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Post by chesterwin on Mar 28, 2009 10:30:54 GMT -5
I've installed a new (aftermarket) piston and rings in my sick SXLAO. Didn't help the compression issue at all. Where do I go next? Crank seals? If so, should I replace bearings or other internals? Although my first time, piston replacement was a piece of cake. How much more difficult will the crank seals be?
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Post by MCS on Mar 28, 2009 10:34:54 GMT -5
Shotgun:
Crankshaft seals will not help top end compression. Have you taken a compression reading with a gauge or are you just going by feel?
I just notice that you have three threads going for your SXL. Pick one and continue with that one so we can follow the progress.
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Post by chesterwin on Mar 28, 2009 13:53:24 GMT -5
Thanks guys. I'm going by feel on the compression. The new piston seems to be making less compression than the gouged up one I pulled out. I'll stick to this thread and when I get it started I'll let you know if the compression improves.
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Post by MCS on Mar 28, 2009 15:05:29 GMT -5
OldZip Wrote: I'd like to hear the answer too  Shotgun:(chesterwin) Did you check ring end gap and compare the after market piston to the one you took out for deck height??? I'm really suprised you found and aftermarket piston so fill us in on the details. With the muffler off, turn the engine until the piston is at the bottom of the stroke. The top of the piston should be even with the bottom edge of the exhaust port. |
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Post by chesterwin on Mar 28, 2009 16:00:55 GMT -5
Has anyone torn a rotor cup pulling a chainsaw too hard? Just kidding. I've got fire and fuel but no combustion. I pulled the muffler and yes the piston is about even with the edge of the exhaust port on the bottom of the stroke. I'm getting real flustered.
The piston is made in Taiwan. When doing a google search I found a guy on Arboristsite that said he had some made a while back but was out of them at the time. He goes by treeman60 if memory serves. I did another google search a little over a week ago and he's selling them in his ebay store for 30 bucks including the rings. The box says they're a motorcycle use piston. Probably for another product the manufacturer puts out. Walt Miller is the guys name if memory serves. I've got the shipping package at work with all that info. if anyone wants it.
Any ideas on getting this saw started?
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Post by chesterwin on Mar 28, 2009 16:08:31 GMT -5
OldZip, I couldn't locate a HL piston but I did find rings in Missouri and got 6 since I have 3 of the sxlao saws plus a 4th that I've been using as a "donor".
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Post by MCS on Mar 28, 2009 17:32:08 GMT -5
Is the plug wet? You say you have fuel. Is it getting to the cylinder? Have you squirted gas into the spark plug hole or through the exhaust port just to see if it sputters?
If you feel the compression is low you are going to have to pull the jug and investigate that piston and make sure it fits the cylinder. The Super XL is and "inch" size piston but just about all motorcycles with that small of a piston would be metric. We talked about this on the other thread, is the wrist pin a closed end pin?
You will have to measure ring end gap with feeler gauges and maybe even have the cylinder and piston measured with a caliper. The piston should be snug in the cylinder - no rings installed - and only have a couple thousands skirt clearance.
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Post by OBR on Mar 28, 2009 17:52:48 GMT -5
Crap...I posted in the wrong thread...go find it if u wanna read it...hehe...but like i said in the other one how did that piston go in...really nice and snug? I say this because being it is aftermarket sometimes idiots think they know wut they are selling but really you get the wrong part...so theres a chance you have a piston from the smaller bored xl-12...probably not probable...but possible, right?
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Post by chesterwin on Mar 28, 2009 18:57:01 GMT -5
Haven't tried mixed fuel but did try starting fluid. First through the plug hole and when checking the piston position on the exhaust port, thorugh it too. I cleaned the cylinder with carb. cleaner. I was afraid to use the acid with no experience. Before assembly, I tried the piston in the cylinder and it was a "slick snug fit". And, yes the plug is wet. I've checked the spark several times as well as resetting the carb. adjustments. 1 1/4 low and 1 high.  ?? The piston fit seems to be great, I don't get it. |
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Post by MCS on Mar 28, 2009 20:17:59 GMT -5
The compression side of these 2 cycle engines is quite simple. It is the cylinder, the piston, and rings. The air that is compressed can enter from either the crankcase or through the exhaust port. Of course it won't run if the air is coming in from the exhaust but there will still be compression. At this point, I think the best thing to do is get a gage reading of the compression and not rely on feelings ;D Even with low compression, the saw should sputter and run briefly if the cylinder is primed with gas mix. How are you testing the spark? If you are doing it with the plug connected to the high tension lead and grounding the base, you should get spark with just a gentle pull of the rope. If you are feel like you are going to dislocate your shoulder pulling to get spark, then there is an ignition problem. You did remember to install the flywheel key, Right? So, by having a true compression value you can decide where to direct your efforts. The saw should run with 50# of compression. After all, saws with compression release start with almost no compression  |
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Post by chesterwin on Mar 28, 2009 20:28:26 GMT -5
Thanks. There is spark with a gentle pull. I yanked out of frustation. I'll see about getting a compression gauge. Should it be read from the first pull? I've seen post from folks quoting the 3rd pull result.
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Post by chesterwin on Mar 28, 2009 20:30:46 GMT -5
Also, I looked at the old piston and it is identical in ring pin setup as the replacement.
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Post by MCS on Mar 28, 2009 20:39:07 GMT -5
It will take several pulls to see the full compression. The screw in type work best on the saws. After you've cranked it about 3 times you should have a good reading. Make sure the gage doesn't leak too. Once you stop cranking the pressure should hold until you release it.
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Post by redsqwrl on Mar 28, 2009 20:53:20 GMT -5
Doesn't the downward travel of the piston PRessurize the trapped fuel charge between the Reed valve and the (for a short time) blocked ports.
Once the the piston clears the ports (exhaust first then intake) isn't this how the fuel is forced into the cyl.
If reeds or crank seal are bad the Compression would suffer.....
correct?
sqwrl
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Post by MCS on Mar 28, 2009 21:11:56 GMT -5
The crankcase isn't a super charger. Yes, it moves the air fuel to the top of the piston, but remember, on these 2 cycles, the intake ports and exhaust port are both open at the bottom of the stroke. The incoming mixture actually play a part in moving the burned gas out the exhaust port. As the piston moves up and the ports close, the only thing that gets compressed is what gets trapped above the ports.
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Post by chesterwin on Mar 28, 2009 21:27:35 GMT -5
I pulled the carb. again to check the reed valves. All seems good there.
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Post by lesorubcheek on Mar 28, 2009 21:55:54 GMT -5
A little late jumping in here, but as everybody has already said, if you're compression is low, its the rings not sealing properly in the cylinder. Were there any noticable scratches in the upper portion of the cylinder above the ports? This is the area where all the measurable compression is being produced.
If comp is low with a new piston and rings, my first guess is the aftermarket rings don't have the proper end gap, or possibly they're too sloppy in the ring grooves. In one book for XL/Super XLs, it shows reccomended end gap at 0.070 - 0.080 with a max of 0.085. These number appear large, but what it means is, there should be a very small clearance with the alignment pins once the rings are in position, If you decide to tear back into it, which maybe you won't, I'd ensure the end gap appears within spec in the upper portion of the cylinder, and maybe measure the diameter of the alignment pins compared with the stock piston.... maybe they are a bit smaller too.
Dan
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Post by chesterwin on Mar 28, 2009 23:10:11 GMT -5
I guess I'll have to dig into my reloading gear to find my caliper. No prob. pulling her back apart to make sure it's right.
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Post by MCS on Mar 29, 2009 8:42:57 GMT -5
Keep us posted. I just have this feeling that it isn't a compression problem unless there is something drastically wrong with that aftermarket piston. A new piston and rings - seated or not - will have sufficient compression to run briefly when primed directly through the spark plug hole if there is a properly timed spark. Did the saw run before you tore it apart?
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Post by chesterwin on Mar 29, 2009 9:51:36 GMT -5
A7, I think you hit the nail on the head with the timing suggestion. I was in such a hurry to get the old girl up and running that after getting everything bolted up I noticed the sprocket rim was pretty worn and decided to replace it. Instead of sticking a screwdriver through the fins of the flywheel like I've always done before, I stuck a piece of rope through the plug hole to stop the piston travel. I thought the socket wrench was slipping internally but it must have been the arm slipping on the shaft!!! This would throw the timing off, right?
1. How do I correct the timing?
2. If the arm is loose enough to slip like that will it affect performance once the timing is corrected?
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?? The piston fit seems to be great, I don't get it.