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Discussion Starter #1
Hi All,

I rebuilt a donated 1985 KTM 250cc MX Stroke from the bottom up and got it in pretty good shape, including new parts from Austria. I have some remaining issues and need guidance.

1. The bike with not start unless I pull the spark plug and put a bit of fuel and then when I kick it over she starts right up. I am feeling like there is a crack or something reducing compression on the motor. I retorqued the head nuts because I saw a bit of oil building up on the front of the motor while it was running.

2. This one may be part of the issue in #1, but it bogs at the top end of the power band and smokes like heck.

3. I am feeling like the best thing to do is do an engine swap because this one may just be beyond rescue. However the bike is complete and in great condition so the engine is what I would need. The down side is that you cannot get the 250cc replacement for the 1985 MX anymore. Is there a 2 stroke that will swap right into it or even better a 4 stroke?

If anyone has a motor that runs and will swap into this I will gladly pay you Tuesday for the Hamburger today. JK... If the price is reasonable and the motor in good shape I am open to your offer. I am in Reno NV USA so I suppose folks in the U.S. would be the ones that might be more able to help me out.

Thanks in advance for any info or offers.

Have a great day.
 

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Hi Mook,

Congrats on the donor bike, that's an awesome deal!

When you rebuilt it, did you do anything to the motor? (i.e. split the case, rebuild top end)
How is the carb? Did you open it up, clean it, and check the jetting to make sure your jettted for your environment?

The oil build up, where is it exactly? I ask because on my 2000 ktm 250exc the powervalve covers will leak black splooge past the gasket, so just wanted to see if it was coming from there and leaking down. When you have the bike started, spray some carb cleaner around gasket to see if the engine RPM changes. This would indicate a leak if it does.

When you mentioned it smokes like heck, is it white smoke? blue?
- Check out my issue when I was smoking like a fire:
Does yours smoke like that? Mine was because I was sucking transmission fluid into the crank. My issue was the case gasket inside, separating the crank and the transmission, was letting oil through. I ended up splitting the case twice. The first time I just used a new gasket and a thin layer of Hondabond as usual but it didnt fix it. So I split it again and added a thicker layer, let it cure a little before tightening the case so it doesnt just squish out. That did the trick.

This is after the fix.

How to test if your sucking transmission fluid into the crank? Start the bike with no transmission fluid but dont go riding! If it doesnt smoke like crazy, then I'd suspect the case gasket. This is how I determined I was sucking in transmission fluid.

Keep us updated!
 

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Discussion Starter #3
Hi Mook,

Congrats on the donor bike, that's an awesome deal!

When you rebuilt it, did you do anything to the motor? (i.e. split the case, rebuild top end)
How is the carb? Did you open it up, clean it, and check the jetting to make sure your jettted for your environment?

The oil build up, where is it exactly? I ask because on my 2000 ktm 250exc the powervalve covers will leak black splooge past the gasket, so just wanted to see if it was coming from there and leaking down. When you have the bike started, spray some carb cleaner around gasket to see if the engine RPM changes. This would indicate a leak if it does.

When you mentioned it smokes like heck, is it white smoke? blue?
- Check out my issue when I was smoking like a fire:
Does yours smoke like that? Mine was because I was sucking transmission fluid into the crank. My issue was the case gasket inside, separating the crank and the transmission, was letting oil through. I ended up splitting the case twice. The first time I just used a new gasket and a thin layer of Hondabond as usual but it didnt fix it. So I split it again and added a thicker layer, let it cure a little before tightening the case so it doesnt just squish out. That did the trick.

This is after the fix.

How to test if your sucking transmission fluid into the crank? Start the bike with no transmission fluid but dont go riding! If it doesnt smoke like crazy, then I'd suspect the case gasket. This is how I determined I was sucking in transmission fluid.

Keep us updated!
Great response nd some things I didn't realize. I did a top end rebuild. Person before me said they couldn't get it started because of low compression. After I pulled the head, jug and replaced the piston with new rings, I cleaned off the gallons of Red RTV they poured on everything. Installed actual gaskets, used new crush washers and torqued to specs per owners manual. Lol... "Newton Meters". I didn't get into the bottom because it didn't looked like it was messed with and the piston rod was solid so the lower bearing didn't scream changing right now.

The carburetor is brand new because I took apart the original and found a bunch of missing parts nd some damage from a previous owner. The floats had one side broken off, jet screens were missing and much more. So I bought new.

It smokes like that but is blue so that tells me it may be the oil in the fuel, but I mixed it 40:1 and it must be the fuel air adjustment being screwed up. It smokes up the neighborhood when it is running. Good thing I am in the country.

The smoke was also coming from the front at the exhaust connection also. I think that is where the oil came fron. The collar that the exhaust lays onto and held by springs was aligned incorrectly and didn't seat up. Because that happened it seems to me that it is not burning all the oil out of the fuel and passing through thw exhaust. It was pretty black oil.

I habe a new Air-Cooled 2-Stroke with electric start and never been started yet. The problem is the cog for the chain is on the opposite side. I have no idea if I can flop that all in the bike to the other side and swap it out. That would allow me the ability to ride the bike and do a proper teardown and bench rebuild. What format do videos habe to be in. I hve been trying to upload and it doesn't like Mp4.

Any thoughts?
Hi Mook,

Congrats on the donor bike, that's an awesome deal!

When you rebuilt it, did you do anything to the motor? (i.e. split the case, rebuild top end)
How is the carb? Did you open it up, clean it, and check the jetting to make sure your jettted for your environment?

The oil build up, where is it exactly? I ask because on my 2000 ktm 250exc the powervalve covers will leak black splooge past the gasket, so just wanted to see if it was coming from there and leaking down. When you have the bike started, spray some carb cleaner around gasket to see if the engine RPM changes. This would indicate a leak if it does.

When you mentioned it smokes like heck, is it white smoke? blue?
- Check out my issue when I was smoking like a fire:
Does yours smoke like that? Mine was because I was sucking transmission fluid into the crank. My issue was the case gasket inside, separating the crank and the transmission, was letting oil through. I ended up splitting the case twice. The first time I just used a new gasket and a thin layer of Hondabond as usual but it didnt fix it. So I split it again and added a thicker layer, let it cure a little before tightening the case so it doesnt just squish out. That did the trick.

This is after the fix.

How to test if your sucking transmission fluid into the crank? Start the bike with no transmission fluid but dont go riding! If it doesnt smoke like crazy, then I'd suspect the case gasket. This is how I determined I was sucking in transmission fluid.

Keep us updated!
I tjink I found what the problem may be. I retarded the timing as it was too far advance I was getting kickback on the kickstart half way through my starting stroke. I am going to advance it a couple of degrees and see if that helps. The oil may have been jnburnt fuel coming through the incorrectly seated exhaust as well as all thw smoke. In turn way too rich. I will update once I handle that. What format of videos are allowed in this forum?

Thanks all.
 

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Discussion Starter #4
Hi Mook,

Congrats on the donor bike, that's an awesome deal!

When you rebuilt it, did you do anything to the motor? (i.e. split the case, rebuild top end)
How is the carb? Did you open it up, clean it, and check the jetting to make sure your jettted for your environment?

The oil build up, where is it exactly? I ask because on my 2000 ktm 250exc the powervalve covers will leak black splooge past the gasket, so just wanted to see if it was coming from there and leaking down. When you have the bike started, spray some carb cleaner around gasket to see if the engine RPM changes. This would indicate a leak if it does.

When you mentioned it smokes like heck, is it white smoke? blue?
- Check out my issue when I was smoking like a fire:
Does yours smoke like that? Mine was because I was sucking transmission fluid into the crank. My issue was the case gasket inside, separating the crank and the transmission, was letting oil through. I ended up splitting the case twice. The first time I just used a new gasket and a thin layer of Hondabond as usual but it didnt fix it. So I split it again and added a thicker layer, let it cure a little before tightening the case so it doesnt just squish out. That did the trick.

This is after the fix.

How to test if your sucking transmission fluid into the crank? Start the bike with no transmission fluid but dont go riding! If it doesnt smoke like crazy, then I'd suspect the case gasket. This is how I determined I was sucking in transmission fluid.

Keep us updated!
Ok. Advanced the timing a bit and it is smoking like a 2 stroke should. Still have 2 issues though. Idle it very high and it will not adjust down. It will not start still without pulling the plug and priming the cylinder with fuel.

Maybe it is needing a hair more advancement?

I feel like this is all related to dialing in the timing with the aftermarket ignition rig.
 

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Getting there! Thats great the smoke is blue, leaning towards the carb settings. What is your sea level and what
Pilot, Main, and Needle are you running? Did you use what came with the carb?

Ahh yes, the exhaust to cylinder connection. Sounds like this the culprit. I usually add a good amount of copper RTV in the exhaust lip in addition to the o-ring to give it a good seal. I have a carbon fiber protector on mine so instead of dents, I get a bent pipe. I rode like this unknowingly until a the next top end and saw the misalignment.

For the videos, upload to youtube and just copy the link into the forum. I dont believe the forum has their own hosting.

The engine donor. Sounds like it might need some custom fabbing, relocating the chain guide and brake mount, is my guess.
 

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Discussion Starter #6
Getting there! Thats great the smoke is blue, leaning towards the carb settings. What is your sea level and what
Pilot, Main, and Needle are you running? Did you use what came with the carb?

Ahh yes, the exhaust to cylinder connection. Sounds like this the culprit. I usually add a good amount of copper RTV in the exhaust lip in addition to the o-ring to give it a good seal. I have a carbon fiber protector on mine so instead of dents, I get a bent pipe. I rode like this unknowingly until a the next top end and saw the misalignment.

For the videos, upload to youtube and just copy the link into the forum. I dont believe the forum has their own hosting.

The engine donor. Sounds like it might need some custom fabbing, relocating the chain guide and brake mount, is my guess.
I am at 5,000 feet in elevation in the Sierra Nevada Mountain Range. The needle and jet(s) are stock to what was in the new carburetor. Any suggestion to size of jet gor a bing 54? I am not sure what groove the needle should be set at either.
 

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Discussion Starter #7
Attached is the video of the bike running, white smoke and it looks like I have an exhaust leak I cannot seem to locate.

Now that the timing seems to be dialed in and I have the air screw is basically only open about 1/2 turn and it seems I have the full power band to wide open throttle.

Now I see why she may not be starting without adding a bit of fuel down the spark plug hole. I am a little color blind, but I assume that is white smoke now. It looks like I may have a crack in the Jug and water is getting into the cylinder, at least I hope that is where it is or one of the new gaskets blew out. It may even explain the high idle like engine do when they raise idle at the time of running out of fuel then they stop.

I suppose I am taking the top end apart again. I would like to find some information on a re-jet the carb for 5k feet though.


1985 KTM 250cc MX, Smoke issue
 

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I am not familiar with the older 2 stroke KTM's, but have 12 years of fixing my son's 2 stroke sx bikes. So some questions:
  • Does this bike have reed valves? If yes, then make sure they close properly and they have no damage, as the 2 stroke KTM's rely heavily on them being all OK.
  • Revving w/o using throttle could also be an air leak between the carbi and the inlet to the motor. To check if this is the case start the motor and spray Ether on the inlet. If the revs change when you do this you have a leak where the motor can suck in air.
  • I watched the video. White "smoke" is steam from water getting in to top of piston. Not sure what type of head gasket your bike has, but all the 2 stroke KTM's I have worked on have an O-ring as head gasket. Tighten the head bolts to same torque. (whatever your bike requires). I have also seen cylinder heads with cracks that allows water in to the piston. Also check that the cylinder head sealing area is even and flat. You can test this in different ways, but I normally put the head on a piece of window glass to check the head's sealing.
 
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