Portinghelp 490

[ola_martin]

KTMaico, I considered the Wheelsmith style spring, but my dealer didn't supply it, so I just went for a set of new washers for now.

I would be very grateful for photos and info of your porting, the bottom end is done now, so fixing up the cylinder is next.


I have matched the cases to the cylinder already, so I'm expecting a massive power increase, either physical or psycological

[JohnnyO]

Disagree, matching the cases can give the bottom end a big lift.

smooth out all the dags in the transfer ports to barrel interface, match to the gasket edge and smooth radius any curves in the transfers. unless you have good porting specs from genuine hot up notes from the period, I dont touch the std timing. The main aim is to smooth any radius's or bends to a nice constant curve if you can and then just smooth the liner to barrel interface and smooth off the outer port edges with emery to prevent a sharp edge catching a ring. its at best a blueprinting process. In some cases of poor casting control, it can give a solid kick.

cant get a pic on ...

Unfortunately  dyno's and stopwatches tells a different story to the 'bum dyno'. Our dyno will measure down to 0.1HP and yet to see any difference in a engine that has had countless hours smoothing and flowing etc etc.
It doesn't hurt but makes no measureable improvement in output.

I can't agree with that either.
From my experience matching the ports gives an increase that can be felt in throttle response and acceleration out of corners.
A dyno doesn't measure throttle response nor does it measure engine performance at 1/4 to 3/4 throttle where a motocross bike spends most of it's time.

[80-85 husky]

Re matching ports etc.... if a reasonable amount of material is removed in tidying up and matching the ports, there will be an increase in power. Just polishing and knife edging the dividers wont do much at all. i did this to my 010 300 and no noticeable increase .

i had a 2000 250 years ago and we ripped into it as it had plenty of fat around the transfers and gasket edges on the barrel. probably removed 2 - 3mm in parts and reshaped the transfers into the barrel into a smooth curve as they had a step like inner radius. the inlet port was cleaned up as it had serious casting dags all over so it was taken out 1- 1.5 mm in parts and the bridge seriously thinned.

it generated a lot more torque and shot onto the power much easier. I was out climbing 450's on steep open grassy hills in 3rd and 4th gear and in a straight line drag against a newish 07 250 it would pull 3 - 4 bike lengths on it. A mate still has it and everyone that rides it is amazed at the grunt it has for an old dunger.

The newer katos have a much tidier casting process and those potential gains aint there! (dammit)