Chris-PA
Where the Wild Things Are
First some background: I've had fun porting my plastic Poulan clamshells. Some have worked out great and I use them regularly, some have been dogs and/or failed, and I learned a few things. But I wanted a strato engined saw, and I figured what saws were what and got a few broken RedMax GZ4000s, which are the best small saws in my opinion. The very first one is actually a clone made by Jenn Feng in Taiwan, and after some minor repairs and a muffler mod it ran great, but this summer it ate a rod bearing and that was it. I stuck a Zenoah engine from a Ryobi RY10532 in it and it runs great. But I picked up a Jenn Feng Troy-Bilt branded carcass with a good engine, and decided to do that engine, put it back in the McCulloch and then fix the Ryobi.
It turns out that the Jenn Feng saws share almost nothing with the Zenoah saws. You can swap the engines, but all the JF made castings and forgings are actually different. As are all the case parts, air valve, filter and the Walbro carb. A couple of other parts will swap, but are not quite the same. When I looked at the Zenoah engine from the Ryobi all I did to it was to remove the base gasket (squish ended up right at 0.020") and blend the lower transfers to match the case. I did not touch the ports as I did not want to screw up the strato function and I know how well they run already. The timing from the Zenoah engine was:
E = 132
I = 137
I(S) = 165 (Strato air inlet)
T = 103
After removing the base gasket on the Jenn Feng engine I measured:
E = 132
I = 144
I(S) = 155
T = 101
I like the strategy of a short exhaust duration to maintain cylinder pressure, and a long intake duration. With strato the short blowdown should not matter as there should be little fuel in whatever goes out the open exhaust port. But I was disappointed that the intake duration was short and decided to port it. Here is the saw it's going in:
It turns out that the Jenn Feng saws share almost nothing with the Zenoah saws. You can swap the engines, but all the JF made castings and forgings are actually different. As are all the case parts, air valve, filter and the Walbro carb. A couple of other parts will swap, but are not quite the same. When I looked at the Zenoah engine from the Ryobi all I did to it was to remove the base gasket (squish ended up right at 0.020") and blend the lower transfers to match the case. I did not touch the ports as I did not want to screw up the strato function and I know how well they run already. The timing from the Zenoah engine was:
E = 132
I = 137
I(S) = 165 (Strato air inlet)
T = 103
After removing the base gasket on the Jenn Feng engine I measured:
E = 132
I = 144
I(S) = 155
T = 101
I like the strategy of a short exhaust duration to maintain cylinder pressure, and a long intake duration. With strato the short blowdown should not matter as there should be little fuel in whatever goes out the open exhaust port. But I was disappointed that the intake duration was short and decided to port it. Here is the saw it's going in:
