Hi,
does anybody have 81 mm pistons , instead of original 77 mm pistons (FJ1200, 1988)? What other modifications are needed, if any? Are other changes absolutely necessary? I'm not looking for extra power (old pistons are out of wear limit), just have a opportunity to buy cheap piston kit of this size. Carpurator changes? Valve changes? BR Jyrki
I believe that is the 1314cc size.
With that, you will need New piston sleeves, the lower case holes will need to be enlarged to accept the larger sleeves. My recommendation on carbs will be to contact RPM and give them your elevation, the displacement of your Big Bore pistons, what type of exhaust you have and what type of air filter(s)
Also ask RPM about the head. I know that the 80.3mm 1297cc big bore kit allows the use of the stock head.
Fred
Quote from: jyrki on January 24, 2020, 03:01:26 AM
....I'm not looking for extra power (old pistons are out of wear limit), just have a opportunity to buy cheap piston kit of this size.
Tell us more about this "cheap piston kit" What does it include?
Just on offer in ebay, for this 81 mm pistons, which needs sleeves, so it's not cheap anymore.
I "think" a 1219 kit is the largest recommended bore only kit for a 1200 cylinder block. A 1250 technically can be done bore only - but the walls will be too thin and will likely go out of round easily.
There are some other "legend car" piston kits below 1219 that also do not need sleeves, but I don't have any knowledge of those specifically. I'm sure Randy or Robert could provide more details
Quote from: jyrki on January 24, 2020, 12:02:19 PM
Just on offer in ebay, for this 81 mm pistons, which needs sleeves, so it's not cheap anymore.
Ok, you understand now.
How about 1219 -kit? Does it need mods in carburators?
Quote from: jyrki on January 30, 2020, 07:01:36 AM
How about 1219 -kit? Does it need mods in carburators?
Probably not. When Yamaha bumped the displacement from 1100 to 1200 (1098 to 1188cc?), they made NO changes to the carbs.
I don't think 19 more ccs is going to need any carb work.
Quote from: jyrki on January 30, 2020, 07:01:36 AM
How about 1219 -kit? Does it need mods in carburators?
It doesn't really make a noticeable increase in power but if your current pistons are shot it will clean up the bores nicely and improve a tired engine. Get a fresh valve job with new valves and seals to go with the new pistons and you'll be all set for quite a while.