Carb Theory - Big Bore Kits and Jetting

[G-Man]

So, the mixture gets leaner, the faster the engine rotates........?
But, (without tuned inlet and exhaust) volumetric efficiency reduces as the engine goes
faster. An engine at 10,000rpm will not breathe twice the same engine at 5000rpm...

Does a 247cc engine need a different A/F ratio at 5000 rpm than a 275cc engine at 4500?

:-)

It would make sense to me that a big bore engine should require bigger carb and ports to do it justice (PW22 on CB72 vs PW26 on CB77) and that the bigger carb (and airflow) then demands a bigger jet to maintain a sensible A/F ratio. If however, you keep the same port size and carb you'll probably end up with similar jet sizes.

There's a possibility that suppliers of big-bore kits are just playing safe and recommending bigger jets to reduce the risk of holing pistons in the hands of 'boy racers'.


G

More theoretical pondering........

At any given RPM, the larger displacement will be moving more air thru the same carb opening, which implies a faster air stream.

Some observations have shown that the faster the airstream across a given jet size, the leaner the mixture.

This would imply richer jetting may be required?

[brewsky]

So, the mixture gets leaner, the faster the engine rotates......
quote]

G,.... The 3 different bikes I have tested with an on board meter did show an increase in up to 2 points in the A/F ratio as the RPM increased from say 3,000 to 8,000 range at constant WOT.

I believe it is called mixture creep or fuel slope or something like that.

I guess it would be possible to calculate the difference between the 247 vs 275cc engines if you knew the "slope" curve formula, but my math skills have "sloped" too far downward over the years to even try! :-[

[teazer]

Back to Carb theory 101.

Air moving through teh venturi of teh carb moves faster than at the carb inlet because the venturi section is smaller. Faster air means lower pressure. That the Bernouli effect that allows planes to get off the ground.

A bigger motor or higher revs means that the motor is pulling in more air through the same size hole, and leads to lower pressure and more fuel picked up by the stronger signal.

The question is whether the extra fuel is proportional to the extra airflow and the empirical answer seems to be - "Usually". Where you might need to fine tune jetting is at high revs and or when accelerating because air accelerates faster than heavy fuel.

[LOUD MOUSE]

Hello there Teazer.
How ya been?
Knowing it was a 2 Stroke I had a bit of a racing disaster.
I replaced the stock HONDA pipe and installed a racing pipe.
The engine obviously had a lot more power as I pulled away from the others.
At about 15 minutes "ZAP" and no nothing from the engine.
Before I pulled the top end I knew what to expect to find!
OH YES there was a 5/8" hole where aluminum once was. )(&_(^%&^(%($#)&@%$&#+_
Talking about air speed/size of hole and pressure I wonder where all that had to do with my problem that day.
Lets see what folks have to offer. ............lm

Back to Carb theory 101.

Air moving through teh venturi of teh carb moves faster than at the carb inlet because the venturi section is smaller. Faster air means lower pressure. That the Bernouli effect that allows planes to get off the ground.

A bigger motor or higher revs means that the motor is pulling in more air through the same size hole, and leads to lower pressure and more fuel picked up by the stronger signal.

The question is whether the extra fuel is proportional to the extra airflow and the empirical answer seems to be - "Usually". Where you might need to fine tune jetting is at high revs and or when accelerating because air accelerates faster than heavy fuel.