Author Topic: Replaced piston/cylinder now won't fire  (Read 1098 times)

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Offline 3000 FPS

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Re: Replaced piston/cylinder now won't fire
« Reply #40 on: December 02, 2013, 11:21:55 pm »
Checked compression with a automotive tester. Could only get it to 95 lbs after lots of pulls. What does that mean? Sounds low to me. Why would that be with a new piston/head. I scored it slightly installing initially but would that do it? Can hardly see it.

I can tell you this if you get a good compression reading and you only have 95 psi then that saw is not going to start.  It is not enough compression.   

Make sure the de-compression valve is out before trying the compression test.
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Offline farmboy

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Re: Replaced piston/cylinder now won't fire
« Reply #41 on: December 03, 2013, 08:46:06 am »
Decomp in got me the other day.  Bumfuzzeled for a min.  Talked ugly to myself for a few.
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Offline wbhburn

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Re: Replaced piston/cylinder now won't fire
« Reply #42 on: December 07, 2013, 01:59:20 pm »
A bumfuzzled, ugly self-talker; must have been a sight to behold!

Offline wbhburn

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Re: Replaced piston/cylinder now won't fire
« Reply #43 on: December 07, 2013, 02:22:11 pm »
So, 95lbs won't get it huh? Pretty sure I had decomp out but I'll do the test again (8 mile trip to town to borrow from Autozone) just to be certain. But let's assume 95 is the correct reading. What then? How can that be with a new piston/cylinder?

Offline wbhburn

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Re: Replaced piston/cylinder now won't fire
« Reply #44 on: December 07, 2013, 04:03:32 pm »
Did second C test with another tester and still got just under 95. Can anyone tell me how this can be?

Offline 3000 FPS

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Re: Replaced piston/cylinder now won't fire
« Reply #45 on: December 07, 2013, 09:46:40 pm »
If it were me I would check the squish on the saw at this point to make sure what you installed are the correct parts.   

You remove the spark plug, then take a piece of solder like you would use for soldering electrical things, and then put the solder through the spark plug hole and bend it so that it touches the cylinder wall.   You want to have the solder facing out toward the sides of the saw and not from front to back.   Then grab the flywheel with your hand and crank it over until is smashes the solder.  Then back it up to smash it again.   
Remove the solder and then measure the very end of it to see how much room there is between the cylinder top and the piston top.   It could be let say some where around .050"s to .035"s.   If there is an excessive amount it will make the compression low and could be the wrong piston.   But you have to measure it to see.
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Offline wbhburn

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Re: Replaced piston/cylinder now won't fire
« Reply #46 on: December 10, 2013, 08:56:59 pm »
Squish test....Check. It came out right at .05 but I'm sure it was less than that since I couldn't get it to go past TDC.

Was the compression check valid with fairly dry cylinder walls?

Offline jmester

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Re: Replaced piston/cylinder now won't fire
« Reply #47 on: December 10, 2013, 09:50:17 pm »
You did put oil on the piston and cylinder before you, reassembled the engine? Use a smaller size solder like for electrical work . Plumming solder is to thick. The engine needs to go full circle to get a correct reading of the squish.
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Offline 3000 FPS

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Re: Replaced piston/cylinder now won't fire
« Reply #48 on: December 10, 2013, 10:01:39 pm »
Also if the squish does check ok, then you are going to have to pull the cylinder off so that you can make a visual inspection of the piston and cylinder.
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Offline Adirondackstihl

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Re: Replaced piston/cylinder now won't fire
« Reply #49 on: December 11, 2013, 11:44:35 am »
How can that be with a new piston/cylinder?

New rings?
Scored cylinder?
Faulty decomp? ( that's where my $ is at )

 

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