My First Carb Tuning
My First Carb Tuning
(OP)
Just started tinkering with the carburation on my 650cc single cyl 4 stroke motorcycle. It has a 40mm CV carb and is jetted horribly lean to meet emission requirements. I wanted beter off idle and slow speed driveability and bought a "jet kit". This consists of a re-configured jet needle and an assortment of main jets. I installed the needle and a 1.5mm orifice main jet. (Stock main jet is 1.4mm.) The bike ran much better, but bogged at 70+. Next I tried a 1.6mm jet and this brought the bog down to 55+. Out of desperation, I took off the airbox cover and went for a test ride. The engine ran strong at all throttle openings and the bog was gone. This result was totally unexpected to me. The airbox inlet is smaller than the outlet to the carb and the box, with the cover on, is tightly sealed. I know that speed tuning of intake tracts can result in performance gains on a race engine, but I am dumbfounded how opening up the airbox can help a stock engine. Ideas?





RE: My First Carb Tuning
Did you get a 1.42mm mainjet? Try that with the stock airbox.
Try the stock mainject with the modified needle, too.
Mike Halloran
Pembroke Pines, FL, USA
RE: My First Carb Tuning
The stock tiny airbox inlet is to control intake noise. Single cylinder engines can make a lot of it if the intake is unmuffled. Obviously that small snorkel can restrict the engine, too, although if the airbox is big enough the effect might not be too large. A lot of non-performance-oriented engines have simplistic, restrictive intake systems.
Two choices: Leave the airbox cover off and deal with the extra noise but enjoy the extra power (this would be my choice!), or else I agree with Mike, put the airbox cover on and go back to the stock main jet.
If you leave the airbox cover off, you might want to consider taking the bike to a dyno-testing place with exhaust-gas analysis to make sure you are not too lean. It's not always easy to detect too-lean conditions on the main jet.
RE: My First Carb Tuning
RE: My First Carb Tuning
If the needle has adjustable positions, try setting it one groove leaner. (Clip in the next higher position on the needle so that the needle sits lower in the carb.) The correct setting is the leanest one that still gives smooth takeoff from a standstill.
My experience with those jet kits, on every bike that I've messed with, is that it's darn near impossible to achieve fuel consumption as low as with the stock set-up. They always err a bit on the rich side.
RE: My First Carb Tuning
RE: My First Carb Tuning
RE: My First Carb Tuning