Carb tuning question - carb leveling vs float height

My daughter has been racing in Briggs Cadet for a year now and recently we bought a 2nd engine so that we have dedicated dry and rain race engines. Her original engine has never been quite the same since we ran it in rather heavy rain (with an intake filter cover of course) at a Ron Fellows race earlier in the year (another reason we wanted a 2nd engine). We had the engine freshened up and dyno tuned and while the results are decent, it still doesn’t seem to be quite as strong as it used to be down the straights. The tuner set the float height to .900", which I just measured to confirm, but since we recently went to a 15deg mount, I got to thinking how that may affect float height tuning.

If the carb and therefore the fuel bowl are tilted, won’t the leading float be pushed up by the fuel sooner and close the valve sooner than intended? Also the fuel that is in the bowl might not be as close to the main jet compared to if the carb was level. Does this mean that a .900" float height might end up being more lean than intended? I have leveled the bowl as much as possible while still keeping the carb outlet opening aligned/unobstructed. I haven’t noticed any cutting out or stumbling but even after a full day at the track the spark plug has absolutely zero carbon build up on it - it looks brand new. That seems a bit lean to me.

I’ve only been working on my kid’s kart for about a year now and while I love every second of it, carb tuning still feels like a bit of a mystery to me. I’ve read quite a few posts on carb tuning but still have a lot of questions. I’m used to tuning DI via software :smiley:

Thanks!

You theory is accurate. However, in application is just doesn’t really matter. When you compare carb angle to g forces, vibration, and sloshing, there is just nothing there to say “adjust X amount for engine mount angle”

For a little transparency, we don’t change our carb settings based on engine mount angle. Ironically, our dyno has a 15 degree mount and our personal karts use an 8 degree mount, so we’re not even consistent in our own stuff.

We did a little dyno testing to prove to ourselves that it doesn’t matter and that we were still doing the best by our customers. Once we did that, I don’t even ask a customer anymore what mount they are running.

Just set it where your tuner told you to and let her rip. If you are a tinkerer and want to play with it, go right ahead. I don’t think you will notice any differences anywhere between .880 to .910.

Remember, its so hard to judge kids’ performance at the cadet level. What you think is engine could be kart or driver or tire or anything. Go out and test, have fun, and do what works for you guys!

Derek Hastings
Ghost Racing Engines

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Hey Derek, thanks for the reply.

That’s a good point about g-forces and sloshing probably being more of an issue than a slightly unlevel carb. I also completely agree about the driver vs engine performance comment too but it’s always in the back of my mind that it could be the engine lol. The issue I have is seeing her put distance on other drivers through the corners only to be walked down on the longer straights and lose everything she gained. It really frustrates her (and me) as they don’t even have to be in her draft to do it; when she is in their draft she can’t seem to close the gap either. :neutral_face: Her teammates are typically running the same gearing and usually running a few pounds heavier than her at the scales. Toe is -2deg total and rear axle is true and all bearings seem free. That’s why I want to believe it is partly the engine’s fault. :slight_smile:

She’s usually in the top 4-5 which is why I want to make sure that nothing mechanical is holding her back. With that said, she definitely has room to improve, no doubt, and I’m sure her exit speeds could be improved. She is moving up to Junior Light next year as she has pretty much outgrown her Cadet kart.

Back to carb tuning, I have two additional questions that perhaps you can provide some insight on:

Idle mixture screw: Adjusting it inwards means a richer mixture at idle and unscrewing it means leaner. From an airflow perspective, when the screw is turned clockwise, where does the air “bleed” to? I assume it must bleed this air back into the already mixed air/fuel from the main venturi, which would result in a leaner mixture. I’ve heard various tuning advice on setting the mixture screw, from backing it all the way to get a bit of extra air into the engine at WOT to setting it at 1 1/2 turns to keep it richer. I can understand the advantage of getting more air into the cylinder but why would you not want to do this? So far I’ve just been adjusting that screw according to the Briggs carb tuning guide but have to wonder if I’m leaving a bit of power on the table.

Fall/winter performance: we are approaching that cooler time of the year in Canada and that means greater air density and dryness. Does that mean the carb should be tuned to run a bit richer (whether by adjusting the throttle needle and/or float height)? What about cool but damp weather, like now in October where there is a lot of drizzle/humidity?

Thanks again for the information. I’ve learned so much over the past year about karting and chassis setup but tuning an engine via a carb is still posing a lot of questions. :smiley:

Nerdy Answer - When you tilt the carb the squarish floats do raise up with less gasoline in the float but that is needed because that helps to keep the vertical distance from top of needle jet to gas level the same. That vertical distance, being maintained about the same, guarantees that the same suction force is needed to pull the gas up into the needle jet.

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