New to karting, having a problem with kart dying under WOT

I recently picked up a lightly used kart with a Vortex Rok GP engine that I’ve taken to two events. The karts worked fine the first event, but at the second event it had problems where it would start up and runs fine on the kart stand but as soon as it’s on the ground if I give it wide-open throttle it dies. Half throttle seems to work okay.

I’ve tried a new spark plug, took the carburetor apart and cleaned it with compressed air and carb cleaner (it was very clean already). The fuel filter looks clean as do the fuel lines.

Does anyone have any suggestions I can try?

My current guess is that it’s either an issue with the fuel lines or filter, or maybe because it dies only on the ground under full throttle it’s a problem with clutch engagement?

Thanks in advance for any help, I’m super new at this.

1 Like

sounds electrical, a faulty earth strap on the old 100s causes this exact issue. Vibrations cause it to cut out and do all kinda stuff that feels like fuel starvation at first

Is the fuel pump working properly?

Or just too lean on the low speed needle.

Check the reeds? (20 char)

I agree. Try turning out the low speed needle and see if it improves. I do not know anything about that engine, but with the Yamaha KT100’s we had to run the low speed jet very rich

Review any work done between weekends. Check if a rag has gotten into the air box. Grounding, reeds or the clutch could be your issue. Was the tube running from side-to-side on the carb replaced? That tube needs the cut at the top. A shot in the dark would be the battery. Good luck.

Trying to understand what you are saying…So the kart will run on the track but only half throttle? If that is the case it probably rules out the clutch. It sounds like not enough fuel or air. Is there air in the fuel line? Is the fuel line / pick up in good condition? Is the vent line pinched off? Is the carb settings at a baseline setting that previously worked? Is the air filter where its supposed to be? IS the airbox clear?

I have had issues with fuel filters and typically don’t run them. I also have had issues with air in the fuel line that were solved by using zip ties on fuel line connections.

Thanks everyone for all the suggestions. Just to summarize what I am going to try and hopefully answer a few questions:

  • Re-check fuel lines
  • Vent line possibly pinched (it looks okay, but is held on by a zip tie that maybe is too tight?)
  • Check / replace fuel filter (might be worth it to check kart without filter to verify)
  • Re-clean carb (it was pretty clean already, but maybe I missed something)
  • Check reeds for any issues.
  • Check carb settings (This has never been touched, I don’t have any different jets)
  • Check low speed needle
  • Check / replace grounding strap (this is pretty new, since it was a used chassis with a new engine).

I’ll report back based on what I find. I took the carb apart again today, and will be going through this list.

try driving it a bit at half throttle like around the parking lot and then see if you can go full throttle, if that doesn’t work maybe the engine is running to rich.

Korey,did you check the gasket that’s between the carburator and the manifold.
There a little hole and this has to line up with the manifold and carburator.
When this 3mm hole doesn’t line up the carburator will not get the vacuum it needs.

1 Like

Just wondering…When you say old chassis, is the fuel tank old? Have you had the weighted pickup out of the tank and checked it? Usually, they are brass but I have seen ones with rust/corrosion to the point they don’t flow much fuel.

Also what fuel are you using and is it fresh?


I wanted to provide an update here, since I believe I have sorted this out at this moment.

Here’s the steps I took today:

  • Re-cleaned carb completely.
  • Undid vent line from zip tie.
  • Closely inspected all fuel lines and fuel filter for leaks or dirt.
  • Inspected reeds.
  • Inspected low speed needle.
  • Checked grounding strap.

Everything looked okay, and I put it all back together. This time the biggest different was that I closed the throttle valve screw all the way.

Once I did all this, I started the kart up on the stand and it was idling (which it normally doesn’t do). I then backed out the throttle valve screw until the kart died. I measured this for reference sake, and then took the kart off the stand and drove to the end of my driveway. The kart pulled away smoothly and going full throttle worked without issue. I repeated the test once.

I then dialed the throttle valve screw out two more turns, and tried the same test. The kart died under wide open throttle. I then turned the throttle valve screw back in to what worked and tested the kart again succesfully.

At this stage I believe the next step is to find somewhere to further test my fixes, but I believe it was this simple. I realize this is probably newbie stuff, but that’s what I am, so I am pleased I made progress and understand the carb a bit more.

Thanks again everyone, I’ll report once more after a real driving test.

1 Like

Just so I understand you when you say, “throttle valve screw”. Are you talking about the idle screw that cracks the throttle open when screwed in or are you talking about the “Low Speed” Jet screw. To be honest, I am not sure which carb your Rok is using.

Does it have a circular disc inside the carb that rotates to let air/fuel in or is like a guillotine that slides up to let air/fuel in?

If it is a disc type it should have High & Low speed needle jet screws on one side of the carb and a throttle cable/return spring on the other? Typically a Idle Screw is on the same side as the Throttle Cable/Armature. It acts like a stop/rest for the armature as to leave the throttle plate inside slightly open. This is completely different than a Low Speed Needle Jet screw that is (of the two, high & low) located closer to the engine. It is responsible for providing fuel from idle to full throttle, where the High Speed Needle Jet screw is most affective at higher RPM. The Idle screw should not affect WOT, where as a too lean Low Speed screw can starve the engine when the throttle is opened.

I am less familiar with the Guillotine type and think you can only adjust the High/Low Needles by disassembling the carb.

Whoa, Whoa, Whoa, My bad, I along with others considering their response have been assuming that it was a Tillotson or Tillotson style carb such as a Tryton not a Dellorto.
That changes everythingn and honestly makes it more difficult.
When you say it dies out does it Yawn or does it gurgle.
I’ve got some experience with a Dellorto on a single speed engine but I’ve found it difficult at least for me.
This changes everything.


Sounds like it might be the older RoK with the VHSH30

Here’s some resources on that while we’re here: