Jetting Equivalent

[gregrob]

Hey guys, I have some new carbs on the way. I'll probably try them out before I get porting etc.

 

 

I'm running 270 mains in my stock carbs right now, what would the equivalent be for these new 34's?

 

 

I'm at 6000ft elevation with the mods in my sig. Thanks a lot.

[gregrob]

No response eh?

[4stroker]

were you running an airbox with the stock setup?

 

the engine is gonna pull a weaker signal through the bigger carbs compared to your stock ones, so your gonna need to go up on the mainjet, probably the pilot too. you will have more airflow through the pods than an airbox, so thatll require bigger jets too.

 

id say start at something like 340s and see if itll pull em

[gregrob]

I am and will be running just filter pods, no airbox to speak of, though I intend to make a diamond plate version just to help with water splashing.

 

 

I'll try 340's but I doubt at my elevation it will work. I think 300's will be rich, but I know what you mean about going up. Those carbs are gonna pull more air, so more fuel.

 

 

 

Thanks for the reply.

[4stroker]

I know what you mean about going up. Those carbs are gonna pull more air, so more fuel.

 

 

 

Thanks for the reply.

 

yeah thats part of it.

 

all 2 stroke carbs work on the same principle... airflow through the carb and over the dump tube causes a vacuum in the dump tube (which the main jet screws into). that vaccuum draws fuel up the tube and into the air stream to mix with the air.

 

now smaller carbs have better air velocity than bigger carbs, meaning they suck harder on the main jet. which means they can get the required fuel with a smaller jet. once you go up in carb bore size, the volume of air in the carb is increased (helping top end power) but the air velocity goes down (hurting the throttle response and low end power) the main jet has to be bigger for two reasons now A) there is more air the vaccuum is less in the dump tube so a bigger hole is required in the main jet to get enough fuel up there

 

a bigger cylinder bore size will increase the air velocity through the carb, (as will the right port job)

 

if you did nothing else to your engine except increase the bore size dramatically, you would probably have to run leaner jetting than before. this is because the bigger pistons, on their induction stroke, would create more vaccuum in the intake tract, sucking harder on the main jet than b4. the bigger bore will also draw MORE air, making you think it will need more fuel to match, but the increased vaccuum has a greater effect than the increased volume.

 

haha hope i havent confused you too much