Maintenance & Technical > KX500 Original

K5 vibrates real bad.

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dirkster:
the cdi box quit on me so i took one off an old yz400 and it works pretty good, the bike was running great for most of the morning no more terrible vibration, been messing with jets and needle position all day to try and get the engine in tune. after a while the bike started to run rough, wouldn't hit powerband, or it would try but fail, sputtering and kicking. now its running hot but the plug is wet? too rich maybe. went back to the setup that was working the best, 176 main, 3rd notch down on the needle. still wouldn't hit powerband. kips valves are opening and closing properly, put in a new plug, cleaned the air filter. this bike is the most temperamental 2 stroke ive ever owned. i just dont get it runs fine for most the day and then it starts to run terribly.   

Foxx4Beaver:
still sounds like ignition/electrical problems to me...
One trick some of the sled guys use when they have electrical/ignition problem symptoms is, pull the spark plug cap off the lead wire, and cut a 1/4" off the end, then just re-install the cap. What happens sometimes, is the end of the lead/wire gets a little corroded, and by cutting off a little. you get a nice clean contact again.

you could/should do a leak down test, to be sure you don't have a bad crank seal...especially the stator side.

big green:
I had a very similar experience to what you are describing on my sons yz 250. The thing absolutely drove me nuts! Could not find the intermittent poor running condition much like you are describing on your bike. No matter what I changed as far as jets or float height or checking reeds and plugs or timing it did not matter when it was acting up it made no difference. Mine ended up being a loose connection on the wire going to the ignition coil. I had the bike apart several times trying to find the issue including this connection and never realized it. I know it has been stated here already but make sure ALL of your electrical connections are TIGHT. You never new when it would act up but it always would. Hopefully this will save you some hair as I am now bald. :-D Good Luck!

dirkster:
took the flywheel off again and saw that one of the wires from the stator, the signal coil if thats what it is called, had been against the crankshaft output and wore through to the wire. took the backing plate off and cleaned all mating surfaces and moved the wire aside where it wouldn't rub on metal and the the bike runs better now, still not running like it was a couple of days ago, decided to get a new stator since the one on the bike is a bit rough and a brand new one is $65 on ebay. Foxx4Beaver, I have a new coil on the bike and the crank seals should be good since i just rebuilt the engine at the beginning of this month with new crank bearings and seals along with the new top end. big green, i checked all connection on the bike and they are all tight and freshly cleaned.
 

Foxx4Beaver:

--- Quote from: dirkster on September 26, 2016, 02:20:16 PM ---the bike runs better now, still not running like it was a couple of days ago. crank seals should be good since i just rebuilt the engine at the beginning of this month with new crank bearings and seals along with the new top end.  

--- End quote ---

you still should do a leak down test, anytime an engine is rebuilt ...just because it has new seals, doesn't necessarily mean it's sealed up.
you mentioned it was running hot...that could indicate an air leak somewhere, causing it to run lean.

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