Changing a 305 to a 250.

[Daddykool305]

Hi, hello to everyone. I have found lots of good info on this forum before.
Now I have a CB77 I have owned for about 20 years. I recently put it back together and it runs pretty well. It needs a few things more to make it a good bike.
The thing is, I have owned it for a long time but ridden it very little.
Although I have been a rider for many years, when the licensing system was changed here, I lost my provisional license. I have been going through the 3 stage process again to get my license. As may be common in other countries, there is a restriction to 250cc machines for the first 2 stages of the process. I used to own several bikes, including one I could legally ride on the streets.

Anyway, I had the thought of changing my CB77 to a 250cc capacity.
This is so I can ride the bike I like most on the street and use it as transport for economical reasons amongst others.
It seems to be a matter of changing the cylinders and the pistons mainly.
I am guessing that the CB77 combustion chamber is probably bigger than a CB72 one and that low compression could result. I see there are some high compression pistons still available from time to time. I have not done calculations, but assume this will help counteract the loss of compression.
I have also read where people have modified other pistons to fit.
I am hoping to either resleeve my original cylinders, or to get a CB72 set of cylinders to use.
Has anyone else attempted this modification, or contemplated it?
I am not sure on some issues, such as are CB72 and CB77 cylinders interchangeable?
Also is there a source of sleeves for STD size CB72 bores-54mm?
I prefer to keep the 305 cylinders and pistons standard, so the bike can be changed back to original if desired later.

If anyone has any helpful suggestions, they are welcome!

Regards to all.

[teazer]

It's not hard to do. In theory you would swap out the whole top end. head, barrels pistons and carbs.

You could keep the carbs without any real problems. After all we use 305 carbs on a 175cc racer. Head is a little more tricky though because of the larger bore squish band.

Valves, springs, rockers cams etc are all the same. So you need barrels and pistons plus a bare head.

If you can't find them locally, let me know. I may have what you need in Melbourne. I used to have tons of 250 stuff and could never find 305 bits for my race bikes.

[Daddykool305]

Teazer

Thanks for your informative answer. It would be great if you can help with some bits. I still have a few avenues to explore, but I think CB72/77 parts are pretty rare around here.

Can I get away with using my CB77 head? Is the problem about bad gas flow because of the different size bores meeting the larger chamber? Would it set up a resonance or result in bad combustion? Or is compression a factor?

I sent you a PM about this.

Thanks.

[davomoto]

What about getting a 250 cylinder, and swapping in 305 sleeves? Then you wouldn't have to change anything else, and it would be a fake 250.

Davo

[Daddykool305]

Davo

That is one possibility I didn't consider, and one I wish I had thought of.
The bike hasn't been road legal for years and a friend previously suggested sticking "250" stickers on it when re-vinning it. Of course someone could read the capacity from the barrels if they were astute.
Considering that the old 305 has less power than a modern 250, I don't know why the rules don't limit the HP, rather than the capacity, but rules are rules. These days they'll give you a speeding ticket for 5kph over the limit.

I am a little torn between keeping the bike original and modifying it. I thought of buying a cheap bike to ride until I regain my license, but with the fuel price, bikes have gone up in price a lot.

Whatever course I take, I need to source a CB72 set of cylinders. And maybe some sleeves.