Rpm & Fuel Issues (New Guy)

Just recently getting into karting with a 206. I have some previous experience in racing but non with a carb. I just bought a brand new engine and am having an issue with rpm on long straights.

I broke the engine in and set the valve lash to 0.02" for both intake and exhaust. My carb tune itself hasn’t had much development (1 1/2 turn out on mix screw + 2nd notch from bottom on needle). Float set to ~0.85" drop set to ~1.045".

The kart seems responsive at low rpm but if I give it full throttle for more than a few seconds I begin to see my rpm drop. It peaks at about 5k-5.1k and continues to drop the longer I stay on the throttle.

My initial thought was I set my valve lash wrong or I have a clogged main jet. I took off the carb, cleaned it and reconfirmed my valve lash settings but no luck.

Any thoughts on what could be the problem? Like I said I’m still new to the magic of carb tuning and would appreciate any pointers!

Have you checked the inlet needle that’s operated by the float? Give it a good clean, even though it’s a relatively new part.

Check the fuel line from tank pickup to carb for air leaks. Also check the fuel pump pulse line for leaks or severe oil contamination.

Has it had this issue from new?

Hey James, yea this issue has pretty much been from new. I’ll give what you suggested a try and let you know

While it sounds like motor, I would also look at the kart, its possible something is moving and causing resistance. The axle could be shifting and rubbing against the brake pad and also binding the chain. Make sure you are not pressing against the brake pedal accidentally. Are bearings spinning free? Are you possibly way overgeared? Just look to eliminate other possibilities too.

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Change your plug yet?

I just fought an engine for two days with what felt and sounded like a fuel issue and it ended up being the brand new plug Briggs sent in the engine…

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Fresh 87E10 gasoline every day will help.

Update 09-14:

Thanks for the suggestions gents.

Last night I tested a couple theories. Most of the fuel lines on the kart are brand new so I replaced the pick-up and breather lines as a precaution. After running the issue seemed to persist, very little oil is visible in the pulse line but nothing crazy (idk how much is acceptable).

I also tested non-engine related problems. I put some paint marks on the brake caliper and got up to speed and swerved / coasted to a stop. The marks were still present so I’m confident the axle isn’t shifting.

I changed the drive ratio from 15/58 to 15/53 and noticed the problem is now happening at a slightly lower rpm (4.4k vs. 5.1k). This makes sense in my head because the engine should be under a heavier load to achieve the same rpm?

Next steps tonight are confirming chain alignment and buying a couple new spark plugs.

Also for reference Im running premium pump gas in it rn (91).

What track are you running at? Maybe we overlooked the obvious…

Sounds like the gearing is too tall and/or exit speed from preceding turn is still very low.

With a 15 front sprocket, the rear gear should probably be in the 60’s, but it does depend heavily on the track.

Ill be racing in Clyde this weekend. The track is pretty straight from what I can tell. I have a 19 front as well, from what ive heard a ratio of 3.4 - 3.5 is a good target for this track.

Very cool. Is this street race new or been around?

Been around a long time. Ran as part of the Clyde fair.

Check your float!
We have experienced issues similar to what you describe from a float sticking twice now. Once when we first started karting, the kart would lose power at the top end of straights during practice session. The other cost us a couple rounds at TSRS Amarillo this summer, the kart would lose power and even die out. Both occasions, the kart tested fine on the stand, but under load would fuel starve. Float was sticking both times.
1st one was the float drop tab was binding and preventing the float coming up at the end of long straights. The motor was actually flooding out when the driver lifted at corner entry. Bent the tab and reduced the float drop a little, float stop sticking at the bottom of the drop. If you look close at the post and tab, you can see how excessive drop could cause this.
2nd one was a slightly twisted float that made the floats not drop properly in left hand turns. The track happened to have some high load on power lefts, so the kart would fuel starve and die in the lefts. Took a penalty and put a whole new carb on at the race. Got home and diagnosed the twisted floats. Only thing we can figure is that I somehow did it putting the bowl back on after post qualifying tech inspection, because it ran fine before that?

Check to make sure your catch can has proper ventilation as well.


^^ Heres your winner. I would bet $1.73 that this is the problem


19 driver of course would be a much higher (taller) ratio and exacerbate whatever issue you have.

+1 on checking floats and catch can (Thought we said already) as I had the exact same issue with a brand new carb only a couple of weeks ago. Although my symptoms were more a long pause in power delivery when getting back on the power and less about a loss of top end power. In my case the needle itself seemed to be catching it’s housing just a slight amount. I exercised it by hand with some WD40 in there, cleaned it with carb cleaner and sent it down the road.


Yep! This is the Sunday main event at the Clyde Fair. this is the 31st year. The track has one long straight and the famous/infamous ‘Shitcane’ to slow the karts at the end of the back straight, the site of an epic party. 206 drivers will hit ~60 mph on the front straight. @Caleb_Schindler, fwiw the guy who organizes the race suggested a 17 or 18 tooth driver and a 63 or 64 tooth axle sprocket. As always, YMMV.
Below is my son Conor (the 0 kart) navigating the ‘Shitcane’ when we raced there last year.


I know you said YMMV but 18:63 (3.556) is 53 MPH at 6050 RPM (Leaving 50RPM to stay off the limiter if that works for the track, leaves about .5MPH on the table)

If the karts are truly hitting 60 in the draft by the finals, you’d need to be at 3.120, which is somewhere around 20:63 or 21:66

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Fair point. I didn’t do the math, I was going on what I was told. My guess is the fast 206 guys (two of which cleaned up at the Elkhart GP) will be approaching that speed since they seem to be operating in a different universe. We mere mortals will probably be closer to what you calculated.

18/56 puts you at 3.11 ratio