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Discussion Starter #1
I have a '74 F11-A dual sport. Are there any heads that interchange with this model engine that have a smaller combustion chamber? 6.4:1 compression is just not enough. Amazes me that anything with that ratio can run. With more compression a little porting can be done and yeild some real benefits from longer duratio intake adn exhaust.

Anyhow, will any other heads fit this bike?
 

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cadunkle,
The only parts that come up on my interchange list are the 73, 74 and 75 F11 heads.
 

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Discussion Starter #3
Are all of those heads the same cc combustion chamber? Any other heads physically bolt to the cylinder? I know the F11-a head is cut away on the cooling fins to clear the exhaust on the left side. Not a big deal to either modify exhaust or head for it to fit. Definitely want more compression on this engine when I am done with it. In stock 6.4:1 form is had maybe less power than my XR200 (4 stroke). Totally unacceptable from a two stroke. I'm used to 2 storke 250cc being fast.
 

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Discussion Starter #4
I was looking at F8 heads on eBay and they look very similar to my F11 head. Looks like it would bolt up and I could take measurements to check. Anyone know CC of the F8 combustion chamber vs F11?
 

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Don't think an F8 head will get you anywhere. I'd be surprised if it fits at all. Higher compression alone will probably only acheive detonation. I think porting, bigger carb and a chamber is the way to go for more power.
 

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Discussion Starter #6
Well the idea is to raise compression and port the cylinder and piston for more intake/exhaust duration. Without raising compression though, it'll just be a turd all around. No bottom end since even lower dynamic compression down low, and not enough flow with stock casting cylinder ports to support the higher RPM that more duration will support.

I'm not about to raise the exhaust port or raise the intake skirt on the piston/lower intake port unless I have more compression. When I saw 6.4:1 in the manual... I was like, how does it run? All my other 2 stroke bikes have been well over 9:1, which seems to be a good explanation why this F11 runs about as fast as my XR200 (4 stroke 200cc) does.
 
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