Compression?

Discussion specific to the various air-cooled KDX models sold in the USA
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kjdad
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Compression?

Post by kjdad »

So I'm to the point that I'm ready to start my 84 kdx. I tried a few times and not really getting a pop or anything. Gas (32:1) and plug are fresh. New air filter. Rebuilt carb and jetted the best I could for now (R-8 jet needle, 35 pilot, 280 main jet, 3mm cut away). I can feel it sometimes rarely feel like it's kicking back on the kick starter. I had a buddy over so we can take turns pushing and kicking since we are both old farts..He seems to think it feels like it low on compression. We tested spark and see a good spark. Found that the first time we tried the float needle stuck as it started pissing out the vent tubes. We pulled the plug and it's been wet each time. I did a cold compression test (in cold weather as well) and saw 120#s after a bunch of kicks. Not great but seems like enough to at least get the engine to pop over.

Thoughts? How hard is your bike to kick over? Can you spin your motor with your arm...not that you ever should. I can.

I am thinking either it's super flooded still after sitting a week..kinda doubt it but who knows. Or it's just a PITA to start when it's low 30 degrees out? That or the timing is super off..not sure how that could be and would expect at least a back fire.
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Re: Compression?

Post by KDXGarage »

Take off the exhaust pipe to take a peep inside the cylinder. It may give you a touch of a clue as to what is going on.

Check the connections. Seeing a spark outside the engine does not mean it is a "good spark" inside the engine under compression.

At least we know it is not the power valves or the liquid-cooling system! :supz:
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decluk13
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Re: Compression?

Post by decluk13 »

A cheap way to test spark strength is to bend the ground on a plug out straight and see if the spark can jump the gap. This is the same idea as an air gap tester and gives a good indication of the sparks "intensity". If it can jump this large gap outside the engine it should be strong enough to fire across the normal gap under compression.
kdxsully
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Re: Compression?

Post by kdxsully »

Mine was hard to start and it did backfire on occasion. 84 motor. I would tilt it over until gas came out the over flow and it would start right up. I know you said you had it overflow but who knows? Are you giving it throttle? From what you’ve posted you should have a running engine.
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kjdad
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Re: Compression?

Post by kjdad »

Not the greatest video..but here is my spark with the drill on high and the electrode bent straight

decluk13
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Re: Compression?

Post by decluk13 »

Spark looks strong possibly a little inconsistent but that could just be the video angle. Can't say I ever used a drill during that check so I only ever saw a few good sparks to know it was

I had a KX that nearly drove me insane with being difficult to start. After tearing apart the carb multiple times, replacing jets, replacing reeds, and convincing myself it had a bad coil it was ultimately that I didnt pay close enough attention to the choke circuit while cleaning the carb. I'm not positive but I think when that circuit isn't working you load up the carb via the pilot until it all pukes into the cylinder at once fouling the plug.

No way to know if it will work for you but that is what got my hard start sorted out.
decluk13
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Re: Compression?

Post by decluk13 »

I also put a small electric space heater blowing on the engine for 10 mins or so out in the garage if I'm going to run the bikes in the cold(40 degrees or less). Usually takes it from 5-6 kicks down to 2-3 on the kx or kdx
kdxsully
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Re: Compression?

Post by kdxsully »

The inconsistency might just be from the fact that the gap is so big. That spark is dancing around. You have spark. You have compression. You have fuel. I think this is a carb issue, if I have to give an answer. One time a little piece of rubber from my fresh cut fuel line got into the carb and clogged the pilot. I went through the whole electrical system convinced that something went really wrong, and it was just crap in the pilot.

Maybe you could put a dribble of gas in the cylinder and see if you get a pop
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kjdad
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Re: Compression?

Post by kjdad »

Just an update. I just went head and pulled the motor. Pulled the jug off and dumped about a qt of fuel on the floor. When the carb needle stuck it must have dumped a ton of fuel in. Since I'm this far I might as well fix a leaking seal..The journey continues
kdxsully
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Re: Compression?

Post by kdxsully »

Wow, glad you figured it out
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