Av8tor1977 thats pretty much it in a nutshell. Thats pretty muck what i figured on doing only i was just gona cut it down in a lathe. Any idea where to get the magnet and the shape, size and type?


I got a chance to do some testing today. I think my results are pretty interesting.

Test #1 stock engine no performance mods. Using old 1 ring piston and newer engine with the twin intake runners per side. This should give a good baseline for a starting
reference point.

Recoded RPM 6300

Test #2 Again stock engine and no perfomance mods. Using two ring piston and newer engine.

Recorded RPM 6330 For a net gain of 30 RPM I actually expected it to be a little more than this.

Test #3 Stock engine no performance mods. 2 ring piston and the older and looks to be higher compression combustion chamber design. If you look at the old crank case and
look at the new one the new crankcase is just a modified version of the old crankcase ports. Therfore the ports should match the ports on the old cylinder just fine. Again the purpose of this test is to see if the higher compression help.

Recorded RPM 6570 For a net gain of 240 RPM!!!!!!!! At this point I was not convinced this rpm gain was due to the higher compression and could have been a result of better ports.

Test #4 Takeing the best combination of test 1-3 cylinder and piston combination. Removing the cylinder base gasket and try to increase the compression ratio some more. With the combination of piston and cylinder I have, when the gasket is removed it causes a piston to cylinder contact. I measured the height of the piston in relation to the top of the spark plug hole. If you take a dial caliper and extend it you can use it for a depth gauge at the end of it. If you hold it on the spark plug hole and rotate the crank when the piston comes to the top you will have your reading. Ok so my reading with the gasket was .610 Without the gasket it was .585 and agian there was contact. Ok so i measured the gasket and figured it moved the cylinder down .030 this means that i had a .005 interfereance fit. So i cut a gasket out of carboard stock that measured .010 . When I installed the cylinder with the gasket and remeasured it gave me a reading of .590 so I now have moved the cylinder down .020 and should have .005 clearance between the piston and the flange of the combustion chamber.

Recorded RPM 6,660 For a net gain of 90 RPM now I am convinced the rpm gain of test three is from the higher compression as is this RPM gain is also.

Test #5 This test takes the best results of test 1-4 and uses the crank from the old engine to see if there is a gain as it looks like this crank has more advance in it. Out of pure curiosity i took the ignition coil from the old engine and used it with the new flywheel. It would hardly run in this combination. Likewise on this test i use the old flywheel. In this configuration it runs great with the old coil but like crap with the new coil. On inspection of all these parts there are two manufatures of thses parts. The newer coil and flywheel is made by walbro. the older coil and flywheel is made by phelan. For some reason they are a matched set and cannot be inerchanged in any combination from new to old. The only way to change the flywheel is to change out cranks. Anyhow so the theory is that the older set up has an advanced timming. When the crank was changed out the seal was left on the outside nezt to the flywheel. I will try and put it on the outside of the inner bearing on test # 9 as this should reduce the crankcase volume.

Recorded RPM 6,670 For a net gain of 10 rpm. At this point i am not sure there is any advance in this setup from the other set up. there isnt enough rpm gain to
conclude this. Since there is a difference in the interchange of these part i now feel there is some timming in the coil assemblies themselves. I am not sure how this is accomplised but i am sure it is something electronic. I would like to check the timming but i need to make a degree wheel and fixture for a dial indicator to find TDC. Once this is located i can then make a TDC timming mark on the flywheel and somewhere on the case for a reference. using my timminig light i can tune the light till the timming lines up at TDC. then all i have to do is look at where the knob on the timming light is and I will have my advance number. I will have to check this when I get time. If I can get an electronic ignition all this info wont even matter but it is still worth doing as i hope guys using the stock ignigtion can gain from this information.

Next I will move on to the larger carburetor. I need to do some modifications to the carb so it will fit this application. The carburetor I purchased is a walbro #387. It has a 12.8 mm venturi. The modification i need to make is: Change the throttle shaft as the old shaft has a better arm to it. The fuel pump cover needs to be changed also as it has the correct idle adjustment for this arm. other than that the only other thing that needs to be done is to remove the throttle return spring. I will proceded with this when i get more time.

All in all i have so far achieved a Gross gain in rpm of 370 RPM