Mastermind
Work Saw Specialist
This thread is the most interesting thing I've seen on this site in awhile.
Thanks Chris.
Thanks Chris.
Reducing Squish on a Poulan Clamshell
- Thread starter Chris-PA
- Start date
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Flattop Burns
ArboristSite Lurker
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It's true. Me and a first cousin are building his 4218 to run better he got for $119 with a case and sharpening kit. Somehow I got interested in winding up the little creature a bit being as it was a tough saw to begin. All we did so far was put two holes in the deflector that were turboconed from the inside and put spacers on the deflector retainer stud to space it a bit farther than it was with just the (now deleted) screen alone. It was an improvement but just the first step.
Mastermind
Work Saw Specialist
These saws were the first ones that I started doing "strange" things on. External transfers, carb swaps, etc.....
If you kill one you've not broke the bank........you can find another and keep playing.
If you kill one you've not broke the bank........you can find another and keep playing.
drf255
BAD CAD
Really great job Chris.
I'm interested in the compression increase as well.
I learned the hard way on my MS250 project that a little dome goes a long way on these guys. You remember the thread.
I'm interested in the compression increase as well.
I learned the hard way on my MS250 project that a little dome goes a long way on these guys. You remember the thread.
Duke Thieroff
Fill your hands, you SOB!
Instead of cutting the jug, why not the engine pan?
Chris-PA
Where the Wild Things Are
I suppose you could grind out the bearing pockets in the cylinder, and cut the edge of the pan down, but then the split between the cylinder & pan would not be in the center any more, and the bearing hole would not be completely round. The distortion would be small though, so it could probably be filled with sealant. I'll have to think about how to fixture the pan so it could be cut down.
Chris-PA
Where the Wild Things Are
After hauling firewood and other chores getting ready for the snow, I got to spend a bit of time in the barn on this project. It was about 22deg. I got the bearing pockets ground out - it took a little while to take off 0.040", but it wasn't hard. I had to go slow and keep checking. Here is what I used:
I used the gauge I made, along with a measurement from that raised ridge that holds the bearings in. I also used the bearings as the seam between the cap and the cylinder is impressed into the rubber. Here's how the gauge fit:
And here's the finished cylinder:
I ended up with 0.021" squish, which is close enough given all the things I had to cut.
Timing with the new piston is (duration/deg from TDC):
E = 150/105
I = 158/79
T = 106/127
Which gives a BD of 22 and a case compression angle of 48. I'm not going to raise the transfers at this point as I don't want to risk damaging the cylinder, and I'm not convinced it needs it.
I think I'd like a little lower case compression angle though, so I'm considering trimming 0.050" off the piston skirt to get an intake duration of 166, and a case compression of 44.
I hope to get the engine together tomorrow.
I used the gauge I made, along with a measurement from that raised ridge that holds the bearings in. I also used the bearings as the seam between the cap and the cylinder is impressed into the rubber. Here's how the gauge fit:
And here's the finished cylinder:
I ended up with 0.021" squish, which is close enough given all the things I had to cut.
Timing with the new piston is (duration/deg from TDC):
E = 150/105
I = 158/79
T = 106/127
Which gives a BD of 22 and a case compression angle of 48. I'm not going to raise the transfers at this point as I don't want to risk damaging the cylinder, and I'm not convinced it needs it.
I think I'd like a little lower case compression angle though, so I'm considering trimming 0.050" off the piston skirt to get an intake duration of 166, and a case compression of 44.
I hope to get the engine together tomorrow.
Moparmyway
Its just a saw
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Great idea on trying it first, before the transferrs get any treatment from the grinder (or cutoff wheel) !
Looking forward to seeing this run !!
Looking forward to seeing this run !!
Chris-PA
Where the Wild Things Are
From what I have read an outlet angle of 90deg is desirable (although I have not thoroughly investigated why). Surely the air in the back corner of the upper transfer will be dead air, which effectively makes it curved anyway. A more sophisticated casting would have a transfer runner that loops further back from the bore and comes in from a larger radius, but this is an inexpensive design.Chris looking at the end of the transfers the 90* shelf at the top of the transfer. Could you gain any air speed/flow by inserting a peice to smooth out that bend? Its essentially a dead corner
Chris-PA
Where the Wild Things Are
It runs! After I got done moving snow I cut the lower edge of the piston up by 0.050" to increase the intake duration, and put it back together. It fit properly in spite of all the cutting:
I had to sand down the lower edge of the intake/air box seal because that part of the cylinder is now lower, and I could not get the bolts started. I little more work with the sanding drum:
Here is the finished port timing:
I didn't have much to cut that was out of the snow other than a small old chunk of pine - I cut that but of course it didn't tell me much! There are a lot of variables that have changed, including the port timing and the new ring which will need to seat. Subjectively it feels pretty snappy and seems to want to rev, but I'll have to reserve judgment until I can get it in some wood. Hopefully I can post s video soon.
I had to sand down the lower edge of the intake/air box seal because that part of the cylinder is now lower, and I could not get the bolts started. I little more work with the sanding drum:
Here is the finished port timing:
I didn't have much to cut that was out of the snow other than a small old chunk of pine - I cut that but of course it didn't tell me much! There are a lot of variables that have changed, including the port timing and the new ring which will need to seat. Subjectively it feels pretty snappy and seems to want to rev, but I'll have to reserve judgment until I can get it in some wood. Hopefully I can post s video soon.
After all the 3d printing and high tech equipment you have access to you wound up using a dremel tool?
Chris-PA
Where the Wild Things Are
The 3D printer is about the only "high tech" equipment I do have access to. Other than that it an old worn out lathe driven by a leather belt, some drill presses and belt sanders. The CNC stuff is all set up for production work and I don't know how to use it anyway (although I have been considering learning to program one). And a Dremel.
macgyver would be proud of chris
Chris-PA
Where the Wild Things Are
Lol, thanks. In fact the 3D printed stuff was just playing around. This could be done with a flat file and a dremel, and the old bearing as a reference. Or at least if you just take off the difference to get the right squish, rather than doing the pop up for which you'd need a lathe.
I think I need to send Chris a non-running Poulan 42/18 in a carton. It has a new piston and rings, has hardly been run, and still will not start. Maybe he can save it or tell me how I can save it.
They are easy to work on i brought back to life my Poulan Pro 260
OK, I'm all ears. After installing a new piston and rings (saw looked like it was straight-gassed), it still seems to have insufficient compression to start, even with a good spark and carb settings as recommended. Saw has very few hours on it and looks brand new. My Stihl dealer told me to stop wasting my time on junk. What's next for me to try?
Chris-PA
Where the Wild Things Are
These saws are so straightforward you should be able to get it running. Check the flywheel for a sheared key/tab, and fuel line which often fail.
Also use the purge bulb to diagnose the lines/carb - it should pull fuel with very few bubbles. If you get air then it is either the lines or a leaking check valve in the carb.
Fuel line routing can get mixed up too - the small fuel line should go to the carb nipple near the end with the cover with the single center screw. The line from the nipple nearest the mixture screws goes to the purge bulb inlet, and the purge bulb outlet goes back to the tank, ususlly through a larger line.
OK, Chris, I'll double check all of that. But, I think that's what I already did in November. Compression still seems nil compared to any Stihl that I own, and that's usually the bottom line. I was very careful when rebuilding the top end and saw no difference in rope pull resistance after completing it. Not much is more depressing than that. I'll report back what I find and show you a Pic of the piston before I started.
