Carb recommendations!

[Twigg1012]

I have a 1990 sp with the yellow 580 motor that has a round body mikuni 44BN on it. The previous owner said it had an aftermarket/modified rotary valve which warranted the larger carb.

However I am having a hard time tuning it after a carb rebuild and would prefer to start fresh with a new one. (it never ran right since I bought it- definitely fueling issue)

From what I have read the SBN is a much better and easier to tune carb than the BN. Should I keep the same size 44mm carb or would I be safe going down to a 40mm? I think stock was 38mm without the modifications.

It also has the external pulse operated fuel pump if that makes a difference.

 

[mikidymac]

44 is way too big for a 580 and changing the rotary valve makes no difference.

 

[Twigg1012]

44 is way too big for a 580 and changing the rotary valve makes no difference.

44 is what was on there before, not sure if other things were done to it. I bought the sbn before you replied so that is what I got now. People also run them on the kawasaki 550 so I figured it's OK?

It seems to work well up to 60% throttle now.

It will run 60%-wot for about a minute then it starts to cough a bit and lose power. Wot after a minute will make a tick/pop noise and lose power more.

Reading about the carb it seems it might need to be jetted higher from the 125 it has now so that's my next step I suppose.

 

[Twigg1012]

Last update!

After reading the SBN's manual I adjusted the high end screw and it's running well now!