Driveway carb tuning

I have the carb cleaned and put back together. The engine starts (with some effort) and seems to run ok.

Since won't be able to go to the track and I don't have a dyno all I can do for is tune the kart idling. When running the engine is it better to get it up to operating temp and let it run for 5-10 min rather than short cycle the engine?

For say, 500' above sea level/blue slide are there good default carb settings to use as a starting point? Based on earlier threads I want to run the float at .900 but not sure about the needle position.
 

ZeroClient

New member
About the only thing you can do in the driveway is verify its not running lean on a quick throttle hit. You can also work on your cold & warm starts, and your slow throttle engagements. Engine should not ever DROP in RPM on a slow throttle hit - like 0 to 1/4 throttle. If it does, its lean. Other than that you must tune it under load. I'd say also if it revs to the noon with the wheels lifted its a good chance you are running too lean.

This I've learned from tuning 2-strokes.

On the track, the least you'll need is an RPM meter. Get going as fast you can out of the last turn before a long long straight, and get your peak RPM. Change jetting richer or leaner, and get that peak RPM at the richest setting possible. You can also use an EGT, and even a WideBand O2 sensor (If you have a 12v power source). Plug reads only on new plugs (less than a couple laps), and head/water temp. Each gives good data. EGT temps should stay below 1300F for a 2-stroke. Probably more comfortable at 1200F. YMMV.

Change jetting, bar'ing class limitations of course, and of course if you have a rev limiter you cant use the RPM peak unless you are also allowed to change gearing per your conditional needs.
 

ClarkSr

Member
Zero, he's referring to a Briggs LO206 where not much of what you suggested is allowed. EG: jet change, egt sensor, etc. They also have a rev limiter set to 6100.
Dad, you will learn next to nothing in your driveway, other then it starts and seems to pick up RPM's when you bump the throttle. Not much else.
Just my opinion of course.
Clark.
 
That's what I thought. It does start (which I was not sure about since I took the carb apart and cleaned it).

I am concerned about carbon buildup - the tailpipe was pretty sooty (I cleaned it to see if it gets all dirty again). Can excessive idling or frequent start/stops where the engine never gets real hot contribute to carbon build up? If I have too carbon do I need to pull the head to clean things up or is seafoaming worth doing?
 
Last edited:

ClarkSr

Member
Dad, you're over thinking this! Just run it, when it appears to be getting a little slower, get someone to do a valve job on it. There are a number of folks on this site who would be excellent choices for that. Or take it to you're local 4 cycle engine guy and let him do it. We're new at this 4 cycle stuff too and getting the valves done brings them right back to life. Otherwise get someone to go over the carb and head at least once a season.
Relax and quit worrying about little stuff. It will be fine.
Clark Gaynor Sr.
 

Yakattack

Member
Thanks Zeroclient!!!
Appreciate the info...
I run a YZ144 and have been experimenting with the carb settings and found what you posted most helpful...
I do not use the stock carb... but a Mikuni SBN... and believe it’s dialed in close...
 

CarlsonMotorsports

Premium User
Dad, you're over thinking this! Just run it, when it appears to be getting a little slower, get someone to do a valve job on it. There are a number of folks on this site who would be excellent choices for that. Or take it to you're local 4 cycle engine guy and let him do it. We're new at this 4 cycle stuff too and getting the valves done brings them right back to life. Otherwise get someone to go over the carb and head at least once a season.
Relax and quit worrying about little stuff. It will be fine.
Clark Gaynor Sr.

There's your answer right there.

I'd be glad to help if you ever need, but this is honestly your best advice with the L206.
If in doubt, have it looked at by a professional. There is little to nothing that you will "tune" in your driveway with the wheels in the air and no load. Having the top of the piston carboned up may actually be an advantage according to some.


-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
29 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
I don't have a kart shop nearby - if I did I would use them. I am toying with the idea of getting a spare engine so I can drop off on raceday with a trackside vendor and pick up later and still have an engine to use for practice. It would be a bit pricey to do this though.
 

flattop1

Dawg 89
Ups man goes everywhere , still same problem of no engine.
A spare head and carb , would solve that.
Spare engine even better.
 

ZeroClient

New member
@yakattack, I'm highly skilled at tuning Mikuni SBN carbs which are the preferred carb for 2-stroke watercraft. The reason you'd be using this carb in a Kart is because the carb can actually run INVERTED and still meter precise fueling - whereas float and bowl style carbs can flood and change mixtures when under hard acceleration like a kart sees in a corner; SBN Carbs are not affected by acceleration nor gravity.

The SBN carb has two external adjustments for fueling. A LOW and HIGH circuit. The Slow Jet should be selected based on your LOW screw dialed in at precisely one and one/eigth turn from seated (1.1~1.2 turns out). If you find a rich or lean spot off idle then you can change the pop off pressure via the spring or N/S size, in conjunction with the LOW jet - but aim the low screw at 1&1/8. The pop off circuit works from 0 through 1/4 throttle. The LOW jet works at about 1/4 to 3/8 throttle, and the MAIN jet controls 1/4 through 7/8 throttle. The HIGH screw controls from 3/4 to FULL throttle. Change your main jet and dial-in the high screw with a target of about 1 turn out. You can test with screws out to 2.5 turns, but be prepared to change jetting to get these screws back to your targets. Trust me it'll run better.

One more thing on the SBN. You need fuel pressure at the pump to be between 1psi (Idle) and 6psi (WOT). If its not there, get it there by adding a 30 Main jet into the return line for a restrictor. Most single SBN carbs actually have a restrictor built into the carb body though.

Im just getting in to Karting, but have extensive tuning of dirt and water machines.

Is it common for folks to use a Mikuni SBN in a Kart? If so thats awesome
 

Jimbo

A trial w/o witnesses is like racin w/o tech
The last time i had a kart upside down i could have cared less if the motor was even running.
The various flat slide carbs work very well in karting applications.
I personally often use the 24 to 33 mm versions.
 

Yakattack

Member
Reason I run a Mikuni SBN 38 carb... is I bought 3 SBN carbs cheaper than I could buy a stock slide for the Mikuni 38mm TMXx carb...
 

Method racing

New member
About the only thing you can do in the driveway is verify its not running lean on a quick throttle hit. You can also work on your cold & warm starts, and your slow throttle engagements. Engine should not ever DROP in RPM on a slow throttle hit - like 0 to 1/4 throttle. If it does, its lean. Other than that you must tune it under load. I'd say also if it revs to the noon with the wheels lifted its a good chance you are running too lean.

This I've learned from tuning 2-strokes.

On the track, the least you'll need is an RPM meter. Get going as fast you can out of the last turn before a long long straight, and get your peak RPM. Change jetting richer or leaner, and get that peak RPM at the richest setting possible. You can also use an EGT, and even a WideBand O2 sensor (If you have a 12v power source). Plug reads only on new plugs (less than a couple laps), and head/water temp. Each gives good data. EGT temps should stay below 1300F for a 2-stroke. Probably more comfortable at 1200F. YMMV.

Change jetting, bar'ing class limitations of course, and of course if you have a rev limiter you cant use the RPM peak unless you are also allowed to change gearing per your conditional needs.
HI I c its a old post but maybe you could help me setup my sbn 38 on a 4 stroke go kart engine. Needle size and pop of pressure to start with. Had it on the motor. But had a huge flat spot when you push it. If you take it slow of 1/4 and push it. It's OK. Thanx
 
Top