Carb Tuning Pilot Circuit

[bob58o]

This is my first time playing with a motorcycle type carb. It is an OKO 24mm (Keihin PWK knockoff), but this is a generic question about tuning pilot circuits when using high stall clutches.

For jetting the idle circuit everybody says to adjust the jets and pilot mixture screw until you have the highest idle RPM. You want no hesitation when blipping the throttle and a quick return to idle RPM after letting off. Sounds simple.

BUT since my clutch engages over 4k RPM and my idle on this build is going to be around 3600 RPM or so, do I set the idle RPM to around 3600 RPM then start adjust pilot jetting and screw trying to find the highest RPM when the idle SPEED adjuster is set high. Seems like I might be onto the slide cutaway/needle portion of the tuning when at 3600 RPM.

OR do I turn the idle RPM down to like 2000 or lower to tune the idle circuit. Seems like this is where the idle circuit is doing the majority of the work, but my RPMs aren't going to ever get that low.

So tune the idle circuit near clutch engagement RPM (even if Idle SPEED adjuster screw is holding the slide open at that point)??
OR
Tune the idle circuit at LOW RPMs (lower speeds than the engine might ever operate at)???


My guess is to make sure RPMS are low and you are operating strictly on the pilot circuit, but it just seems strange since my idle will probably be a combo of pilot circuit + slide cutaway + needle?


Thanks in adVance.
-Bob Vance, Vance Refrigeration.

 

[MTNSleder]

I just took a piece of MIG .35 wire and twisted it into the jet to make the hole bigger... How complicated can it be?

 

[bob58o]

I just took a piece of MIG .35 wire and twisted it into the jet to make the hole bigger... How complicated can it be?

That doesn't even start to address the question I posed. If I asked how to make a jet bigger, then maybe?

What speed do snowmobile clutches engage? and what speed do you tune the pilot circuits on them? Try to be helpful. Take some aderrol, then get back to me.

Engine plus Carriage = Motor Car
Engine plus Bike = Motor Bike
Engine plus Boat = Motor Boat
Engine plus Sled = SnowMobile (Why is this not a Motor Sled? And where is that Aderrol?)

 

[MTNSleder]

What speed do snowmobile clutches engage? and what speed do you tune the pilot circuits on them? Try to be helpful. Take some aderrol, then get back to me.

I thought we were talking about a predator carb I AM no help here... I have Computer fuel injection..

 

[KartFab]

the idle circuit only affects only up to what the 1/4 to 1/3 throttle position? A lot of the mini bike people that have been drag racing have their idle at the standard 1600-2000 rpm, but then just crack open the throttle to 1/4 to 1/3 so its partially engaging the clutch and they are holding on the brake so it takes the slack out of the chain, then when the flag drops its full throttle and let go of brake.

 

[bob58o]

the idle circuit only affects only up to what the 1/4 to 1/3 throttle position? A lot of the mini bike people that have been drag racing have their idle at the standard 1600-2000 rpm, but then just crack open the throttle to 1/4 to 1/3 so its partially engaging the clutch and they are holding on the brake so it takes the slack out of the chain, then when the flag drops its full throttle and let go of brake.

Good point. This will probably be easier on the driver's garter springs too. And make it sound meaner at idle. LOL I know it will idle at 2k, but it didn't seem too smooth.

ADDED:
BUT what about non drag racing situations, where I might be cruising along, let off the gas (no brake), then want to crack it wide open again... Do I really want to rev from 2k to 4k before it grabs? Seems like for non-drag racing, I would want idle set near engagement right?

 

[Whitetrashrocker]

I would say tune the carb to what it wants. Dis regard the stall of the TC. One you screw the idle stop in to raise the Rpm's your not in the idle cuircit anymore.

I'd prefer a lower idle so when you stop and sight see the engine isn't at a high idle burning more fuel than needed and the noise level is higher.

 

[itsid]

I'm not sure why you'd want to set the idle that high tbh..

there's no benefit to speak of.
as long as the clutch is not engaged (not even partly) the engine revs up in split seconds..
and the difference is minimal (if at all noteworthy)

All difference can be taken care of with a preloaded throttle (as Grant said)
and applied brakes

So what would one gain if one sets the idle speed to 3500++ rpms?
premature engine wear.. ?! Is that a goal?

Now I think what you want is either throttle range.
And yes, that kinda makes sense...
BUT keep in mind that your engine is revving up much quicker than usual anyways, and that carbs like these are meant to rev engines from idle (1200 rpms) to full speed (~12000 rpms on a standard motorcycle) ...

As long as the enigne is able to idle solidly the lower the better IMHO.

Oh and while you're cruising (assuming you're cruising 'at speed')
it doesn't matter what your idle is set to much anyways..
to get to the speed the clutch was engaged and thanks to it's inertia it'll still be engaged
if you release the throttle as long as the axle spins fast enough to keep the driver above engagement speed it'll fight the engine (trying to idle slowing down the vehicle)
during that phase the engine is still revving above engagement speed no matter what throttle position it's in.
Once below it'll quickly slow down to idle indeed.. but again.. it revs up w/o load rather quickly, and I doubt you'd see much of a difference really.

just saying

'sid

 

[ezcome-ezgo]

GAB on idle settings.

 

[bob58o]

Great insight guys. Thank you. It all makes sense.
I'll set idle to a lower speed. The answer to my question was not obvious to me cause I was doing it wrong.

https://www.youtube.com/watch?v=IQpPdkd0B6M

 

[bob58o]

OK so I let it warm up earlier and tuned the mixture screw/ idle speed screw to get the idle around 3600 RPM. The air screw was 7/8 of a turn out to give the highest "idle" RPM at this time.

I went back and warmed it up again. I turned the idle down. The engine doesn't like idling below 2000 RPM. It seemed violent. It hurt me to watch my baby go through that.

Maybe too much advance for 2k? She liked 2300-2400 RPM better.

When tuning the air screw for idling around 2400 RPM, it seemed like 1 3/4 turns gave me the highest idle RPM. 1 3/4 Turns is within the 1-3 turn range, so the idle jet should be pretty close.

It is very strange twisting the throttle and hearing/feeling the engine rev from 2400 to 4200 before you move. There is a lot of wrist twist in those 1800 RPMs, especially when twisting slowly to avoid flipping over.

 

[Whitetrashrocker]

You got it mang.