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Homelite st275 weedeater low compression


boostedtt91's Avatar
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09-20-15, 09:07 AM   #1  
Homelite st275 weedeater low compression

A friend gave me a homelite st275 weedeater that didn't run. Been working on it, the fuel lines were all ruined and spark plug was bad so got all that fixed and was able to get it to fire for a second with starter fluid. Spark is good, fuel system works good now and still won't start or attempt to. Finally got a compression gauge today and the highest it will go is 50psi which I'm assuming is way low and why it won't start.
Is there anything I can do to help the compression or is there even rebuild kits available for these things still. Idk if you can just replace the rings or what usually is involved in fixing low compression in these type engines.
Any advice would be appreciated.

 
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09-20-15, 11:16 AM   #2  
Normally low compression means the piston is galled which made the ring stick and the cylinder scored. Since that's caused by insufficient lubrication, it also sometimes means the bearings inside are shot. The parts alone will cost more than a new unit. If you take the exhaust off and look in the cylinder you can get a better idea. It could just be a stuck ring.


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09-20-15, 03:19 PM   #3  
Everything looked ok when I took it completely apart. I talked to the owner of a small engine repair shop and he said 50psi should be ok, said 50 to 80psi. Anyway to fix a stuck ring if that's the case.
I have good spark and won't fire with starting fluid or anything so the compression is the only other thing I know of that could be keeping it from starting

 
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09-20-15, 05:18 PM   #4  
Wow, a small engine guy said 50 psi is okay?! That is absurd. Make a mental note not to take your equipment to that place.

90 is pretty much minimum. It can run with compression a little lower but it will be at the end of it's life at that point. If the piston is galled enough to stick the ring, there is no fixing, only replacing. I know this forum is about doing it yourself, but for a trimmer that's worth $40-50 if it was working flawlessly, it sounds like you're partway there in parts and several times the value in labor. They are disposable units. I'd suggest buying a $80 weedeater at Walmart and you'll be way ahead of the game.

Here's how it goes with these disposables: You see broken fuel lines, so you spend half an hour and $5 changing those. Then when you put gas in it and start priming it, the primer bulb breaks. You spend another halft hour going to the shop to get one and back and $5 more bux. Then you get it to sorta prime and maybe start for a moment and fiddle with it for another half hour before you realize the carburetor is just not gonna work, so you go back to the store and get a carb kit and spend another $20 and 2 hours getting it, carb cleaner, taking it apart and cleaning/rebuilding it. Then if you're lucky, it works. If not, you try again and again until you resort to buying a new carb for $30. So you get it fired up and running half decent but by this time you've pulled on the 10 year old pull rope so many times that it breaks, then you have another hour taking the engine off and clutch apart to get to the pull rope. Another trip to the store and $5 and that's fixed. Maybe the bump head and flex cable are okay, maybe not. Assuming they are, you go use the trimmer for 2 months and something else breaks. At this point you already have $15 less than a new trimmer (more than a new one if you count gas in your car going for parts), and several afternoons of frustration invested. If this sounds like I've been there before, I have. Over and over.


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09-20-15, 07:22 PM   #5  
Anyone that tells you 50psi is fine should get in another line of work because they don't know much about small 2-cycle engines. Go to Wally World and get the $59.96 special and you will be ahead by a long shot, then run it with a rich mixture like 32:1 and it will smoke a little but last a long time, a fouled plug is a lot cheaper than a scored cylinder wall. Have a good one. Geo

 
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09-21-15, 03:45 PM   #6  
ok thanks guys. I know 50psi isnt good, he said 50-80psi it should still be able to start. He didn't say it was good per say, just that it should be able to still start.

 
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09-21-15, 04:28 PM   #7  
The kicker with a two stroke is - the threshold is being able to start those things. If it starts you can usually use them for just about anything but winning races.

An interesting note on a two stroke is a lawn mower will often start on lower compression (within limits) than a chainsaw or weed wacker. I assumed that was because of the blade driving it through the cycles.

Of course if the cylinder/piston is scored up it's pretty well dead.

 
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09-24-15, 04:17 PM   #8  
I took it apart to check the cylinder to see if there were any obvious signs of problems and the cylinder walls look perfect, no scoring or marks anywhere. Is there anything else that could possibly cause low compression? I'm assuming these have a head gasket of some sort, do they go bad or could that be a cause?

 
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09-24-15, 04:37 PM   #9  
Were the bolts that hold the cyliner on loose? Is the ring loose on the piston?


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

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