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OS GGF-120SII Conversion

Aug. 10, 2015

This OS FS-120SII engine started as a CDI conversion some time ago. The old fuel regulator was not working and parts are not avialable. It was waiting for me to make the parts to mount a Walbro carb to run gasoline and spark ignition. OS released the G5 glow plug in the mean time and I decided it was time to finish this project and give the plug a try in this engine.

A spacer was required for the manifold to make room for the much larger carburetor. This part was machined on an Emco PC Mill 125. This is the first part I made on this machine and was glad to see it go well, other than the feed/speed error on the drill.

This is the manifold adapter for the Walbro carb. It was turned manually, then the flange was cut in the small CNC mill.
The manifold spacer and carb adapter on the engine.
The new backplate was first turned manually.
Then, the back was CNC milled.
A carb mount of Nylon plastic was milled again on the CNC, parted off in the lathe, then milled flat.

Two holes were drilled manually on the small flange to thread into the new backplate.

Everything bolted together. The carb position was almost perfect. The manifold spacer was shortened by 0.5mm to make it right.

Another view.

The engines starts easily and runs well with the plug lit. It misses and drops a lot of RPM when plug heat is off. I think increased compression ratio will improve running.

Prior to increasing compression ratio, I machined the glow plug seat. This allows the plug to protrude deeper into the combustion chamber. The plug depth now matches FS-120SIII. Test running showed improvement at full throttle, but not enough improvment at idle to be reliable. The next step will be more compression.

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