Carb jetting?

[Dave]

ok so my 83 cm 450c has a really bad hangup at around 4k which goes away completely with more choke so i figure i need to up the jets. problem is after lots of searching i cant find anything resembling a stage one jet kit. anyone know where i could find one or approximately how much id have to go up? they are keihin carbs and according to bike bandit it has a #38 slow jet, a #115 main jet, and a #72 fast jet. help?!

[BobTheBiker]

What model are your carburetors? CV, CVK, etc? you dont want to bother with some "stage one" shit. just go up one size on your main jets, or shim the needle up one washer on the slides and see how that affects it.

I'm doing this exact method on my ZX10 when I get everything semi sorted out.

[Dnyce]

yea, fuck the stage shit, and another thing-just cuz bikebandit says it has xxx jets, dont trust it. its an 83, chances are some1 changed em already. dont wana kick yourself for buying jets u already got, so check em first.

[Dave]

do the jets themselves have numbers on them? my question was how faar up i should go and on what jets. and i think they are cv not sure

[Dave]

ok let me ask a ew more dumb questions i thought of later, im assuming a larger # means more fuel? and which jets need to go up? im assuming idle is fine and maybe the slow since it only hangs at 4k?

[No Worries]

At 4K, your bike is on the jet needle. And your carb is probably a CV (constant velocity), as my 79 Suzuki is one of the last years before CV carbs. The jet needle is pulled out of the main jet by differential air pressure. It's probably sticking. In other words, the carbs are dirty. Specifically, the diaphragm/piston and walls, and possibly one or more air bleeds.

[Dnyce]

bigger number, bigger hole :dthumb:

no worries has a gd point that i just assumed-did you try cleaning em already? thats usually the 1st thing to do with carbs before you start changing stuff.

[Dave]

ive cleaned them about 4 times and put extra attention towards making the needles move smoothly. found a recall notice today concerning needles actually. may have to see if i can find the replacement versions. might be my problem

[HRCNICK11]

If it has a excelerator pump it might be cloged. The excelerator pump has very small holes that are hard to clear if cloged. The pump will be round with 3 screws on the bottom of the carb next to the float bowl normally on the same side as the throttle linkage. If the pump is not cloged.

I would lift the needle(richen the needle circut) in the carb and try one size larger pilot jet you may need to adjust the mixture screw in a bit to get the idle back.

If your slide has a diafram on it it is a CV carb. CV carbs the slide is not directly connected to the throttle linkage it moves by engine vacume the throttle controls a butterfly between the slide and the engine. If it has one of these it needs to move smoothly and should pump air and have a vacume in both directions controled by a small hole near the top. Factorys jet kits sometimes adjust the size of this hole on sportbikes. Dynojet adjusts the spring.


Does it do it as you open the throttle or at steady throtlte.

Also this web site has lots of info. http://www.jdjetting.com/

[Dave]

does it on both, always around the same spot. It does have a accel pump and im starting to wonder if the diaphram for that is hosed. I think im gonna pull the carb again tonight or tomorrow and post some pics of the next teardown

[HRCNICK11]

The web site I posted tells how to adjust the pump timing also.