Chris-PA
Where the Wild Things Are
I really didn't mean to port it, it just kinda happened. And it's only a little bit modified! I had it on the bench waiting for the new intake boot ( http://www.arboristsite.com/community/threads/poulan-pro-pp338pt-check-your-boots.234456/ ), and I decided to pull the muffler and check out the cylinder. There were several stripes on the cylinder wall that looked worrisome, and I checked on line to find that a new cylinder cost $17 - so I pulled off the old one to see how bad it was. It turned out to be pretty much nothing, only one minor one could be felt at all, and that was down by the transfers.
And since everything was covered with snow this morning, I did measure the timing before I took it apart.
Duration/angle from TDC:
E = 142/109
I = 145/72.5
I(Strato) = 152/76
T = 107/126.5
BD = 17.5
Case Compression = 54
The exhaust port width was only 43% of the bore - really tiny!
So what to do with it? It has always run quite well and I didn't want to do anything radical to it, so I decided to leave the timing alone, even with the large case compression angle. So I widened the exhaust port to 50% of the bore.
However, I noticed something strange on the strato ports. While the duration of the strato inlets was 152deg when the piston notch opens to the ports on the intake face, the transfers were still mostly blocked by the plastic inserts they use to create closed transfers. These don't fit that tight to the piston (the ring does not hit them), but that still forms a narrow slot.
Here is the intake configuration:
The piston isn't visible in this shot, but before the lower edge of the piston opens into the case on the lower intake, the slots in the piston open into those upper ports. The slots are almost straight on the upper edge:
There's a good 2.5mm until the piston slot clears the lower edge of the plastic insert and it can flow into the transfer runner. Here are the inserts:
They got smoothed a bit, and the upper edge got cut back to allow the strato ports to flow:
Here is a shot of the modified insert installed - it is harder to see than I hoped, but the lower edge of the transfer outlet is now even with the strato air inlet:
This shot shows the inserts and the transfer runner that is formed between them and the casting:
And since everything was covered with snow this morning, I did measure the timing before I took it apart.
Duration/angle from TDC:
E = 142/109
I = 145/72.5
I(Strato) = 152/76
T = 107/126.5
BD = 17.5
Case Compression = 54
The exhaust port width was only 43% of the bore - really tiny!
So what to do with it? It has always run quite well and I didn't want to do anything radical to it, so I decided to leave the timing alone, even with the large case compression angle. So I widened the exhaust port to 50% of the bore.
However, I noticed something strange on the strato ports. While the duration of the strato inlets was 152deg when the piston notch opens to the ports on the intake face, the transfers were still mostly blocked by the plastic inserts they use to create closed transfers. These don't fit that tight to the piston (the ring does not hit them), but that still forms a narrow slot.
Here is the intake configuration:
The piston isn't visible in this shot, but before the lower edge of the piston opens into the case on the lower intake, the slots in the piston open into those upper ports. The slots are almost straight on the upper edge:
There's a good 2.5mm until the piston slot clears the lower edge of the plastic insert and it can flow into the transfer runner. Here are the inserts:
They got smoothed a bit, and the upper edge got cut back to allow the strato ports to flow:
Here is a shot of the modified insert installed - it is harder to see than I hoped, but the lower edge of the transfer outlet is now even with the strato air inlet:
This shot shows the inserts and the transfer runner that is formed between them and the casting:
