Looking for help. We have a KA, new last season with about 6 hours on it. Our first race of the season was a rain race and it seemed fine. In the second race of the season 2 weeks later we had an issue where the motor would run fine at times and then have no power mostly out of the turns and sputter and not want to run very well.
I did a trackside cleaning of the carb with spray cleaner and replaced the diaphragm, and checked the reeds still has the same issue. New plug, new gas, same issue. At home, I did a compression test and got 160, which seems fine as another freshly rebuilt motor has 175. We also eliminated the kill switch. So really the only thing left is the ignition components but I was led to believe they either work or don’t. Plus they are expensive to replace. So I am looking for other ideas or thoughts.
Did you try a different carb?
I will consider that.
At home I cleaned the carb again with the ultra-sonic bath so I was going to go with that first. Does anyone know if it is normal to have the carb chambers full of gas?
Do a pop-off test on the carb. At what pressure does it pop-off at and does it hold pressure afterwards? Also take a look at the pickup tube in the fuel tank.
This is more of a general comment vs specific to the KA, but ignition issues can present exactly as you describe your symptoms here.
The coil or stator heat up and issues occur, everything seems fine when they cool off.
How does the rpm reading look when this issue occurs?
One way to narrow down the source of the issue or at least confirm or eliminate the ignition as the issue is to check your RPM trace on your data logger. If the ignition is breaking up, you will have a distinct section of the graph that jumps up and down much more than when it’s running smooth. If it is not Ignition related, the trace will be smooth, but have a declining slope when the event occurs as the engine decelerates.
How does the spark plug look? Does it look like it’s fouling? If the needle is not sealing off the flow of fuel when you are off throttle, it can overly richen the mixture during corning off throttle, then it will take a moment to clear the plug before it starts to pull smooth again when you get back on the throttle. Like James mentioned, you can check the pop-off pressure to see if the needle is seating properly. It should pop-off then bleed down a little and hold. If it’s not holding, then there is your culprit.
What would be the fix if this were to happen? I have seen this happen in the past where after the pop-off occurs it will continue to bleed down and applying more pressure it will not hold. Typically if I release all pressure and check again it will work but for whatever reason, without releasing all pressure it will not hold.
RPM are lower than normal, not sure if this is a cause or result of the situation.
Heat doesn’t seem to be a consistant factor as it will happen at start and other times it will be fine at the start.
On a pumper carb like you have, it means the Needle is not sealing off the seat. The Needle is the metering device that seals off incoming fuel at the Seat. It gets pulled away from the seat as the demand for fuel increases. You can compare it to how a float carb works. As the pool of available fuel drops, the floats drop opening the path for fuel to enter. On Tillotson Carbs, the needle has a rubber tip and the seat is contoured to be sealed off when the needle tip contacts it. The lever that opens and closes the needle iis controled / regulated by a spring. You can change the pressure needed to separate the two surfaces by the spring rate / tension on the spring. This is known as Pop-Off.
There will be a static pressure at which point the needle will not budge. At a point slightly higher is where it will finally “Pop” releasing pressure until that minimal static pressure is reached again and it seals back up again. You can think of it sort of like tire grip. A tire can grip a surface to some degree. You can often push this limit until it finally breaks traction and starts to slide. It will not regain traction until it reaches that base line of grip.
Personally I like E C’s description of how Pop-Off works and it relates directly to your carb.
Thanks for sharing that video. I have always done the first method but it is typically with a dry carb and even in the video you see it leaks down. The second method with a fluid in the chamber it seems to act as a seal a helps with the leak down. I will give that a try.
Still unclear what to do if it leaks down but has a target pop-off, say 10psi.
If the carb doesn’t hold pressure, replace the needle and seat.
Had similar problem – finally replaced ignition module and wire: problem solved!
What item are you referring to when you say ignition module? The stator and magneto or the coil?
It’s rare but not uncommon for the KA100 carb have a problem with the needle and seat. I’ve experienced it 2x in 3 years of racing KA, maybe 90 days of racing. Not sure if the needle gets stuck open or closed but on the track the kart will lose power intermittently but still continue to run. Replacing the needle and seat or the process of replacing the needle and seat resolves the problem. At same time I replaced the pumper gasket, if it wasn’t new. Replacing the seat requires a very thin wall socket (8mm?) . It is strange situation because there is no way to proactively prevent this from occurring, as least that is what I’ve been told.
Larry this is an interesting situation. Wondering how you verified it was the needle and seat? Pop off test?
Robert – Ha! Yes, I never went back and changed back to the suspect needle and seat to verify that really was the problem. The 1st time the problem occurred, I called my engine builder and he said to replace the needle and seat and the problem went away. He said it was rare but not an uncommon problem. The 2nd time it happened he was at the track and did a pop off test and I believe it wouldn’t hold pressure. It’s sort of disturbing to think that there is no preventative measure to this, as even a new needle and seat could fail, according my engine builder.
No, is the answer to your question. There shouldn’t be fuel in the pulse line. Remove the carb and reed cage if your engine has one, remove the engine from the kart and pour out the fuel. Check for fuel in the pipe as well. Could you post a picture of your engine.
Put a brand new carb on and it fired right up perfectly. Thanks
Another problem that I have had happen since I started running these motors, say I find myself in an unfortunate situation, could be an off, spin, or pinched off where I have to get out of the gas, but the result is always the same, the motor bogs down and sometimes takes an agonizing amount of time before I can get back up to speed. I think this is fairly common but what I don’t understand is why this happens and can it be avoided?