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Discussion Starter · #1 ·
Hi All,

Wish I knew what I was doing when posting. I'm old and a virgin! - at this anyway. I have a 1980 Honda Super Dream, handed to me to have a go at Motogymkhana - yes I'm a rookie at that as well.

The SD is a 1980 CB250N. When I have finished mucking about in car parks, I have half an idea to cafe racer it and it seems not a bad plan to enlarge the engine to a 350cc.

I believe the engines are designated K3 (1973ish) so was hoping mine at 1980 might be a K4 as I believe that designation is used in lightweight race bikes. So that is the background to my question as to whether a 350cc pot can simply be substitued for a 250cc one, or perhaps the existing 250 one rebored to 350cc.

All observations gratefully received.
 

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Hi.
The pooper dream is an entirely different animal to the K series. It has a three valve head and a (troublesome) balancer shaft. You would think that Honda would make improvements over the years, not really so. The cream of their 250/325's were the 69/70 K1 K2's, all downhill after them.
Yours was actually designed as a 400, so its weight was apropriate, the UK market 250's had the weight, but not the cc to lug it all around. 400 engines are still reasonably readily available, straight swap and at least something aprroaching a reasonable performance. Not Hondas finest hour.
 

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Discussion Starter · #3 ·
Thanks for that very helpful info/advice. I will be using it for a bit yet, but maybe I will look at boring it out to 350. Maybe also there are also some suitable pistons with the same small end size as the existing 250cc con rod. I am not too worried about performance. I have a few bikes - dont we all - including a BMW HP4, 1998 R1, 2000 R6 as well as 3 little 50cc demons from 1957 (Benelli) and late 60's (Puch). It more the final look and fun of the challenge that motivates me.
 
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