The 10+ year KX500 rebuild thread

[derek badger]

Looks Ace Mr Badger. have you tried swapping the HT leads over? Its a common mistake. That and the cam timing.
If its not that, it could be that you've installed the wrong power band.

HTH.

Naah, I reckon its the starter motors too old and knackered.

100%

[derek badger]

Small update.

Fueling is still not right. The carb was either flooding or not giving fuel. I stripped and cleaned another Mikuni VM I had (38mm rather than 40mm) but it still did the same. Kicked over, runs for a second then stops. Kick again, runs for a second then stops. Repeat until a sweaty mess. I then noticed the joint between the silencer and the expansion chamber was dripping burnt two stroke oil. I took the pipe off to find it was full of premix, this was never going to help things and it was probably being sucked back into the cylinder.

I've had the reed valve assembly off and pumped the crank case out again. I've left the whole intake assembly off and the piston at TDC for a few days and went back to looking at the carb. I've gone back to the VM40SS and took the needle valve assembly out, but it doesn't match the one from the Mikuni parts diagram, its a push-fit with o-rings rather than a screw fit with sealing washers. However this type of fuel valve is used on their TM flatslide, so I've ordered a new one from Allens Performance and I'll set the float height with a vernier and try again when it arrives.

Two things to come out of this are that a) I'm sure the ignition is okay now and b) I had a horrible feeling the crankseal had gone after being sat for so long as the fuel I pumped out of the crank case was kind of a murky black, but I'm pretty sure this from the fuel that was washing about in the exhaust. I never decoked it during the original strip down/rebuild. The gearbox oil level has also not changed or discoloured.

[derek badger]

Fucks sake. This is doing my head in now. Opinions please...why isn't it running? Plug is coming out a touch wet ever now and then, but this is all it will seem to do. Weak spark maybe?

[KungFooBob]

Have you tried a new box fresh plug?

[Taipan]

Am I right in thinking the engine has been overhauled? Crank seals deffo okay? What is the ignition type? Not one of those ones with the old fashioned stator plate that could move is it?

[demographic]

Check the coil, and make sure the woodruff key isn't sheared and the flywheel is way off and altering the timing.
I lapped the flywheel onto the crank end with grinding paste with mine so its a better fit and less likely to slip.
The timing on mine can be altered by moving the stator backplate, after removing the flywheel.
Its only got a few degrees of movement but if the flywheel has slipped you could be a long way out.

If the engines flooded hold the throttle open a bit when kicking over to weaken the mixture (although throttle open allows much more air in and you have to compress that with yer foot so its a double edged sword) but if its not flooded then just use choke and don't hold the throttle open. Thats nicer to kick over and the choke circuit works better that way.
Thats with mine which has a Keihin PWK 39, I'm not sure but thought the Mikuni doesn't have as good an idle circuit? Not sure about that though and it could be the Keihin PJ carbs I'm thinking of.
Plug gap right?
Could the choke be blocked? Not very likely admittedly and more likely to be a blocked pilot jet as those feckers are tiny.

If it were a fourstroke I'd easy start the shit out of it but I'm a bit wary of that on a premix twostroke as its not got oil in it.

[derek badger]

The fact that it fires once means it cant be that weak
Is it a big fat blue spark or a weedy yellow one?
Is the engine earthed properly (if its mounted on rubber bushes you need a proper earth, a wire from battery -ve to a casing screw will do for testing)
Blocked pilot jet? If you squirt some easy start / lighter fluid down th einlet does it fire more than once?
Is the ignition adjustable, could the timing be out

Carb has been stripped and cleaned twice. Jets are good and the correct size. I've stripped the paint from the coil mount and the top engine mount. There are no rubbers mounts.

Spark is blue, but it's not exactly fat, but the iridium plug has a very small electrode.

Easy start makes no difference.

Have you tried a new box fresh plug?

Yes. Standard and Iridium. The Iridium one fires better both are the same grade.

Am I right in thinking the engine has been overhauled? Crank seals deffo okay? What is the ignition type? Not one of those ones with the old fashioned stator plate that could move is it?

Engine was rebuilt 15 years ago and not been run since. I too wondered about the crank seals, the gearbox oil level hasn't changed or discoloured.

Ignition is CDI with a new (15 years ago) stator plate/ignition coil/hall sensor/whatever you call it. I've tried 3 coils on it. I've checked the resistance in all and they vary, but are in the ball park. The ignition coil on the stator plate, while being new doesn't read right though. A new one has been ordered.

[derek badger]

Check the coil, and make sure the woodruff key isn't sheared and the flywheel is way off and altering the timing.
I lapped the flywheel onto the crank end with grinding paste with mine so its a better fit and less likely to slip.
The timing on mine can be altered by moving the stator backplate, after removing the flywheel.
Its only got a few degrees of movement but if the flywheel has slipped you could be a long way out.

If the engines flooded hold the throttle open a bit when kicking over to weaken the mixture (although throttle open allows much more air in and you have to compress that with yer foot so its a double edged sword) but if its not flooded then just use choke and don't hold the throttle open. Thats nicer to kick over and the choke circuit works better that way.
Thats with mine which has a Keihin PWK 39, I'm not sure but thought the Mikuni doesn't have as good an idle circuit? Not sure about that though and it could be the Keihin PJ carbs I'm thinking of.
Plug gap right?
Could the choke be blocked? Not very likely admittedly and more likely to be a blocked pilot jet as those feckers are tiny.

If it were a fourstroke I'd easy start the shit out of it but I'm a bit wary of that on a premix twostroke as its not got oil in it.

Flywheel and woodruff key are fine. When I'm trying to start it the choke is on and I'm holding the bars rather then the throttle. Plug gap is fine. It's almost as if it's trying to run but as soon as it starts sucking fuel through the carb it dies. Hence my thought about a poor spark as the plug has been damp after 5 minutes of kicking it.

I hope to Christ it isn't the crank seals...

[demographic]

Do you have a manual that shows the resistance of the coil?
I have if you need me to ratch the numbers out.

[Skub]

I've had strokers that wouldn't kickstart,but did with a bump/tow. At least one occasion it was down to a weak coil.

[Taipan]

Chokes are certainly not unknown to cause issues. Its deffo working alright and not sticking on or suchlike? I'm not familiar with these bikes though and dont know how critical choke operation is on them?

re: crankseals, isn't there a leak down test you check with? If its never run since rebuild, then there wont really be any oily giveaways.

Have you kicked it over in the dark and got someone to watch for errant blue flashes? Hopefully the new coil will sort it?

When are you getting chain and sprockets on. Tow starts pump everything how it should be and you can play with throttle openings etc.

[derek badger]

Do you have a manual that shows the resistance of the coil?
I have if you need me to ratch the numbers out.

I've got the Clymer and the Kwak service manual, but cheers fella.

[demographic]

Do you have a manual that shows the resistance of the coil?
I have if you need me to ratch the numbers out.

I've got the Clymer and the Kwak service manual, but cheers fella.

Even just making sure all the points where the wires or the coil connect to the frame are free of paint can help.

Its obviously firing for a few seconds mind.

[derek badger]

OEM coil and new plug cap ordered.

[KungFooBob]

For £500 plus expenses, I'll come and set it on fire.

[derek badger]

For £500 plus expenses, I'll come and set it on fire.

Knowing my luck it probably won't light...

[Skub]

The KX is just being a tease,it wants a lengthy thread to entertain us all. You'll get there man.

You'll be posting in the 'Where/when did you last ride your bike' thread before you know it.

[derek badger]

The KX is just being a tease,it wants a lengthy thread to entertain us all. You'll get there man.

You'll be posting in the 'Where/when did you last ride your bike' thread before you know it.

I might even get to start a "Post a picture of the hedge have you wheelied manically though" thread.

[derek badger]

Sprocket arrived from Talon. I had to go down to a 13 on the front to get the best chain fitment, this gives a final drive ration of about 3.31.





And no, before you ask. It's still not running...

[Skub]

You'll be able to bump it/tow it now.

Vids pleeze.