661cm low compression but still runs strong…

[OG-Forestry]

I’m trying to find out if this is normal, or if it’s time for rings, or if I possibly have a air leak.

I have a 661cm that will barely make 100 psi of compression. Yet it pulls the 36” bar no problem and runs great It also seems to be running lean. No scoring on the piston or cylinder that I can see by looking through the exhaust port. Also the inside of the exhaust is whiteish colored and there is no carbon build up in the exhaust port.

I also pulled a compression release plug out of one of my dad’s old “hopped up” (woods ported) saws to make sure that it wasn’t the compression release. I also checked the rear head stud bolts and they were still tight. I will check the front ones tomorrow I didn’t feel like pulling the muffler tonight.

I bought it new in 2020 or 2021 it’s the mtronic version 3 I believe. Out of the box I put a max flo filter, bark box, and i fully gutted the muffler its lives with a 36” bar on it and sometimes a 42”. The saw doesn’t see many real hard days as the majority of its work load is chunking down big trees while pruning, or falling trees around houses.

 

[el33t]

I’m trying to find out if this is normal, or if it’s time for rings, or if I possibly have a air leak.

I have a 661cm that will barely make 100 psi of compression. Yet it pulls the 36” bar no problem and runs great It also seems to be running lean. No scoring on the piston or cylinder that I can see by looking through the exhaust port. Also the inside of the exhaust is whiteish colored and there is no carbon build up in the exhaust port.

I also pulled a compression release plug out of one of my dad’s old “hopped up” (woods ported) saws to make sure that it wasn’t the compression release. I also checked the rear head stud bolts and they were still tight. I will check the front ones tomorrow I didn’t feel like pulling the muffler tonight.

I bought it new in 2020 or 2021 it’s the mtronic version 3 I believe. Out of the box I put a max flo filter, bark box, and i fully gutted the muffler its lives with a 36” bar on it and sometimes a 42”. The saw doesn’t see many real hard days as the majority of its work load is chunking down big trees while pruning, or falling trees around houses.

What tester did you use to measure the compression? Did you compare the measurement result with measurements taken with the same tester for other saws?

 

[OG-Forestry]

I used a brand new compression tester I purchased off of Amazon, and yes I did the worn out 460 had 105 psi, and an all original flathead 066 that runs pretty well for a retired falling saw hit 125 psi.

Also the 661 held 100psi without dropping.

’m in the process of turning this into my backup saw, my 066 is losing the rod bearing.

 

[Woodpecker]

Your compression tester is the problem

 

[Shane¹]

I would agree with @Woodpecker probably compression tester is inaccurate. Does the compression tester have a Schrader valve in the end of it?

 

[OG-Forestry]

I would agree with @Woodpecker probably compression tester is inaccurate. Does the compression tester have a Schrader valve in the end of it?

No it does not…

that would make sense though, because I believe the flat top is woods ported and that could be why it makes higher compression than the 661. Also I tried it several different time even putting some oil on the threads of the compression tester in case it was leaking air or not sealing tight and it repeated the same numbers every time at least it’s consistent I guess.

 

[LRFIX]

Whatever the compression tester tells you, the engine is telling you it's running strong. I'd be more concerned with letting it run lean for too long, if it is.

 

[Stump Shot]

Your compression tester is the problem

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This

Automotive testers with their adapters add too much volume to an engine small as a chainsaw, that the outcome is greatly affected, unlike the large car engines it was designed for. Which is why all the major makers offer a tester specifically for smaller two stroke engines. One of these should be acquired if an actual psi reading is to be noted.

Whatever the compression tester tells you, the engine is telling you it's running strong. I'd be more concerned with letting it run lean for too long, if it is.

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and this
A pressure/vac test is much more important to a two strokes health, than compression readings. I generally only check for compression when trying to diagnose a failure and pulling the engine over with the starter "feels" light to me. Sometimes a quick look at the piston through the exhaust port, which is super easy on a Stihl, can show damage that no further testing be necessary, as you can see the problem.

 

[Gavinmalloy18]

It’s a little pricey but the echo brand compression tester is a good choice.

 

[Gord]

took apart one last week that had been streight gassed- piston had a few marks on it but not bad but the crank was black and big end bearing was half siezed on the rod and reay to loose all its rollers- strange, usually the cylinder takes the beating on a streight gassed saw

 

[OG-Forestry]

I would agree with @Woodpecker probably compression tester is inaccurate. Does the compression tester have a Schrader valve in the end of it?

Whatever the compression tester tells you, the engine is telling you it's running strong. I'd be more concerned with letting it run lean for too long, if it is.

Gotcha and I am kind of concerned with that although I wasn’t sure if that was typical of the Tronics this is the only Tronics saw I’ve ever had or ran.

/\
This

Automotive testers with their adapters add too much volume to an engine small as a chainsaw, that the outcome is greatly affected, unlike the large car engines it was designed for. Which is why all the major makers offer a tester specifically for smaller two stroke engines. One of these should be acquired if an actual psi reading is to be noted.


/\
and this
A pressure/vac test is much more important to a two strokes health, than compression readings. I generally only check for compression when trying to diagnose a failure and pulling the engine over with the starter "feels" light to me. Sometimes a quick look at the piston through the exhaust port, which is super easy on a Stihl, can show damage that no further testing be necessary, as you can see the problem.

thank you for that explanation that makes a lot of sense as to the volume issue. I will have to look into that. I mainly bought a compression tester because there’s a ton of old running falling saws laying around and I was going to compression test them to see which ones were in the best shape.

I will have to look into the pressure/vacuum testing I’ve rebuilt two strokes, but I’ve never really dove deeper into the theory of how they work until recently. So I do appreciate this information.

 

[OG-Forestry]

took apart one last week that had been streight gassed- piston had a few marks on it but not bad but the crank was black and big end bearing was half siezed on the rod and reay to loose all its rollers- strange, usually the cylinder takes the beating on a streight gassed saw

Ouch

 

[OG-Forestry]

I would agree with @Woodpecker probably compression tester is inaccurate. Does the compression tester have a Schrader valve in the end of it?

Whatever the compression tester tells you, the engine is telling you it's running strong. I'd be more concerned with letting it run lean for too long, if it is.

Gotcha and after taking a second look, I don’t think it’s running lean like I thought I think I may have freaked myself out