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Must the ignition switch be off when compression testing?

Started by zoltar, November 19, 2017, 04:56:35 AM

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zoltar

I have a Stihl MS-261 C-M on which I want to do a compression test (for fun/science, as mentioned in another thread ;)). One question first though: I have heard that the ignition should be off to avoid electrical damage. Without a spark plug connected when you pull the starter, it is said that the voltage could go higher than normal, and do some damage. The MS-261 C-M has more electronics than your average saw, so I thought it would be a good idea to check with you guys first.

The problem is that I can't easily lock it in the ignition off position, since the button is designed to pop right back to start after you push it to stop. Has Stihl designed this saw so that it regulates the voltage to safe levels even without the spark plug?

sablatnic

Put a plug on the wire and lay it to ground on the cylinder, and you are safe.

Don P

Having not too long ago put a new coil pack in a Honda car from a non grounded compression test, yup ground or disable the ignition when compression testing.

Gearbox

Just hold it in your hand and pull the rope with the other . That way you check the spark and compression at the same time . {Caution this is a joke }
A bunch of chainsaws a BT6870 processer , TC 5 International track skidder and not near enough time

sawguy21

old age and treachery will always overcome youth and enthusiasm

Ada Shaker

Quote from: zoltar on November 19, 2017, 04:56:35 AM
I have a Stihl MS-261 C-M on which I want to do a compression test (for fun/science, as mentioned in another thread ;)). One question first though: I have heard that the ignition should be off to avoid electrical damage. Without a spark plug connected when you pull the starter, it is said that the voltage could go higher than normal, and do some damage. The MS-261 C-M has more electronics than your average saw, so I thought it would be a good idea to check with you guys first.

The problem is that I can't easily lock it in the ignition off position, since the button is designed to pop right back to start after you push it to stop. Has Stihl designed this saw so that it regulates the voltage to safe levels even without the spark plug?

Stihl won't divulge any information due to intellectual property rights. When you think how a spark plug works it provides a spark through thermionic emission, the dielectric being the air gap the spark plug has. Grounding the plug lead directly to ground removes the thermionic emission circuit out of the equation, will it cause damage?, who knows?. Might be best to connect the spark pug into circuit and ground with a separate wire to ground on the engine somewhere.
If it hangs to the left, your likely to be a Husqvarna man.
If it hangs to the right, your likely to be a Stihl man.
Anything else is an uncomfortable compromise.
                             AND
Walking with one foot on either side of a barbed wire fence can become extremely uncomfortable at times.

zoltar

I think my compression tester is broken. It's brand new, but it doesn't hold pressure. After pulling the starter, I see the needle moving, but it drops to zero again in less than a second. No way to build up pressure. I'm sure I tightened it up good (but not so tight that it would damage the O-ring), so something is definitely wrong with it. (And yeah, I haven't opened the decompression valve on the saw, so that's not the reason why I couldn't build pressure ;)).

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