Jetting PWK 24 and 28

[nordic quad]

Ok here is the deal,airtemps just went from low 50s to high 70s overnight and the 28mm carb on the Access 100 suddenly ran wery rich on mid and top end,it came jetted with a #52 pilot,neddle in the middle and a #120 main.Our Apex 100 with a newer engine is running a #40 pilot and a #112 main with a 24mm carb,So my question is do a bigger 28mm carb need bigger jets than a 24mm to obtain the right fuel flow?We are running 98 octane pumpgas with 40:1 mix and we have low humid air.

Stein A

[Coley'sdad#8]

Stein,
yes, more air requires more fuel to get the same mixture as on your 24 carb.
Now dont freak out and start changing things, this is really easy IF the bike WAS running just fine before the cooler temps went away. pilot should not be out of range and can be adjusted with the screw, turn it out about 1/4 turn or until the rpm's rise.
drop the needle 1 position (raise the clip) this will lean the mid range out , now go test it warm and see if the top end is still fat, it will probably be fine and was just trying to clean out the excess fuel from going through the midrange.
If it is still fat on top put the next size smaller main jet in it.
Hot humid conditions dissapate oxygen and make jetting fat.
Cold low humidity conditions condense oxygen which dissapates fuel and leans it out.
thats why alot of night races the bikes seem to be running "peak" because it has cooled down the air going in the motor leaning it out slightly.

[nordic quad]

Thank you for the reply Chris,your answer confirms my own ideas,i dropped the needle one clip and will keep the pilot,i also have a #118 and #115 main to try at the track on saturday.Got some new sparkplugs so i can do some plug chops.Thanks again.

Stein A

[bulldogfallon]

The brand of the carb also plays a large factor in the jetting, so a 28mm carb could take a smaller jet than a smaller or even same size carb.

Just something to conisder for other applications.

[nordic quad]

Thanks Gary,this is our first year running 28s,our 24mm carbs are Keihin and the 28mm are Sunworld both kinds are PWK.
I will keep close watch on the sparkplug color.
Stein A

[marsrace2]

In my experience when we switched from a 24mm Kehin to a 28mm Kehin I had to go up about 10pts on the main jet and the pilot was not much different. So based on what you are saying I would guess you will be in the 122-125 range on the main jet and a 42-45 pilot.

[nordic quad]

Thanks Mark i am going to start with the needle pos.and start from there,i am just going to bring a range of jets to the track and see what works best.We got the 1st race in the Regional championship comming up next weekend,going to use this saturday to get everything tuned in.

Stein A

[MK Racing]

Hi,

In the Netherlands we run;

28mm Stage6 Carb - Pilot 38 second position and 138 Main.
28mm Sun World - Pilot 40 second position and 135 Main

[nordic quad]

Thanks guys i will report back our results from the track tomorow.We have learned from the past that we usually need to drop the needle a clip and put a smaller main from the factory settings.

Stein A

[nordic quad]

Went to the track today did some carb adjustments but was not happy with the performance,decided to change every thing to the same set up we have on the Apex 100 only with the 28mm carb,went with higher stall and higher rpm power band.Jetting stayed the same,only dropped the needle a clip.Thank you for all your advices guys.

Stein A

[Logan #34's Dad]

Okay, I got the answer! Here is what you need to do!
You should decide to change every thing to the same set up you have on the Apex 100 only with the 28mm carb, go to higher stall and higher rpm power band. Jetting stays the same, only drop the needle a clip.
HEHE!


:devil:

[nordic quad]

He He :D Spot on.:devil:But it like you alvays say,the power is in the right clutching.