I’m new to karting, having bought a used TAG cart with a “freshened” Leopard engine (pre-09). It runs great — for a while. After three or four sessions (10 – 15 minutes each) it begins to stumble around 9000 or 10,000 rpm. Leaning the high speed helps a little, but only for a few minutes. Then the stumbling gets worse until the kart is hardly drivable in a few sections of the track (the stumbling always occurs in the same parts of the track — starting with a moderate straight). The plug looks fairly good, some light brown, some soot/oil, a little wet. The flexible is dry. The engine sprocket is VERY wet, but that may just be chain lube. (I use a lot.) I can’t duplicate the problem on the test stand.
Any chance that a leaky crank seal could cause this, with the crankcase gradually expanding as the engine heats up? The water never gets over 120F or so.
Make sure you’re fuel pickup is working correctly and not having any bubbles in the line. Make sure the connection to the coil has a good contact, and if all that looks good and you continue with the problem, you could have a bad power pack. I have had a similar experience and switched power packs and the problem went away. Also, make sure all the wires coming out of the power pack are good and aren’t cracked or broken.
Thanks for the suggestions, Michael. I found a break in the grey wire that goes to the ignition coil. The ends were held in place by a dab of RTV, but the conductor was clearly broken. I expect that will fix things. I’m actually surprised that the engine ran at all. Thanks again.
Sorry to say, but fixing the broken wire did not fix the problem. The search continues.
I have a spare motor from the kart’s previous owner. I’ve swapped carbs and, next time out, will see if this isolates the problem. If not, I’ll swap motors. If that fixes the problem, good; if not, it’s probably the ignition box. At least that’s my thoughts. Your thoughts? Again, this problem occurs only after about 20 or 30 minutes of running and doesn’t come on quickly, but builds until the engine doesn’t run well at all.