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What would be the drawbacks of opening up the intake ports and widening the exhaust port a few mm? The exhaust has room to be widened some... is 72% a reliable maximum? What about a window in the piston? what do you guys think?

i suggest gordon jennings book. it goes into detail about all the stuff your asking about

Thanks Clyde... That book did answer most my questions...

http://www.edj.net/2stroke/jennings/2st ... ndbook.pdf

I think I am just going to clean up some casting imperfections and run it... I would hate to sacrifice the mellow power delivery it has now, to gain top-end it really doesnt need... maybe i'll just open it up a lil bit... lol

Having had one of these my first thought was "why?". But making the stock porting more efficient makes perfect sense. There are some in the historic racing world who would know what to do, but you might get an "expert-only" result. If your suspension is stock I do know a much better place to spend your time and money...

redrocket190 wrote:Having had one of these my first thought was "why?"

mainly for hillclimbing and some dune use... Its my only dirtbike at the moment... I have some '87 forks for it..

I say DO IT! too many people are afraid to bust out the grinder. its just an air pump...grind away my man!
I'm not too familiar with the '84 ports, but if they are like the '85-'86 then you want to open up those boost ports going from the reed cage to the transfers. shape them more like a strait shot to the transfers and make them big enough to fit your thumb through. knife edge the transfers and exhaust bridge and widen the exhaust. you don't have to change port timing to see some great results, and infact a basic clean up with some knife edging and opening a few things up should keep the motors characteristics close to the same but yeild an improvement all over. messing with CR, crankcase volume and port timing is where you can really F up.
good luck mang!

dannygraves wrote:I say DO IT! too many people are afraid to bust out the grinder. its just an air pump...grind away my man!
I'm not too familiar with the '84 ports, but if they are like the '85-'86 then you want to open up those boost ports going from the reed cage to the transfers. shape them more like a strait shot to the transfers and make them big enough to fit your thumb through. knife edge the transfers and exhaust bridge and widen the exhaust. you don't have to change port timing to see some great results, and infact a basic clean up with some knife edging and opening a few things up should keep the motors characteristics close to the same but yeild an improvement all over. messing with CR, crankcase volume and port timing is where you can really F up.
good luck mang!

Danny thats about exactly what I had in mind... the 480 intake is very similar in design to the 500's(from what ive seen in pictures) besides it uses a smaller 250 size reed cage... the boost ports could be opened alot.... and the exhaust is bridgeless... I think I will do some mild porting... I see these cylinders for sale cheap on ebay all of the time if I dont like it... my only concern is it becoming pipey... i kinda like that on 250... but I can picture that being dangerous on a big-bore... Have you ever tried any piston mods?

I'm getting ready to. I was told that cutting the intake cut out 5mm higher is a good idea and cutting a frowny face looking cut below the rings on the intake side the width of the upper boost port and 30* in height (approx 6mm) will help get some more power out of the beast, thats the next thing I'm going to try!
now, I was told on those old finned bikes a carb swap can be a huge improvement!

for a long time this photo remained clasified top secret . now you can use it as you wish- might give you some idea of how wide you can safely go

mines wider

mines longer

dannygraves wrote:mines wider

i dont see that you can go much wider

well, I did! came dangerously close to hitting water jacket! and narrowed my bridge too.

Thanks for sharing the info guys... I wont be able to make my exhaust as large because mine doesnt have a bridge...

dannygraves wrote:I'm getting ready to. I was told that cutting the intake cut out 5mm higher is a good idea and cutting a frowny face looking cut below the rings on the intake side the width of the upper boost port and 30* in height (approx 6mm) will help get some more power out of the beast, thats the next thing I'm going to try!
now, I was told on those old finned bikes a carb swap can be a huge improvement!

Have any pistures of the piston mod? I saw a picture on here somewhere of a GSS piston that had a round hole in the intake side...

dannygraves wrote:now, I was told on those old finned bikes a carb swap can be a huge improvement!

I have a 38mm pwk i was going to use... I also have a 39.5 pwk I could try too...