KT100 running rich

[Mark Lozon]

I have a fresh rebuild (according to the seller) with a nice set of hone marks and a new 52.35 piston so all looks good there. I have install a fresh stock WB3A carb. low side settings are at 1-3/4 and the high side at 3/4 turns out. Spark plug is a NGK B8ES. Exhaust is a 4 hole can. Head temp is after 5 minutes is 315 egt is 400. I know this is low due to the extra fuel. But my problem is even after turning the low side to 3/4 out I have fuel being pushed out the can. The engine will idle at about 4500 but is sluggish to react to throttle feed (due to the extrat fuel). Do I have a bad carb or what ? This engine has NOT been broke in yet. Share the wealth guys

[Walt Gifford]

When your carb is set for racing it will be too rich to let the engine idle. 3/4 of a turn on the high is way too much, try 1/16th to 1/8th open and set the low at 2 turns. If this is a bone stock off the shelf carb it needs to have the pop off set at 10 psi and the fulcrum arm set .050" below the metering gasket and you also need an aftermarket fuel screen with an open mesh.

Hope that helps,
Gif
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FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[Mark Lozon]

Walt will the engine load up during a break in on the shop bench ?

[Walt Gifford]

If you want it to idle turn the low to 1/2 turn open and screw the idle adjustment screw in until it coil binds on the spring and it should idle about 4,000 rpm. Do not go out on the track with that setting.

You really don't need to break it in unless the builder used a very tight piston clearance and most builders use plenty of clearance. Just take it to the track and keep the revs down in the first track session, that's the best way. Idling on the stand doesn't put any pressure on the ring.

Gif
_________________
FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[al nunley]

Would you like to try something different?
Let the engine, mounted on the kart, idle at a rich low end setting. The high speed doesn’t really supply a lot of fuel at an idle.
Let the engine just sit there and idle. Lot’s of smoke. At some point, if they aren’t set already, the engine RPM will jump up. I believe this is the point where the rings seat.
The next time at the track, make the first practice session a cautious one, and then race it.
A good compression tester is essential. A good engine, with a “good” tester, will show about 160psi pressure.
Set the high speed needle, for racing, at about ¼ - 3/8 open. Set the low speed at about 2 - 2 ¼. The final setting will depend on if the carb has been blueprinted, where the fulcrum arm is set, what fuel your running, the air density and what exhaust you are using. It depends a lot on the air density. We’ve run with the high speed closed on a cool, really low air density summer day.
If you just want to get in some laps, a lot of this is less important. A KT will gun just fine with less compression, or with a stock carb, or with a carb not set just right. In any case, be cautious.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

[Walt Gifford]

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