Idle /pilot jet question

[str8dum]

When I warm my bike up and its idling at 1800 rpm and I pull the choke, instead of bogging out, the revs increase to 2500 rpms. When i close the choke, the revs drop back down to 1800 or so.

Does that mean that my pilot jet is wayyy too lean? Its the 39 that came with the oko 28.
Thanks!

 

[Astrolopitec]

Sounds like it.
When I used to have an 28 OKO I used to run a 55 pilot and 155 main. Of course that will vary from cylinder to cylinder and I was running a 96cc.
For now richen your hidle by tightening the air mixture screw. But a bigger pilot seams in order.
Much more important though. Are you using an apropiate main jet ?
The 28 OKO is very popular. Perhaps other members can give you a guide line for 70cc's.
I think Pinky runs a 28 on his EVO.

WAIT !
I just reread your post. The shoke is ON when you pull the plunger UP. And yes RPMs should be higher. When you push DOWN the choke to turn it OFF for riding. the RPMs should go down to your seted hidling speed.
The way you descrive it . You are running normal.

 

[str8dum]

Yes with the choke on, you are cutting air and increasing fuel (ie choking it ) . This should cause the motor to bog out just like your motor should bog out if you turn your air screw all the way in). With the choke on, my rev;s increase which is the opposite of the expected effect.

Main jet would have nothing to do with the idle circuit bc the needle has it closed off.

Sounds like it.
When I used to have an 28 OKO I used to run a 55 pilot and 155 main. Of course that will vary from cylinder to cylinder and I was running a 96cc.
For now richen your hidle by tightening the air mixture screw. But a bigger pilot seams in order.
Much more important though. Are you using an apropiate main jet ?
The 28 OKO is very popular. Perhaps other members can give you a guide line for 70cc's.
I think Pinky runs a 28 on his EVO.

WAIT !
I just reread your post. The shoke is ON when you pull the plunger UP. And yes RPMs should be higher. When you push DOWN the choke to turn it OFF for riding. the RPMs should go down to your seted hidling speed.
The way you descrive it . You are running normal.

 

[tiba_karotsu]

my set-up runs fine with 39 pilot jet & 138 main jet but my OKO carb is tuned & not run out of the box.

 

[ex-racer#2]

Yes with the choke on, you are cutting air and increasing fuel (ie choking it ) . This should cause the motor to bog out just like your motor should bog out if you turn your air screw all the way in). With the choke on, my rev;s increase which is the opposite of the expected effect.

Main jet would have nothing to do with the idle circuit bc the needle has it closed off.

The main jet affects everything else to a certain extent. With a new carb that is not set up perfect yet the needle and needle jet may be too big and the main can affect the areas normally controlled by the needle and the pilot. In addition to richening the pilot try raising the needle just to check the effect and if the needle is restricting. The other possibility is the carb dia is so large compared to the motors consumption that it can't create enough vacuum at idle or low rpm.

If the motor is warm / hot the motor should bog when choke is applied unless the choke is too small and isn't adding enough fuel.

If the revs increase or it finally is able to pull when choke is applied when warm/hot you are definately running lean. When in doubt always go richer on main and then tailor the fuel flow downwards with needle changes and or pilot changes depending on where it is rich. The key being main is the one you need to set first as a lean main can make the needle seem too low or the pilot lean as you whack the throttle from idle to full. The proceedure is main,needle (and possibly needle jet if waay off) and then pilot. Test again and do the whole routine again. As it gets closer you may only need to change one variable.

*******************************************************
Since I am still using the stock 12mm carb on my 2001 BWS I change my needle clip and then have to change the main again due to the fact that I am limited by the stock straight taper needle. I get the main spot on but then dropping the needle affects the main at the very top as the needle is so low it is restricting the main at full throttle. Time for a 17.5mm or 19mm carb as the carb diameter is restriciting the airflow to such an extent that the carb/motor is hyper sensitive to air pressure and temperature changes.

Too small a carb (as in my case) or too large a carb can cause big tuning issues as it is always working to the extremes of its usefull range. It may work killer at top end (big carb) or super responsive off the bottom (small carb). The point is they become hyper sensitive to changes. Basically one change may seem to have absolutely no effect but the next equally small change has a dramatic effect and the next change in sucession is in completely the wrong direction again. The jetting "LOOP" becomes very small and sensitive and very hard to fine tune or it changes with the slightest of temp/density changes.