Confirming a damaged piston/cylinder?

[zuren]

A few moons ago I acquired a little Homelite LX-30 Bandit that never really ran right, but as long as you had some patience, it would run and cut. My patience has run out, so I started looking into why this thing starts hard and once it is warm, have to really work to keep it running. After running it for a while and it does stall, it is nearly impossible to restart.

I started with fresh fuel, cleaning the carb, new spark plug, and new filter when I first got it. Lately it has given me all sorts of problems starting; 15+ pulls later and some feathering of the choke, it will sputter to life. All the pulling is getting old. I always wondered about the compression but didn't check it until today. The spec I found for the family of Homelite saw to which this belongs is 115-145PSI. When I measured the compression on my saw, I got 90PSI cold, 85PSI once it was hot. So it seems that my compression is low...even below what many consider the minimum needed to even have it start and run at all.

I removed the spark plug and muffler, and did not find anything glaring. I did find a small burnt strip on the cylinder wall opposite from the exhaust port. The ring seemed free to move. I see no scoring or physical damage, but it is hard to see. Is that little burned strip enough evidence to support the low compression?

Thanks!

 

[Pat Burton]

A few moons ago I acquired a little Homelite LX-30 Bandit that never really ran right, but as long as you had some patience, it would run and cut. My patience has run out, so I started looking into why this thing starts hard and once it is warm, have to really work to keep it running. After running it for a while and it does stall, it is nearly impossible to restart.

I started with fresh fuel, cleaning the carb, new spark plug, and new filter when I first got it. Lately it has given me all sorts of problems starting; 15+ pulls later and some feathering of the choke, it will sputter to life. All the pulling is getting old. I always wondered about the compression but didn't check it until today. The spec I found for the family of Homelite saw to which this belongs is 115-145PSI. When I measured the compression on my saw, I got 90PSI cold, 85PSI once it was hot. So it seems that my compression is low...even below what many consider the minimum needed to even have it start and run at all.

I removed the spark plug and muffler, and did not find anything glaring. I did find a small burnt strip on the cylinder wall opposite from the exhaust port. The ring seemed free to move. I see no scoring or physical damage, but it is hard to see. Is that little burned strip enough evidence to support the low compression?

Thanks!

 

[Pat Burton]

Might want to try a wet test. A bit of oil in cylinder then test compression. A significant rise would indicate ring/cylinder wear.

 

[farmguywithasaw]

The rings could be stuck from piston damage. Also check your tester on something you know is good. An air leak seems more likely for your symptoms

 

[Saw Fixr]

One issue that often gets overlooked when talking compression, is your particular altitude. Depending on where you live, compression may be lower and still completely fine. 130 psi @ sea level, can be significantly lower for every 1000’ in altitude.
Case in point....I live @ 9300’ in the Rockies, west of Denver. Pretty much, all of my saws have about 90 psi (on my particular compression gauge), and run strong. There’s just no air up here. 160-180 psi in say, North Carolina , will probably be at least 20-30% less in Golden Co....
Your exhaust port pic looks pretty good. Doesn’t mean there’s something in there that you can’t see though.
Maybe you just need a carb adjustment??

One of these days, I’m going to do a compression test on my best running saw at different altitudes, maybe in 1000’ increments. I have a 5000’ elevation change in 12 miles.
A while back, i think it was Brad Snelling, that posted an altitude compensation factor. Maybe someone still has it?

 

[lefturnfreek]

Possible cold seize. By the color of the piston it's got tons of blow by, pull the cyclinder.

Keep on .... Runnin' Loads !!

 

[zuren]

Might want to try a wet test. A bit of oil in cylinder then test compression. A significant rise would indicate ring/cylinder wear.

The rings could be stuck from piston damage. Also check your tester on something you know is good. An air leak seems more likely for your symptoms

I will give both of these a try this weekend. I'm guessing the procedure is remove the spark plug, pour a bit of 2-stroke oil in, reconnect the compression gauge, and give it a few pulls?

One issue that often gets overlooked when talking compression, is your particular altitude. Depending on where you live, compression may be lower and still completely fine. 130 psi @ sea level, can be significantly lower for every 1000’ in altitude.
Case in point....I live @ 9300’ in the Rockies, west of Denver. Pretty much, all of my saws have about 90 psi (on my particular compression gauge), and run strong. There’s just no air up here. 160-180 psi in say, North Carolina , will probably be at least 20-30% less in Golden Co....
Your exhaust port pic looks pretty good. Doesn’t mean there’s something in there that you can’t see though.
Maybe you just need a carb adjustment??

One of these days, I’m going to do a compression test on my best running saw at different altitudes, maybe in 1000’ increments. I have a 5000’ elevation change in 12 miles.
A while back, i think it was Brad Snelling, that posted an altitude compensation factor. Maybe someone still has it?

Altitude is not an issue for me since I'm at 600-700 ft. above sea level.

Possible cold seize. By the color of the piston it's got tons of blow by, pull the cyclinder.

You are seeing the cylinder wall in my pic. The parts of the piston I can see look very good.

 

[farmguywithasaw]

I will give both of these a try this weekend. I'm guessing the procedure is remove the spark plug, pour a bit of 2-stroke oil in, reconnect the compression gauge, and give it a few pulls?

A table spoon is enuf

 

[Brent Nowell]

One issue that often gets overlooked when talking compression, is your particular altitude. Depending on where you live, compression may be lower and still completely fine. 130 psi @ sea level, can be significantly lower for every 1000’ in altitude.
Case in point....I live @ 9300’ in the Rockies, west of Denver. Pretty much, all of my saws have about 90 psi (on my particular compression gauge), and run strong. There’s just no air up here. 160-180 psi in say, North Carolina , will probably be at least 20-30% less in Golden Co....
Your exhaust port pic looks pretty good. Doesn’t mean there’s something in there that you can’t see though.
Maybe you just need a carb adjustment??

One of these days, I’m going to do a compression test on my best running saw at different altitudes, maybe in 1000’ increments. I have a 5000’ elevation change in 12 miles.
A while back, i think it was Brad Snelling, that posted an altitude compensation factor. Maybe someone still has it?

Now that sir is a very very good point!!!!

Sea level is 14.7psia
10 000 feet above sea level is approx 4psia

Technically I would think you could add 10psi onto your numbers for comparison to a service manual.

 

[lefturnfreek]

....You are seeing the cylinder wall in my pic. The parts of the piston I can see look very good.

I have seen a few that are damaged out of view from the port, always on the clutch side. Pull the pot, what have you got to loose ...

Keep on ... Runnin' Load's !!

 

[Pat Burton]

reconnect

May want to pull over a few times to spread the oil over the rings and cylinder before using tester