My First Carb Tuning

[swall]

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?

 

[MikeHalloran]

The rise in absolute pressure with a lower inlet restriction changes the tuning of the CV carb ... completely, everywhere. As does making a large change in main jet size.

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

 

[BrianPetersen]

What happened is that you went too rich on the main jet. Then, when you took the airbox cover off, it reduced the amount of restriction in the intake upstream of the carb. The reduced intake vacuum pulls less fuel through the fuel jets, and for various reasons this will mostly affect the main circuit.

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.

 

[swall]

Yes, I plan to try the stock (140) main jet with the modified jet needle. I put 160 miles on so far with the modification and have observed about a 10% drop in fuel economy. Don't have access to a dyno, but do have a fellow engineer/biker with a portable oxygen sensor exhaust gas analyzer such that readings can be obtained under different riding/engine loading conditions.

 

[BrianPetersen]

Normally, fuel consumption during day-to-day operation is mostly determined by the slow jet and needle position settings. Usually those jet kits don't require changing the slow jet (sometimes called the "pilot" jet).

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.

 

[swall]

I think I have it figured out, looking at Mike's first post from a vacuum standpoint rather than a pressure standpoint. With the airbox intake restriction, the bottom side of the diaphragm sees more vacuum than it otherwise would. Thus, at any given throttle opening, the piston will not raise as high as it otherwise would, you get less air flow to the engine and it runs rich. The piston is the main restriction to airflow in a CV carb until vacuum raises it out of the way.

 

[drwebb]

The needle is attached to the carb piston, so won't a lower rise of the piston reduce fuel proportionally to air?

 

[swall]

Drwebb--yes, but. The kit has a smaller diameter needle and bigger main jets. So the lessened piston and needle rise still meters more fuel to the engine than the stock set-up.