250 to 350 conversion

[Spikecat]

Hi, has anyone done this, and was it worth the trouble, cheers

[Wolfie]

Yes and it's no trouble, just get the 350cyls, heads n pistons and bung them on your 250.. Nothing else needed..
Here's one I did earlier..

pic220.jpg

You will get a significant power boost but not to a full 350 spec, that gets expensive.. [/color]

[Spikecat]

Thanks, sounds good , will give that a go soon.

[Spikecat]

Hi, is there enough of a power increase for a sprocket change, maybe a few less teeth. Thanks

[Wolfie]

Oh hell yeah, if you're touring you can go as far as 400 sizes but it will wilt on inclines or a decent headwind.

I was on 15/38 for years but I started fiddling recently and cant remember what the back is now that the front is 16.

I have a table somewhere where I calculated all the ratios.

[Neil the Don]

Hi, is there enough of a power increase for a sprocket change, maybe a few less teeth. Thanks

If you fit the 350 csrbs as well it should jump from about 26 as a kh250 up to around 45 hp as a 350.

[SAM]

My S2B has 46 rear sprocket. So just hits 90 on 8,500 perfect for most situations.

[Wolfie]

The Yellows are the Standard and option sprockets, and the pink are my S1 variations.. looks like I'm on 16/41 at the moment. That is way above anything other than grand touring and she gives in at around 6K RPM at the moment. I need some more fiddling,
She went on a Dyno a while back, he said that power is there apparently with a hole at around 6K right where I don't need it, once pushed past that it kicks in again and she will go to almost 11K.. What!!??

Ratios.PNG

[mraxl]

Here is a good way to see the effects of gearing:
https://www.gearingcommander.com/
Select "Triples" as model. S3 will be the same as 250 for internal gearing.

[scouse]

This is not a rude question
How heavy are you?

This might affect what gearing you can use

Gary T

[Spikecat]

Hi, I'm 12st 2lbs at the moment, I have a s1c, just wondering if the higgspeeds on it at the moment would be effected ,as they were designed for a 250, thanks

[SAM]

Higgspeeds work very well on 350.mine had them. Performance improvement very noticeable.

[robh1f]

A mate of mine did it back in the day...….with 350 carbs on his KH250. That guy on the Suzy GT550 didn't know what was going on as we all took off from the lights.It was proper quick...…..but petrol consumption was silly

[Treeman]

Don't bother with the carbs, as said above, the petrol consumption is silly. I got cought out twice thinking I had a problem with my bike, only to discover that I had run out of petrol (a full tank) at circa 45 miles I was a lot younger and faster in them days though

[Wolfie]

I also got caught out several times after the conversion, the 350s were thirsty little buggers... although these days with my lighter throttle hand I seem to go for ever on Reserve..

Maybe my expectations have been tinted by having the H2..

[MickTriple]

Yes and it's no trouble, just get the 350cyls, heads n pistons and bung them on your 250.. Nothing else needed..
Here's one I did earlier..

pic220.jpg

You will get a significant power boost but not to a full 350 spec, that gets expensive.. [/color]

That is one handsome S1c ? , I did have one exactly the same colour for about 5 years in my youth (how did it survive). Did wonder why they didn't replicate those lovely upswept pipes on the H1 for example ,do look good in that picture