What causes low compression

[sunfish]

You should be at 140+ psi. Like said, your gauge might be wrong.

Could be a bad aftermarket kit? I like OEM parts mo better.

 

[HuskStihl]

Make sure you don't have the choke or decomp engaged. If everything checks out, no seal/boot leaks, measure the squish. If it is more than .030 with the base gasket in place, you can simply remove the gasket and replace the cylinder over a thin film of yamabond 4. If the squish is between .024 and .030 measure it again without the gasket. Of you getter than .018 it good to go. If the squish is less than 0.024 with the gasket, don't bother. As has been said. AM will frequently but not always have looser specs. That being said, if it not looking for pro performance, 120 psi will certainly cut wood.
Good luck

 

[redunshee]

I just installed a new meteor piston and rings on my 028 Super after cleaning up the cylinder. Comp went from 90 to 170 so I'm pleased.

 

[fin460]

Okay i'm wanting to learn all I can about working on saws, as I have no experience, but I'm mechanical and like to learn. On a saw how do you do a vacuum test, when is a test needed, and what tools are needed. Also how do you measure squish?

 

[3000 FPS]

Hi fellas

Replaced the scored piston and cylinder in one of my project saws, a husky 351, and the compression is still low. The saw runs, but was just expecting a bit more.
When I get home I will compression test it and post the results.

So, what things would cause low compression after replacing piston, rings and cylinder.

If it were me I would check the squish first since it is so simple and quick to do.

 

[Matt_M]

Ok guys. Thank you all for your suggestions and replies. It just proves how little I know and how helpful everyone on the forum is. I have never heard if squish until now.
Will be pulling it apart and having a play and measure. Another thing to learn which is great.
I will post the results once I've had a tinker.
My correction too, the saw is a 350 and not a 351 as I previously said.

 

[sunfish]

Ok guys. Thank you all for your suggestions and replies. It just proves how little I know and how helpful everyone on the forum is. I have never heard if squish until now.
Will be pulling it apart and having a play and measure. Another thing to learn which is great.
I will post the results once I've had a tinker.
My correction too, the saw is a 350 and not a 351 as I previously said.

Bummer it's not a 351, which is basically same as a 353. I gave my brother a 351 a while back and it's now his favorite saw.

350 isn't a bad little saw, but it does have a plastic case, where the 351/353 have metal.

 

[Matt_M]

Yep all plastic, but for $20 on eBay I couldn't resist!!

 

[bucknfeller]

Just curious here, did the kit you put in your saw come with a "dished" piston? That's the way the later 350's came from the factory, they run lower compression intentionally. You can replace those 45mm dished 350 pistons with a flat top piston from a 353, and it will instantly boost the compression, and make a better running saw.

 

[sunfish]

Yep all plastic, but for $20 on eBay I couldn't resist!!

Hell yes! A friend just gave me a 350 that only needs carb work. I'll run the crap out of!

:msp_biggrin:

 

[sunfish]

I need better friends......:msp_biggrin:

He just brought it to me, said it started running rough so he bought a new one. Looks like a new chain on it. I'll get it running right and it'll become my 'tractor saw'.. :msp_smile:

 

[Matt_M]

View attachment 295462
So, this looks like the start of scoring doesn't it?
Small fine scratches down the piston.

Might have to zoom up a little.

Saw had only run a few mins as in haven't reset the carby as I don't have a tacho yet. But, in saying that, it looks like there is clearly another reason for the scorched piston as you guys said.

 

[HuskStihl]

View attachment 295462
So, this looks like the start of scoring doesn't it?
Small fine scratches down the piston.

Might have to zoom up a little.

Saw had only run a few mins as in haven't reset the carby as I don't have a tacho yet. But, in saying that, it looks like there is clearly another reason for the scorched piston as you guys said.

I can't tell for sure without more pictures, but if it is what I think it is, you are lean, either from a leak or just the tune. I have a tach, but you don't one to tune that saw. First, go about a full turn to a turn and a half ccw on the high speed jet for safety, then run it wide open... Actually, just search for saw tuning videos, 08F150 has a really good one. If you're not confident the tune is good, or at least not lean, don't run it

 

[Matt_M]

So I bought a leak down tester and have got 40% leakage . It's whistling out the exhaust. Did a search and read that leak down tests aren't really any use of 2 stroke engines ? Doesn't sound right. I did a test on another 2 stroke engine and didn't have any leak - well , less then 10% anyway.

So, what causes leaking through the exhaust ?
This is a new P/C and rings.

It's probably assembly error. ?

Can anyone suggest a good book or thread that has diagnostic info ?

Matt

 

[HuskStihl]

Pressure testing a saw requires you to block off the exhaust port. Most fashion a seal out of metal plate and inner tube rubber screwed into the muffler bolt holes

 

[Matt_M]

So with a two stroke you are basically only testing leak through the rings ?

 

[CTYank]

So with a two stroke you are basically only testing leak through the rings ?

Noooooo, the rings are out of the picture. Intake and Exhaust ports are sealed.

Leakage then can be at crank seals, case gaskets, impulse line.

The rationale is that such leaks can make it difficult to "charge" the crankcase with fuel & air to make it difficult to start, AND lean out the charge in the crankcase, leading to melted aluminum. All very bad.

The rings are a potential victim not perp.

 

[Matt_M]

How come one engine leaked and the other didn't when at TDC? Does this come down to engine design ?

 

[mad_marz]

Be sure your compression gauge is accurate, whenever I suspect my gauge is out of whack, I have a "control saw" (or calibration saw if you will) that I check it with. Its also important your gauge has a valve in its tip, or it will read too low.
Is this an aftermarket kit or OEM? Some aftermarket pistons have an incorrect deck height, so its always a good idea to check the squish before reassembly.

 

[Hulpio]

How come one engine leaked and the other didn't when at TDC? Does this come down to engine design ?

Sounds like you have some automotive tester connected to spark plug place. With 2 strokes air leak TO crankcase causes lean mixture AND that can cause engine destruction. In 2stroke test you look for leaks from main bearing seals, crankcase gaskets etc all surfaces which face crankcase. There for sure is good vids YT if you search chainsaw vacum test and chainsaw pressure test etc.