Help with KA100 Engine Compression Issue

New to the Forum and hoping you can help with a question on an engine issue. I seem to have an issue with compression of the engine. When the spark plug is out, it seems to relieve the pressure allowing it to crank ok. However, once the spark plug is in, the pressure built up in the piston doesn’t allow the crank to turn. Any ideas?

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are you sure you put the correct size and type of spark plug ? and if you are wrong with that maybe the spark plug stops the piston .

do you see any fuel pumps out when you crank with out spark plug ? coz a flooded engine could cause a hydraulic stop if you crank it up .

the other cause is maybe a no good battery or starter problems if you have those on your engine .

It’s not spark plug, could maybe be a flooded engine. How would I check that?

Welcome Ryan.

If nothing has changed since the last time it ran it’s probably the battery. It can be started from an auto battery too. Give us some more info about the motor and describe what is happening.

If you crank it with out the spark plug you sould see fuel liquid bust out of the spark hole on your finger . Or the reed cages will be wet as hell

Usually, a flooded engine happens when you crank the engine and it doesn’t fire. So you have fuel going in but its not being burnt. Its not too common with the KA but can happen.

Should there be that much pressure built up in the engine that it doesn’t allow the rear axle to spin freely by hand turning it the wheels? I don’t remember it doing that before. There is a new battery but we just replaced the crank shaft key because it broke before and I’m concerned there so much resistance there that it will do it again if I try to fire it up. I’m trying to figure out a way to post a video to show what exactly it’s doing…

If the axle is spinning when trying to turn the engine over, something is wrong with the clutch. The axle should spin freely until the clutch is engaged and the clutch is only engine when the engine is running and at a certain RPM.

Are we sure this isn’t a clutch problem and not an engine problem?

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It could be a clutch issue, any idea on why would it be engaged when the engine isn’t running?

Assembled wrong, broken spring or disc, a rock or something stuck in the clutch.

If the kart is indeed direct drive right now and the clutch is not disengaged, I would start by disassembling the clutch and making sure everything is in the right order, as well as cleaning out the assembly.

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If you can’t pull the clutch drum off the crank easily, then the drum and clutch are probably toasted. Perhaps something got too hot on your previous session.

Tell us more about this. I don’t recall a key being used on the KA. It sounds like you have some issues for sure. Pictures or a video would help. Probably the easiest way to do that would be to put the video on youtube and then put the link here. Is this a new motor or new to you? Have you had it running?

Quick edit…there is a key on the magneto side but that is for timing not really a sheer key.


Had it happen just this weekend. Warmed up fine. Went to grid for morning warm up, wouldn’t start. Then barely cranked. Coil went bad from the pit to the grid… :man_shrugging: Starter struggled after a few attempts because she was pumping plenty of fuel, but no spark.
Missed the session, carried it back to pits and checked it out. Wet exhaust, wet plug, so we have fuel. No spark at plug, so put a new plug, still no spark. Put on a new coil, and woo hooo, spark. Cranked it briefly with plug and exhaust off and fuel line disconnected, put it all back together, and started right up.

Definitely should not turn axle when you attempt to start. For sure some type of clutch assembly or damage issue.

I always carry a spare coil, I think its one of the weaker parts of this KA package. Even when they are good its not the strongest spark. If you race at all in the rain be prepared to replace the coil.

The timing of the failure was good/bad. It failed for morning warmup. So we didn’t miss a race session. But it worked on stand warmup and failed on the grid. No way we were getting it figured out and changed in time to make the session. :man_shrugging:t2:

Overall I am not a fan of the KA package so far. Lots of money for an engine that is finicky, has a weak starting system, and requires frequent service. You would think we could do better.

What specifically are you finding finicky and what frequent service are you doing? As a 2-stroke racing engine it’s pretty much bulletproof, the carb require minimal tuning, rebuild intervals aren’t bad, the clutch requires nothing. KA is by far the most reliable and foolproof 2-stroke I’ve run. Especially compared to the Yamaha it replaced.

I literally let mine sit for months at a time and it almost always just fires up right away and runs great when I dust it back off. I even went to throw my battery in the charger after it had been sitting since SuperNats in November and it was still fully charged. I often forget to drain the carb or rebuild the carb over winter. I don’t remember when I put a spark plug in last. Maybe I’ll eat my words when I go out tomorrow and it doesn’t run :sweat_smile:

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Yeah, I have to side with TJ on this as well. I think you said you previously ran 206 so I can see some of your points but compared to a Yamaha specifically or other current racing 2 strokes its a good package at a reasonable price. As for the frequent service that’s a mater of point of view. We find that we do a top end every season 10-12 hours with little drop off and a top and bottom every other season but others go longer.

as for the starting system, what do you find weak?

@tjkoyen and @mtbikerbob, we only have about 14 hours and 1 top end in on the 2 stroke so far. In that time we have had starter brushes fail twice, coil fail once, and a random fail to start that we could not diagnose but went away after pulling parts to check and putting back twice. For a motor that costs 3k I would expect more reliability. The starting system in particular seems weak and unreliable.
Find it particularly frustrating to fire up on the stand, then go to the grid and NADA, zip, zilch, no fire. Missed session and find bad coil back at pits. The coil was installed new 14 hours ago. Is that all we can expect on a $110 coil?

This may be track dependent but I think I have about 45 hours total with several rebuilds and still running the original brushes. Have you done the RTV trick?