I’m currently fighting some motor/carb issues. I’m trying to troubleshoot by swapping carbs and reed cages from motors that are working.
I’m currently having an issue where the motor starts, and runs for a second or 2 with no throttle input, and then screams to full RPM with NO throttle input. I have to pull the plug boot to get the motor to stop.
I checked the carb and the throttle body is not engaged when this is happening.
That doesn’t sound good. I had something like this happen a while back with a Leopard. I forget the reason but I think I would start with the basics.
Make sure the butterfly is actually closed or has returned to the idle position. Does it snap closed to the stop?
Set your needles where they should be (1 turn out on the low and 1 plus 10 min on the high is my baseline). Also, did this happen after a carb service? Make sure the H and L needles are in the correct position as they are not the same. Maybe also make sure the serviceable parts in the correct order.
Make sure your fuel line is not restricted. (Line is primed, no clogged filter, etc)
If a motor is starved for fuel it will rev so if this is happening with other carbs I would make sure your fuel is flowing good, make sure the tank is vented.
I investigated the butterfly, and it wasn’t returning to the full closed position. It doesn’t have the high rev issue now. However, when I would rev it, pull the brake, and rev again it wants to bog/die.
Put the original carb back and on it won’t start. It’s a brand new rebuilt carb, but I will go through it and look for issues.
Strikes me as a possible air leak. Crankcase, crank seals, base gasket and head (gasket if fitted) are items to investigate. A pressure/vacuum test is a good way to check. It should hold 5psi for 5 mins or more.
The bog could be a carb adjustment.
Your original carb I would have to guess has an issue not letting fuel in.
How much time is on the motor since its last rebuild?
The air leak James mentioned is a possibility, I would also wonder about compression. We have had a motor where the ring lost its springiness and basically had no compression. It would run without a load but any load it would not run.
Fresh rebuild. Zero time on it.
That sounds like a significant air leak. Check the gaskets between the reed block and the crankcase and the carburetor. If you idle the engine, and spray a little starting fluid around the engine to clean it, you’ll find your air leak!
So after trying combinations of 3 carbs and motors with no luck getting any started, I decided to run a new fuel line. I blew fuel into the line and hooked to carb. I still had to cover the box to get it to pull fuel and start.
Once started, it was creating crazy amounts of bubbles in the fuel line. This is causing the motor to bog/cut at low rpm.
Plugged pulse hole. Make sure the reed cage and carb gaskets are installed correctly and not covering the small pulse hole. Also look for silicone on the reed cage… just in case
That would be normal even if priming the line.
I don’t know what to make of that. I’ve never seen this happen. Yes, a little bit of air in the line at the highest point happens but not what you described. James is thinking the pulse line is worth a look. Here is a little education video on the subject of pulse lines.
This guy does very informative videos on 2 strokes
I’m thinking air leak somewhere in the tank. The same issue was happening on 3 different motors/carbs.
Going to buy a new tank/lines and see if that fixes the problem.
Before buying a new tank I would try feeding fuel from another source and see if it does the same thing. I would also track back to what has changed from when everything was working correctly to when you had the issue.
You said the motor had a fresh rebuild, did it have any run in / dyno time before mounting in the kart?
The issue started when my sone had to go off track to avoid an wreck. came back on and the motor started to bog/act irratic. He pulled off.
I assumed motor/carb issue and put the new motor/carb on. We couldn’t get the new motor/carb to stay running and missed the final. Then this week I have been trying motor/carb combinations to troubleshoot what the issue might have been.
The fuel line was old, and I couldn’t tell if fuel was being pulled to the carb, so I put on a new line. That’s when I noticed all of the bubbles in the line.
It could be be “sucking” air from the line in any one of the connections. Top of the tank where it meets the pickup line especially. I blew a motor once with that same problem.
You can try pressurizing the line and tank with some air (close off the vent) and listen for air leaks. You don’t need much pressure. I run spring-loaded clamps on all connections. You can also use safety wire too.
John, I would be surprised the off caused the issue. I have found a spin or an off causes the motor to bog and it takes some coaxing of the throttle to get it going. It may have been loaded with fuel. Also, make sure the air cleaner is clear too.
Bought a brand new tank with internal line. Hooked it up, and sold all of my problems. First time I’ve had this happen in 8 years of racing.
Ironically, the line in the tank broke when I tried to pull it out.
Glad you figured this out. If I had to guess your tank is probably okay but your lines were not. I had a similar issue last night with a crack in a fuel filter. I couldn’t figure out where the air was coming from till checking all connections the filter broken in half. Air is good, air in fuel is bad.