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What is the compression on a new two cycle string trimmer? What is the lowest compression where it will still run? The compression on mine is 45 psi, trying to determine how much life is left in the old gal

Can vary depending on model and brand. 90-120 would be in normal range for most units.

WOW Looks like toast to me. New Trimmer time.

OR Partstree.com for a new top end, bout 1/2 the cost of new trimmer, (excluding yer labor.

fritz1255 said:

What is the compression on a new two cycle string trimmer? What is the lowest compression where it will still run? The compression on mine is 45 psi, trying to determine how much life is left in the old gal

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What kind of trimmer is it?

Use 100 psi as a bare minimum here as I have only seen one unit in 12 yrs that even try to run below 100 psi. New compression varies by manufacture and the equipment but 120-150 psi is quite normal. But if the unit squish have been modified can be over 200 psi at times. Now the Stihl 4 mix system has minimum compression of 87 psi but it is actually a 4 cycle trimmer.

Modern 2 cycles may only have short blocks available which can cost well over 50% of a new unit especially the cheaper ones. And this is if you do your own work. Here I just kill off most units that cost under $150 because by the time I charge for a PNC and labor it is already 3/4 of that if not more. An example is my Poulan 4218 chainsaw. It cost new $139 plus tax and the parts alone ran $98.

Getting an accurate secondary pressure reading an small 2 strokes can be very problematic
Compression testers with flexiable connections can expand to a volume substantially bigger than the volume of the combustion chamber
Some times it takes 100 pulls till a compression tester stops going up and that is how you have to do it , just keep on pulling with the trigger wired flat chat till that tester stopd moving
Then post the number .
Using a tester according the instructions ( 6-10 pulls ) will never give an accurate reading on anything under 100cc

fritz1255 said:

What is the compression on a new two cycle string trimmer? What is the lowest compression where it will still run? The compression on mine is 45 psi, trying to determine how much life is left in the old gal

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If it still works keep using it until it dies.

bertsmobile1 said:

Getting an accurate secondary pressure reading an small 2 strokes can be very problematic
Compression testers with flexiable connections can expand to a volume substantially bigger than the volume of the combustion chamber
Some times it takes 100 pulls till a compression tester stops going up and that is how you have to do it , just keep on pulling with the trigger wired flat chat till that tester stopd moving
Then post the number .
Using a tester according the instructions ( 6-10 pulls ) will never give an accurate reading on anything under 100cc

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Mostly Bull....

It all depends if you are using the right compression tester or not. It is primarily the placement of the schrader valve and it has to be a schrader valve with the correct spring rate. Those from tire valve stems usually have too heavy of spring rate. If it is in the wrong location you will never get a correct reading. Many automobile compression testers has the schrader at the gauge, plus the schrader spring is too high for small engines. The schrader valve must be at the end of the hose at the cylinder end; otherwise, the hose must be filled each time before it can even try to open the schrader valve..

I use an Actron compression gauge and usually get a correct reading within three to five pulls even on the 21 cc engines and it has about a nine inch flexible hose. You never will find me pulling 100x.

My string trimmer is a Toro Model 51934. I also have a two cycle mower of 1980's vintage that also has compression of 45 psi and it runs just fine. Why the difference? I can't rule out the possibility that my gauge is bad, I suppose.

Yes compression your gauge is not design for this smaller engine. And if you are only getting 45psi on the mower it is definitely not reading right as even 4 cycle walk behind mowers with compression release needs to read at least 65-70 psi. Two cycle equipment must operate at a higher compression in order to ignite the fuel mix as it has higher ignition point than straight gasoline.

Just trash or recycle that unit as parts are no longer available for that 25cc unit. You couldn't repair it if the PNC is bad. Most of the other parts are also NLA. Now the carburetor may still be rebuilt but a new one out of the question.

What to do is try a little fuel added to the cylinder and see if the trimmer engine will try to run. If it does then it is carburetor problems; otherwise, it is compression or ignition timing which would unrepairable due the lack of replacement parts.

I think the new husqvarna 322l are not even making 90psi. Probably keep the compression low to reduce wear would be my guess. I asked the tech at local dealer and they had no idea what psi should be.

StarTech said:

Yes compression your gauge is not design for this smaller engine.

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Right, isn't there a purpose built compression tester for small 2 smoke engines?

YEs
However they are expensive because they are genuine mechanics tools
So you won't find them where cheapskates shop like ebay, amazon, Harbour Freight, Walmart or even Lowes.
hey will come as a set with a lot of different spark plug adapters and in particular different length ones because if the adapter protrudes 2 turns into a combustion chamber it can increase the effective compression ratio if you think about it it is the same as shaving a head to bump up the compression ratio .
The same goes for adapters that are too short so add volume to the combustion chamber .
Don't know where mine came from as I have had it for 50 years & I got it second hand from a 2 stoke race bike tuner .But Gunsons comes to mind .

I have the same Toro but the blower version. I am not a fan of the 25cc engine at all. It has high compression and it is just hard to start in general. I have been futzing with mine for about a month already trying to repair it and it seems like it is ready to just give up the ghost. Hopefully I can get by with it until I can save up for an Echo blower.