Walbro carb on a JICA

[Kirk Deason]

I ran my first race with the JICA this past weekend and had a tough time. I apparently am lacking some fundamental understanding of how my carb operates and what I can do to adjust it.

Twice when exiting the pits (running fine on the stand, etc--yes I know, no load, not revving it, just making sure it ran.) I had the low speed open 1/4 and the high speed at 1 full turn open. When i hit the gas to exit the pits, it bogged really badly and barely ran --i got onto the track and it quit even though i was giving it full throttle. ) Next time out, I made it onto the track by futzing with the carb and being much more gentle on the throttle. this time I made it around on the warmup and it seemed to be running great until i came up on the field (Which was CRAWLING up to the start)--I got slowed down to match their pace, and I stepped on the go pedal for the start --and then the kart dies again. Pulled into the grass to watch another race go around without me.

Then in the final, I leaned down the high speed to about 1/2 open until i was on track and accelerating and gradually opened it up (ran ok) as I came up on the field for the start, I leaned the main down again until we started picking up speed i opened it up. Much better luck this time. Still wasnt perfect on the front straight as it didnt clear it's throat until about 14k. Midrange was good.

Root questions:

1. Should I keep the high end closed or just cracked open until i am moving along at a decent clip?

2. Should I close down the low end some more until I am moving and the engine is pulling cleanly? THEN open it up to about 1/4 turn?

3. I think I am confused as to the correlation between throttle position and engine speed--obviously (now) I realize that if the engine isnt spinning high enough to NEED the fuel then I should have the high end leaned down and it doesnt matter one bit whether my foot is buried or not.

Conclusions: I can figure this out but I don't want to blow one up to learn.

NO, I do NOT have an EGT.
70 degree day at about 5000 feet
Plug looked safe like a mocha coffee

When I was running and in the MEAT of the power, the motor absolutely RIPS and sounds the business. I was offered the option of a Rotax just so I could make the main (shudder). I am glad I declined.

Thanks in advance for everyone's opinion and input.

[TJ Koyen]

Only 1/4 open on the low?

Can't remember the Walbro settings, we mostly ran the Baroni carb, but found this on another forum.

http://www.karting.co.uk/notice/Message.asp?FID=2&MsgID=0000406892
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[Kirk Deason]

I had it backwards.

Sigh. OK back to square 1.

My LOW setting was at 1 and my high was at 1/4+.

When I step on the gas and it bogs/dies, is that from too little or too much fuel?

[Jim White]

The low is the one closest to the engine.
You need to make sure the engine is not "loaded up" when you pull out. As you pull away start to open up the low needle and feel the response. Now close it down a little. Hows it feel? Now repeat. If you can, take a a relative easy lap and keep tuning the low (richer first) coming out of the corners, each time getting on it a little harder. Eventually you will find the spot where the engine really wants to pull off the corner. Cruise into the pits and note that setting. Now richen it about 1/8 turn more to be safe.
Now you can go out and try to run a little harder down the straights. Do the same with the Hi speed needle as you go down the straight. Always richen first. Eventually you will get to a point at higher rpm's that if you open it a little it will fall off and sound funny (four cycling). Turn it in just enough to get it to run clean again. Come into the pits and note you settings.
That should get you pretty close. Think about where it feels like it has peak torque. Thats where it needs the most fuel. Remember always go richer first till you get the feel for it. It's easier and cheaper to change a fouled plug than a seized piston

[Steve O'Hara]

Jim,

Learn to use smoke signals
Really... simple rule of thumb... if your motor boggs when you step on the gas and you don't know the difference in sound between a lean bogg and a rich bog, turn around and look at the exhaust... if the motor is rich it will be blowing out a cloud of white smoke... if it is lean there will be no smoke.
At 1/4 on the HS and 1 on the low I would lay big odds it was starving for fuel... way too lean. Usually if you run the HS at 1/4 you will need the LS needle at a minimum of 2 turns to run in the track.
Exact settings depend on the pop off pressure and arm height settings but the first thing to try is to just open the low to 2 turns.
Steve O'Hara

[Jim McMahon]

Only thing I would add is that a pop-off gauge is an essential addition to your toolkit. Not even for checking the absolute pressure, but at least for checking that the needle is indeed sealing, opening. If you have a leaky valve your motor will tend to load up with fuel.
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[gregg boyce]

Kirk,

Start it at 1 on the low and about 1/4 on the high. You're too lean on the low. As you're leaving the pits open the low up to ~1 1/2. Then lean the high to ~1/8 (BUT BE VERY CAREFUL) to achieve top RPM.

If you leave the low at 1 it will fall flat on it's face until you richen it up. Since you're in CO you may not have to go all the way to 1 1/2 but you'll be close.

Good Luck,
GB
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[Kirk Deason]

Thanks everyone. That's the info I was looking for. And yes, Gregg, it was falling on its face--BAD. I'm hoping to get back out again this Sunday because when it was running it was a blast!

[Ted Hamilton]

If you get sick of the walbro, I have a Tryton Vamec F3 just sitting on my bench that will wake the JICA up a bit, I imagine. Both needles are easy to access and tune too. There's a document out there about carb tuning too, talking about tillotsons but the same principles apply. Al Nunley might be the one who has that. If you do a forum search on Al Nunley and Carb Tuning, you'll probably find it. Good luck -- it all gets a bit easier once you've done it once or twice. Only seems hard when it's information overlaod. Imagine trying to add choking the engine and direct drive too -- fun fun!
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[Brian Mead]

Kirk - Those are great little motors, as Gregg said being that high up open the needles up more thatn usual. I used a Baroni once on that motor and it was a monster. Remember a walboro was designed for a chainsaw, not a racing engine. It pumps too much at the top and not enough at the bottom. Open the low up it'll wake right up. We ran at 17000 all day, I've heard stories of 18-19 k, at that level, that's a rod replacement fairly frequently. there's parts and engines all over the place, you can run them for a long time.