Topic: Slow to Fast Jet Transistion - rpm or throttle (cb400f)

[bggann]

Okay - I rebuilt my carbs over the weekend and put "stock" jets in (75/40 f/s). This resulted in hesitation and missfire on accel and also on decel - probably too rich (I'm at 5-7K feet) - but the idle was so nice...

Yesterday - I put the smaller jets back in (68/36 f/s) - and the hesitation is gone - yea!

But - idle is not so good... Little rough, threatens to die.

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My question is - should I put the 40 slow jets back in- that would (probably) bring back the nice idle, but would it contribute to the hesitation I saw.

Do the slow jets control ONLY idle, or do they also contribute to  "mostly closed throttle, higher RPM" mixture - say 3-4K but cruise with little or no throttle.

In other words - if I put the richer slow jets in, will I fix my idle, but also bring back the hesitation - or do the fast jets control all of that.

I can just try it - but I'm starting to worry about how many times I'm removing my airbox - and changing them in the bike is a PAIN.

Bg

[eurban]

Throttle opening size has a much greater affect on which "system" of the carb is dominant in controlling the A/F mix, than what RPM you are running at.  If you are cruising with little or no throttle then the slow jets are certainly a big player in the mix. The carb's systems do overlap considerably making it, for instance, possible to affect 1/4 + throttle performance with both the slow jet and the main  . . .There are a number of approaches to carb tuning and I can't say for sure which is best.  I usually try to sort out my main jet first and use runs (examining both performance and the plugs) at WOT (or near so).  Since it is essentially impossible to find needles with different taper than stock for our carbs, we are left with either using the stock needles as they are, or modifying them in some way.  I am not dedicated or skilled enough for needle moding so I compromise and keep the stock needles as they are. Next I sort out the needle clip position by examing plugs and performance at somewhere around 1/3 throttle.  Moving on,  it doesn't seem possible to find slides with different cut aways than stock so again I compromise and leave the slide cut away alone.  Finally I play around with the idle circuit  being both critical of idle and off idle performance.  If performance improvements are still be made with the screw approaching either 1/4 turn out (turning in) or 3 turns out (turning out) then I would swap to either a larger or smaller pilot and try again . . .The concept here is adjust the individual carb system based on testing at the throttle setting where it is most dominant.  Overall performance seems to be the best with this approach. . . .Here's a pic that TT attached to an earlier carb tuning thread that should help you visualize . . .

[bggann]

Thanks eurban.  This is as I kind of expected, and the picture is helpful.  I'm afraid that what it tells me is that the slow jet was the one controlling the hesitation I was feeling because at higher starting throuttles, it was hesitating less.

I may give it a shot this weekend anyway and change the slows back to the richer one... it's only time..

bg

[Tim2005]

Have you tried cleaning up the bottom end with the airscrews (it's easier than swapping jets)? That said, stock 400fs are a tad rich low down so your altitude will only worsen that- you could try a set of 38 pilots (from a 550, iirc) as you are finding 36's too lean. BTW, I'm assuming you're on stock airbox, if you're running pods you could be trying to get the jetting right for a long while.

[bggann]

I have 'fiddled' with the airscrews - but I've not really worked at it - but that was this morning when it was still cool.  I'm going to do that tonight when I get home and the bike is warmed up good.

I am using stock airbox/filter and with what I hear about pods - ain't going there.    This is a daily driver - so reliability is more important than the last ounce of power.  Jetting is a pain....

BG

[StrongPerf]

How did things turn out?