Briggs 206 Jetting rules on Main Jet

People keep saying that, but I’ve read the rules at the Briggs website, and the rules say that’s not so! Are the rules in error? Am I miss reading them? Everything is possible.

The rules say “stock unaltered jet” then has a go no go. So in theory you can buy a million jets and find some that are bigger and smaller but you cant change out the jet to a bigger/smaller jet size.


From the 206 ruleset:

  1. Carburetor & Intake Manifold
    The B&S stock carburetor part #555658 is the only carburetor permitted. ‘Walbro’,
    ‘Briggs’ diamond logo and/or #590890 etched in the body are additional visual
    indicators. No alterations allowed unless stated below. All parts will be compared to a
    stock known B&S part for eligibility. This includes the nozzle, emulsion tube, jets,
    float, float needle and all other carb parts. It will be allowed however to adjust the
    float height by means of bending the small tab on the float arm.
    A slight chamfer around the choke bore ID
    (air horn) may be present. 1.149" no go Tech Tool A7.
    Both idle and main jet must remain stock, as shipped from the

Maybe you were looking at the Outlaw rules Al.

This is what happens in a spec class. At least jets are cheaper than buying Spec Miata cam shafts. :grin:


Please be mindful that for the sake of simplicity, especially for newcomers we’d rather not bring ambiguity into certain topics.

If you find consensus on something is opposite to what you’re seeing yourself, consider if that debate belongs in a separate topic.

I’d encourage people to flag posts for a topic to be split up if you think it makes sense to do so.

Carry on!

Stock unaltered is pretty clear cut.
Even if the jet passes the go no-go test you can get a DQ if it was altered

I don’t know for sure, but doesn’t Briggs make more than one “stock” jet? They say “stock” and then they go on to explain what “stock” is. I don’t know about the rest of the country, but herein Austin Texas, the air density can easily change by 10% over a years time. I’ve calculated that would require a plus or minus 1 jet size. For instance .038" to .040". And another thing; if the factory can drill a hole, you have to believe there are many other people that can work
the same hole and make it look the same way.
And something else; changing a jet is not only easier than adjusting the float level, when you change the hole in a jet, you know exactly how much!
I’m not advocating cheating or anything else, I would never do that, but the rulebook is the rulebook and that’s what I’m going by. You can hardly call it cheating if it says in the rulebook you can do it.
As far as changing of float level, go ahead and do it, with my blessing, just tell me how you calculate how much to change it for a given change in air density. “People want to know” lol

You’re kinda stirring the pot and splitting hairs Al. The whole idea of the 206 is to not get into the weeds and just drive the kart. As such it’s designed that certain things might look worthwhile on paper, but at the track there’s more to be gained with chassis than tinkering with engines.

Not for the carburettor supplied with the 206, no. There is one jet.

3500 people seem clear on it :smiley:

Appearance is just one check. Pin gauge is also done.

Most people just drive the kart and get on with it… That’s the spirit of the package. Some get into the weeds with air density and other things… Yet at the same time I’ve seen racer of similar driving level literally bolt one on out of the box and been just as fast.

There’s lots of things one can do. The real question is are they worth doing when the overall performance package is being considered, including driver and chassis.

95 main jet, unaltered within, spec
Then if you get the chassis right you are ahead of the game.

I bet I could figure out how to get a jet to flow more and still pass inspection. But then I’d be better off figuring how to do something else. Like tires. I mean really if you are going to game it, bend or break a rule wouldn’t tires be where you do it.

Okay, I give! Still, I’m wondering; why do you prevent a jet change and yet allow a carburetor float level change? If a jet is subject to inspection, why not the float level? I don’t know for a fact, but I’m pretty sure it’s easier to change a jet (and probably a lot easier to inspect) than it is to change the float level.
And why am I the bad guy, being accused of raining on everybody’s parade. lol.

I am not really agreeing or disagreeing with you on what should have been done or not done rule wise. I am just in the “it is what it is” mode. I don’t care, it doesn’t matter, it’s the same for everybody. Dave Klaus is not going to call me and ask what my opinion on it is. I just bought spare carb parts to have in spec replacements and started listing out what my carb adjustment options were.

I’d guess that if you would mandate a float change rule then you would have to mandate a motor mount angle rule. Then some chassis wouldn’t be able to fit a LO206. Clutch is a somewhat open choice too.

There is the Ignite series which I think has some stricter rules. Since I am not near it I never looked at it close.

A friend has been wanting to do a Spec kart class with a somewhat fixed frame and LO206. I like the concept, but spot a couple of small to medium hitches.

For the most part the LO206 is a great option and a lot of fun for the dollar and effort needed to do it.

I I think James is trying to keep the answer as “simple” as possible to give the guys something actionable that doesn’t have any gray area.

For an experienced guy looking to play around within the spirit of the rules, your idea may well have merit!

Keeping it “simple”, presupposes… something.

As does overcomplicating it :wink:

On a practice day we did mess around and get the exact jetting that the weather called for just to see what could be gained. Turns out it was negligible. Even playing with the float and the “correct” jet wasn’t enough of a gain over handling adjustments. When you get down to it it’s a 10 hp engine with a 6,000 RPM limit. You’re not going to see huge increases with engine tuning. The chassis is the key in the 206 class, hence why that discussion is all over the place.

That is kind of the deal. Engine can only be so right or so wrong.
Chassis, Clutch/gearing and Tire PSI all probably trump Carb.

[quote=“Easy22, post:15, topic:4759, full:true”]
On a practice day we did mess around and get the exact jetting that the weather called for just to see what could be gained. Turns out it was negligible. Even playing with the float and the “correct” jet wasn’t enough of a gain over handling adjustments. When you get down to it it’s a 10 hp engine with a 6,000 RPM limit. You’re not going to see huge increases with engine tuning. The chassis is the key in the 206 class, hence why that discussion is all over the place.
“The exact jetting the weather called for”? Interesting! How was the exact jetting arrived at?
When I went to a race, late in my racing career, there was an air density gauge in the trailer and an EGT (exhaust gas temperature) gauge on the engine.
One thing I wonder about; because clone engine jetting is only available in .001" increments, how do you adjust for a 1% change in air density? On Longacre’s website they say that you should adjust the jetting to match any change in air density. They say, a 1% change in air density would then call for a 1% change in jetting size. If you’re using a .038" jet, a 1% change would be a .0382" jet. Not exactly, but real close. Changing a jet by .001" would be about a 5.3% change in area with a .039" jet.

You should go to their website and read up on the air density gauge and how to use it, it might give you a better understanding of the air density gauge.
“Tuning is tough” (Al Nunley)

I think folks are well up of the rudimentary principles of jet size and RAD. Thats not really the point or an education.

The question is the actual real world impact on performance as it pertains to the particular engine type being discussed… lower output OHV four strokes.

We’re comparing apples and oranges with the engines and yes it makes a difference.

One engine is a four stroke with a very low specific output. The other, high performance two stokes with 3-4 times the specific output.

These two engine types respond vastly differently to the changes being discussed and that’s really the key point… you can whip out your RAD gauge and experiment if you want. Tune your jetting to 0.00xx if you like. The question is… is it a good use of your time when you’re being left for dead in the turns.

At least in my experience, jetting changes have a much bigger impact on two stokes. It has an impact on lower powered OHV four strokes for sure, but not nearly to the same extent. A change that behaves like a a nuance on a four stroke OHV can make the difference between a screamer and a dog on a two stroke.


Assuming you were to try to get more out of a LO206 and leave the jets alone what would you adjust on the engine?
Can you explain any aspect of float height vs jet change?

For general knowledge:
One thing to keep in mind is tuning advice on an LO206 also varies with the slide and ignition. When you hear a guy say never, ever let it hit the rev limiter, he is probably tuning for a non-adult 206.

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I’m sorry I shouldn’t have used the word exact. We were using an air density gauge and Longacre’s guide. We also had a whole slew of jets from back in the alcohol animal days when we were changing jets constantly. Looking back at my notes we were off by 1.3% on what the “exact” jet should have been. We were using our digital calipers to try and adjust the float to make up for the missing 1.3%. Who knows if everything lined up and we hit that magic “exact” setting then we may have found 0.5 seconds.