Re: Assistance with baseline carb set up, 1970 T250 w/pipes
Posted: Fri Mar 15, 2013 2:56 pm
That's good to know. I have found a few times that changing main jets more than a few steps starts to influence mid range. Needle jet and needle are modulating flow form the main jet during partial throttle conditions. Change the main jet and it changes fuel flow at part throttle.
We also found on the dyno that needle changes still change full throttle fueling an a Keihin carb where the area through the needle jet was many times larger than the main jet and even allowing for the extra air and frothing, that was quite noticeable. I have not tried the same thing on a Primary type needle jet to see if that affects them as much.
IIRC, on the old Rotax 256 AFI Aprilias the rule was more than one size main jet requires a different needle. More than two requires a change in needle jet as well. Those were strange devices though.
We also found on the dyno that needle changes still change full throttle fueling an a Keihin carb where the area through the needle jet was many times larger than the main jet and even allowing for the extra air and frothing, that was quite noticeable. I have not tried the same thing on a Primary type needle jet to see if that affects them as much.
IIRC, on the old Rotax 256 AFI Aprilias the rule was more than one size main jet requires a different needle. More than two requires a change in needle jet as well. Those were strange devices though.