Is it possible to runs slip-on cans (road legal), with a K&N air filter, set up on a dyno, with no jet kit?
Yep
What adjustments can be made on the dyno without re-jetting? (Carbs and idle mixture I presume) Also, what benefits will I notice by just changing to K&N? (Fuel economy, smoother accel etc).
The dyno for that price is probably just a diagnostic run. It with tell you where the carb needs changing for proper running through out the rev range.
Most road legal cans ran with a K&N filter are normaly a bit weak around the 4K zone. Normally not enough to cause damage but occasionally you do get a bogging feeling on the acceleration as you pass through it. On the up side you wil get better economy on a sedate moterway run.
Personally I'd have the Dyno run done and then do the carb adjustment myself. Most likely you'll need a 0.7 mm shim put under the carb needles to get ride of the mid range flat spot.
Or should go the whole hog and get it re-jetted?
Have a play first and see how you get on
Pete.l
