Head Swapped Suzuki FA50

Alexander Gasiorowski /
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Hi All,

First time poster, long time lurker!

This past summer I picked up a non-running 1984 Suzuki FA50 from a friend and nursed it back to health, installing a 60cc big bore kit from DOS cycles as well as a MLM Sidebleed pipe from treatland. With these mods and some knobby dirt bike tires the bike topped out 34mph on flat ground. Satisfactory! But I can never leave well enough alone. The cold winter months leave my mind brooding and I began hunting for more performance mods.

To preface, I'm just a hobbyist messing around with his first two stroke engine, I'm not well learned in the ways of two strokes, but I'm keen to understand through advice, reading, and experimentation.

The original cylinder head has a low compression domed combustion chamber that is 41mm in diameter at the base to match up with the original 41mm bore. This results in a 2mm step protruding into the chamber when a 45mm piston is installed. Even so, the piston is recessed enough in the bore such that the piston dome doesn't interfere with the head. I don't have qualitative data to prove that this 2mm ledge on the head has any quantitative impact on performance, but my hunch was that it probably interfered with flame propagation, as it left a shadowed ring of air/fuel around the perimeter of the combustion chamber.

With this in mind, I began looking for a cylinder head that would fit the stock cylinder and would be better suited to the 45mm bore. I found that in a cylinder head from a early 1970's Suzuki A80. The A80 has a 47mm bore and, to my layman's eyes, a nicer combustion chamber shape. The bolt holes line up perfectly with the original head, but I had to cut the top fins to allow it to clear the frame.

Now that I had a suitable head in hand, I took the cylinder jug and the head to the machine shop. Milling 0.020" from the head resulted in a perfect 45mm base diameter and a cylinder head dome that almost perfectly matches the 45mm piston dome. I CC'd the head before and after. The original combustion chamber volume was 9cc. After milling 0.020", it came out to around 7.5cc. To bring the piston's dome all the way out of the bore, I also had the cylinder decked 0.065".

Today I put it all together and fired it up for the first time! There's clearly a ton more compression. I estimate the static compression to be around 12:1. It runs great and there is no mechanical interference. I filled the tank with some 110 octane leaded fuel, but it knocked on initial startup as it burned through the 87 still in the lines. A road test is pending; there's still a foot of snow on the ground!

Let me know if you have any questions, comments, or tips! I'm keen to hear any thoughts about my hunch on the head ledge in the combustion chamber.

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