Converting to 100cc?

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TygerTung
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Converting to 100cc?

Post by TygerTung »

In New Zealand we have a class of racing called 'Bucket Racing'

You can use a 100cc water cooler 2 stroke, a 125mm air-cooled 2 stroke with a maximum of 24mm carburettor, 150cc 4 stroke, or 100cc 4 stroke turbo or supercharged.

The rules are that engines must be based on non competition engines and you must use normal petrol, Ie. Avgas.

I was thinking that a KR250 would be a good engine to use as it is quite high performance, and we have a spare parts bike lying around.

I was just going to use one cylinder only and sleeve it down to 100, but my mate Nigel had the idea of making two 50cc cylinders.

I have worked out that if you were going to make 2 50cc cylinders that you would have to modify the crank as there isn't any pistons which are small enough to be 50cc with the current stroke.

If the stroke was reduced, perhaps there wouldn't be enough port timing?

Also how much thick cast iron can you put in the bore before heat transfer becomes an issue.

Any ideas would be great please.

Cheers,

Sam
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Top-shaggy
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Post by Top-shaggy »

Sounds like a fun project you have there guys, something for KR-1R and TwoStrokeInstitute to comment on perhaps (and they are local'ish)

With my limited knowledge I would use a water cooled disk valve AR125 engine and reduce the bore / stroke to match.. ;-) kr250 engines are like rocking horse poo in the UK

Hope it goes well, please post some pics and let us know yeah


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ScottaKR
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Post by ScottaKR »

Apart from the massive amount of engineering involved in getting a KR250 down to 100cc, there's the problem of weight, as the KR engine will be heaps heavier than something that was designed to be that capacity.
I'd go with Top-Shaggy's idea of starting with something along the lines of an AR125, as youll get the same amount of power for a heck of a lot less effort than would be involved in getting the KR's engine down to size.
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Post by Luders »

Please don't butcher a KR engine, they are rare enough as it is.

Use an AR or TZR 125 engine as suggested.
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Post by TygerTung »

We don't have any AR or TZR125's in New Zealand, they were never imported here as you were alowed to ride a 250cc on your learner licence, so there was no demand for a bike like that.

I don't really want to import one from the UK as the cost would start getting well out of control.

There are however a few KR250's lying around.

I can't see the KR being that much heavier, they are not a terribly heavy bike.
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ScottaKR
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Post by ScottaKR »

So what do the bucket racers over there normally run? I can't imagine they'd start that catergory over there if there were no bikes that were locally available that were eligable.
The KR250 isn't terribly heavy for a 250 of that era, but is nearly 30kg heavier than a AR125. I know you don't have the AR over there, but that's just an example of the difference.
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Post by TygerTung »

There is many 125's and 150's here, but we didn't get any watercooler 125 2 strokes.

Most people run Suzuki FXR150's,h***a CB125, h***a GL145s, h***a CBR150s, Suzuki AX100, h***a CG125, Suzuki GP100, GP125,Yamaha RX100, RX125, h***a MB100, that sort of thing really.

I used to run a destroked h***a XL185 engine with 6 speed box etc in a custom frame, but I got sick of the valves dropping all the time with the agressive camshafts and high revs. Also 4 stroke engines are a real pain to work on with all the extra bits in them and camchains etc.

I am just really keen to run a 2 stroke due to the increased reliability.

My friend and I have a spare parts KR250 lying around that I could use.

Can also probably reduce the weight of the bike somewhat considering all the surplus stuff on there which wouldn't be needed for the track, could remove one of the front brakes etc, as 1 should be plenty.
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Post by Luders »

What about an oversized RXS100 engine?
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Post by TygerTung »

Because it's a crappy air cooled 100cc. It's not designed to be a high performance engine.

It's designed to be a reliable commuter bike engine and that it does very well.

I am actually riding around on a Yamaha RX125 at the moment, and while it goes fine for around town, the gearbox ratio's are all wrong and it's only 5 speed. Not especially fast either.

The KR250 on the other hand is designed for high performance and good handling. That's why I think it would be a better option.
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500bernie
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Post by 500bernie »

Hi Tyger
By the time you have sleeved the barrels down to 50cc it would effectively be air cooled :evil:
But if you go ahead post plenty of photos as it would be very interesting:
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JanBros
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Post by JanBros »

I'd say it is a waste of a good engine and a lot of resources/money if you go ahead with that plan ...

you'll need a new de-stroked cranck , and you''l end up with a cranckcase volume that probably will be a bit too big.

you'll need complete new cylinders :
* exhaust is way too big for a 50cc
* cylinder wall's would be very thick, ok for cooling ?
* exhaust/transferports are too high for destroked cranck, + because of the thick wall you'll never be able to get the angle of the transfer ports right.

So basicly all you'll be able to use unmodified is the KR250 gearbox ...

Personaly, I'd search for any 125cc single cylinder 6-speed gearbox, that has a 50mm stroke (or less, bore would be 56 than), and put on an MX 125 cylindre with a 54mm bore (or less if you could find any). than you are already down to about 114cc, and than put in a sleeve of about 50mm so you end up just below 100cc.

plenty piston's to be found from 48mm to 50mm (80 up to 100 cc bikes), and as you can see : you'd end up with a perfectly square engine which is ideal for 2-strokes :wink:
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Post by Luders »

](*,)
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Post by TwoStroke Institute »

You would spend more money trying to get a KR 250 down to 100cc than you would to build a 100cc engine from scratch.
'TZ350' has got a reliable 30HP from his GP125 and that kills the opposition if it finishes the race. A KH 125 would be a better start 6 speed a bigger disc and 50mm stroke, just fit a RG400 cylinder and away you go. Think that has been done before too.
If I had the time the ultimate would be to build a mini Rotax 256, using ebay sourced KTM 50 cylinders/cranks/primaries married up to a 80/85mx clutch/gearbox. Would be small enough to machine from billet and bolt together like 60's Kreidler/Jamathi engines. A homebuilt Vtwin Simpson 100 made 40HP in Germany
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