300/440/550 Tuning questions for a Mikuni BN on our 550

D

Dan Rudd

Location
Michigan
My son saved up his money and bought a real nice 86 550. The motor had been previously built up. It has a Westcoast head, Coffman half pipe, Westcoast waterbox, aftermarket intake (unknown brand), as well as all the handling goodies (plate, grate, etc etc...). The carb is a BN44
We just had the top end done w/new pistons/rings and bore job because it was way out of specs.
We put it together and in the water and are now tuning the carb. It's running pretty good but needs tweeking. We're starting with 1 on the H and 1 1/8 on the L and 1 on the Pilot. I'm somewhat comfortable with tuning the H and L but not the least bit familiar with tuning a Pilot jet. We seem to be running rich on the L side, but I'm OK with that for now until the motor has a little more time on it.
My confusion come in to play when I'm trying to tune the pilot jet. Where does that fit into the puzzle? When do I tune that? and for what responses?

Thanks,
Dan
 
cookn

cookn

Kamikaze
Location
where you live
I believe in the BN's don't have jets like an SBN. The pilot jet you are talking about is probably the needle and seat. It regulates how much fuel is coming from the fuel pump. The BNs are all about the screw tuning. You just have to play around with them and what works one day might not the next. This is just my experience. I really like the SBN 44 on the 550, and with a ski as modded as your sons it would really work well.
 
N2 H2O

N2 H2O

Previously Tbone550
Location
Charlotte, NC
The pilot actually affects the mid range on a bn. As others have said, I highly suggest a sbn44. Out of 4 550 I have only 1 runs well with a bn.
 
D

Dan Rudd

Location
Michigan
Hey guys, thanks for the info. It seems everywhere I ask, I get the same answer....dump the BN and go with the SBN. Man, we were tuning on that thing all afternoon and just couldn't get it dialed in. We're pretty comfortable with carbs and carb tuning since my son races karts and has to tune his carb on the fly, but this BN has got us stumped.

Anyone know where to find a good SBN at a good price? Keep in mind thats it's coming out of a hard working 14 year olds wallet (maybe I'll help a little too)
Thanks,
Dan
 
N2 H2O

N2 H2O

Previously Tbone550
Location
Charlotte, NC
pwctoday.com There are a lot of parts for sale and you should be able to find a sbn44 for $75-$100. You will also need an intake spacer. Jre has them on ebay or you can probably find one with a wtb add on pwctoday.com
 
D

Dan Rudd

Location
Michigan
Tell me about this intake spacer. His BN has a spacer between the carb and manifold which is about 1/4" thick. Is this what you're refering to?

Thanks,
Dan
 
screaming440

screaming440

The Old Skool Professor
Location
KC/Lake of The Ozarks
here is some stuff i have had sitting in my tool box for a few years)

judging from the note on the front i picked it up when i was working at a jetski shop probably around 1990 or so.

anyway it should help

View attachment Tuning the Mikuni.pdf

we have run the bn for years without problem. if eveything else is right on your ski you should be able to get it to run right. if your having problems its a good idea to start from the ground up. reseal the motor and rebuild the carb.
 
You must log in or register to reply here.
Top Bottom