F1 Primary Ratio

[neilg]

I have read a reference, in the book by Peter Shoemark, that the F1 had a different primary drive ratio to all the other models. Having compared part numbers between F1, F2 and K7 I can't see any differences that would allow this. Can anyone shed any light on this for me.

[hairygit]

The F1 and F2 have lower ratio primary compared to all the previous models (Sandcast to K6) The K7 uses the F1 engine but with awful PD carbs. The lower ratio was about letting the engine pick up faster, slightly improving acceleration to counter the power loss over the years from detuning to comply with emissions regs.

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[neilg]

From that then I have to assume that the book is wrong as it quotes the F2 and K7 as being the same ratio as Sandcast-K6 at 1.708:1 and only the F1 at 1.985:1.

[K2-K6]

I've an genuine Honda manual for F2 and K7 which states the primary at 1.986 reduction with the K having 2.733 final and For 3.071.

I thought from how Honda paired them in the same manual that the K7 is sister to the F2,  with basically smaller valves, less duration cam and pumper carbs ( PD42A for F and PD42B for K).

It's the F1 that appears to stand alone but more related to the K6 engine wise,  I've always thought.

[neilg]

I was always of the impression that the F1 and K7 were sisters so to speak with the F2 being the odd bod with different head and block. I feel one of my head aches starting now 

[K2-K6]

May be wrong but thought one of the differentiations is the final drive output shaft bearings that were changed to larger size to manage durability when using the larger O-ring chains fitted to the 2 and 7 models. Not sure what the F1 uses.

Honda as far as I can see deals with these two as a pair and just describes the differences when needed. I've not seen the barrels and head castings together so not sure if they are the same with different valves and seats.

[neilg]

No the F2 heads have lots of differences and the block has 2 of the oil drains unused with the casting different to block them off at the base.
Incidentally the F1 has the "old" cases without the extra added to allow the bigger final drive bearing, but I believe the rest of the internals are basically the same as the K7. This is why I initially queried the primary drive ratio as I'm mixing and matching bits between them.