IAME Leopard start

Good afternoon
Preparing the kart for a practice day tomorrow and I usually go down on my check up ritual. When it was time to fire up the engine, it started right away but them it went skyrocket on the RPMs (no pressure on the gas pedal at all!). I had to disconnect the spark plug to stop it before blew up. Of course now it will not start. Letting it cool (113 temp) now, before I start checking things out.
Any ideas why the sudden “max RPMs” with no foot on the gas?
Last started about a month ago with no problems. Stored on a vertical stand, if this makes a difference.
Also, before I pulled the spark plug wire, I pressed the gas pedal and nothing, moved the throttle linkage and nothing as well. Pulled the cable as last resort.
Help is appreciated.

Gotta be inside the carb.

In which way? The linkage? Internal?
Thanks

I’d say internal since you wrote linkage and pedal position is normal. I don’t know what exactly internally, but it can’t be anything else.

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It’s not 100% clear what you mean here… But it may be as simple as the cable getting snagged somewhere and holding the throttle open? Check the throttle system from pedal to carb. Remove the airbox so you have good visibility to the throttle plate itself.

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With the gas pedal, I pressed it and release but nothing. For the linkage, I moved it counter clockwise (like if the cable was pulling it) and no difference. Good idea about removing the air box to make sure is opening & closing.

Will reving close to the 17k RPM damage the reeds?
I cleaned the carb and everything looked good, but still will not start.
Just checking before I start tearing things apart.
Ideas?

It should not. Max RPM is 17,500. Definitely sounds like a carb issue. Did you pull the airbox and make sure the actual throttle plate is not loose or binding?

The engine needs air, fuel and spark to run. It had all of those or would not have revved so high, but now I wonder if by pulling the spark plug you may have fried the coil. Coils don’t like firing with no ground (normally the plug). Find something metal to put into the spark plug cap with the other end a small gap away from a good grounding source (the engine or top of the spark plug on the head). Turn the engine over and see if there is a spark jumping the gap. Remember, the gap should be around 2 mm or it may not spark.

If the engine hits its” natural” rev limit without a load it can crack or chip the reeds which would make it harder to start.

Sounds like you need to check each item in detail.

Also, I think the the 2mm spark gap that Greg mentioned only applies to when you’re looking for a spark outside the engine. When running it should be somewhere around the 0.6mm mark.

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Thanks for the info. Yes, I pulled the spark plug, connected it back while resting/touching the chassis and saw spark (4 attempts) each time, so I am assuming the electrical portion is working. I pulled the strainer cover and found some dirt on the fuel inlet screen. Checked the pedal cable (fine with no obstruction), and I can see the throttle opening & closing as the pedal moves, the throttle lever/return spring (free movement), and I can see fuel going to the carb (eventually fuel dripping from the intake support after several attempts to start the engine.
Now I want to check the reeds for any cracking with a light.
For reference, I haven’t done a carb rebuild in 2-1/2 years, but in that same time I only have about 2-3 hours of use.

Is the battery fully charged? I had an issue once when the battery was too weak during cranking to properly fire the spark at the same time and the engine would not start.

Yes, battery was fully charged (had that problem before :rage:). I have taken the carb out and in the process of cleaning and checking it. I will try again this weekend (will charge the battery all day Friday) and see.
Thanks

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The saga continues. Here is a list of things done so far:
-new carburetor
-new reeds
-new spark plug
-new fuel line (I can see the bubbles to the carb)
-new gaskets (manifold, reeds, carb).
-I can see the bendix engagement.

I can see spark on the spark plug when I try to start the kart.
The mychron shows 16916 as the max RPMs when I started the kart and went to full acceleration.

What else am I missing? Will I have spark if the coil ia damaged (by suddenly pulling the spark plug cable to stop the engine)?
How about the fuse?
Any tips/suggestions are really appreciate.
Thanks

Things you may not have tried:

  • Make sure the pulse line hole didn’t get covered over by the gasket between the carb and reed cage.

  • Remove the air filter. With the starter engaged, choke the engine a few times. Stop if fuel starts running out of the carb. Remove the spark plug to see if it’s wet.

  • Trying an external starter.

  • A compression test.

Thank you for the tips.

Chocked the carb until it started to drip fuel; nothing. Hole/gasket alignment is good. Battery at 13.02 volts.
When I try starting it for a longer time (say past 15-20 seconds with the starter button) and modulating the gas pedal (quickly stabs to the pedal ) I hear a fast blow off (like a turbo wastegate) from the exhaust once or twice for each try. Any ideas?
Going to borrow an external starter to see if that works.
Thanks

How did the plug look?

A silly thought, the engine is flooded. Remove the carb and reeds, then tilt the engine forward.

Silly thought two. Try starting the engine in the dark. Maybe the spark wire or cap is arcing to the cylinder.

As my engine guru once explained to me, a 2-cycle is a simple beast. To run, it needs air & fuel, compression and a spark happening at the right time.

I took the carb and reeds a few days before I ordered the parts, so say about 5 days without them on the engine (I put a cardboard cover over the hole for protection) so I will figure it was enough time to dry any fuel inside the engine. I will try an 1. external starter tomorrow and 2. will squirt fuel in the carb and then inside the piston/jug. Hopefully one of these steps will work.

Not to be pessimistic, but when you had your hand over the carb inlet to choke the carb how did the suction feel? Was it strong or weak? Just before my MY09 cut loose, the engine revved up and then lost power and started sputtering. After tearing it down, I found that the bearing seal behind the clutch had disintegrated allowing an excess of air to enter the engine beyond the carb. This might explain the sudden over rev and subsequent stall without being able to restart.

It should feel like the carb was going to suck flesh off your hand. An engine is basically a very efficient pump. If a seal cuts loose, then there will not be enough fuel in the airstream to properly combust and the vacuum (debatable terms here) will be significantly reduced at the carb intake. Hence the no start. Compression will be effected less, because it happens above the rings and only relies on air entering the crankcase.

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Thank you for the input. After the steps above, and if no start, I will look deeper and check your suggestion.

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