Weird engine problem got me stumped

[maniac57]

Okay guys, I'm at a loss here...
The last two days, my bike has been acting very strange. It runs fine for about ten minutes, then begins overheating so badly it refuses to cut off when the kill switch is pressed! It loses power and sounds like the muffler is clogged! (it's not) The cylinder gets hotter than the exhaust!
Nothing was touched prior to this problem and it was running great before now.
I've changed the plug,swapped the head (no difference) the plug wire, disconnected the kill, checked the stator key (good) swapped the cdi and also the mag coil with no change.
The head looked fine with no strange buildup or anything that would cause a hot spot leading to pre-ignition, yet it actually diesels with no spark once it starts acting up.
I pulled the plug wire and rode it another mile today before it finally died!
I'm stumped. Like I said, the mag stator key is not sheared or damaged so I can see no reason for this. Everything in the engine LOOKS fine. What else could affect timing so badly?
What am I missing?
This is the first air cooled two stroke I have ever seen that was overheating without MAJOR timing issues or mechanical damage.
HELP!

 

[2door]

I was going to suggest carbon deposites glowing and causing diesleing but you say you replaced the cylinder head so that rules out my theory. How did the piston crown look?
What fuel to oil ratio and what oil?

I'm assuming a stock CDI and not one of those over priced aftermarket things with adjustable advance.
Any change in riding style or terrain or weight? Any possible obstructions of air flow over the cylinder and cylinder head?

Tom

 

[maniac57]

No to all the above. The piston crown looks fine. Light brown carbon deposits, nice and smooth with no jagged stuff to glow and cause dieseling..
Same conditions since last good ride. Three days maybe.
Same tank of fuel, same oil and ratio...
I just had an idea!
Would bad fuel possibly cause this? The fuel jug has been sitting outside so possibly it got contaminated or adulterated with something nasty somehow? I can't imagine how but like I said, I'm stumped! I'll check this in the AM.

 

[Goat Herder]

I know last winter when it was ridicoulasly cold out side. I said to my self that I had a ''forgiving'' excuse to play with my timing on one of my two strokes. Boy howdy even in the cold winter that pig got pretty hot fast!

 

[maniac57]

That's what I'm baffled by. How could the timing change so drastically from simply sitting in cold weather? When it first starts, it runs fine. No indication of any timing issues....until it goes wonky.

 

[Goat Herder]

Woundering if a CDI going bad could do somthing? Just A thought.

 

[maniac57]

Already swapped it out, no change.

 

[2door]

'Garage Grimlins' it's the only explanation.

Tom

 

[bairdco]

an air leak could cause it. did you re-use the head gasket when you swapped heads? take a good look for signs of the head gasket leaking, cracked intake manifold, carb, etc...

 

[maniac57]

I checked for carb leaks and everything looks good. No signs of any air leaks when it fires, no high idle, plug looks good, head is good and tight, no oil leakage showing. Both heads act the same so I don't think that's it.
I am baffled. It almost sounds like it has a cold when it starts doing this....No popping as revs come down, every rev gets ignition, just slowly loses power and begins to diesel. It's a VERY strange sound coming from a two stroke!

 

[a.graham52]

could it be a lean condition causing this? starving for fuel?

 

[maniac57]

Unlikely as the carb is still set rich for break in.

 

[bairdco]

i had a motor that dieseled and found a cracked manifold. reading up on it it seems the two main causes are coked pistons burning or airleaks.

check the base gasket maybe?

 

[BigBlue]

Clogged jet, air intake or muffler. If you left the fuel petcock on for an extended period, the muffler may have excess fuel clogging the muffler.

Moisture in fuel.

Chris
AKA: BigBlue

 

[scotto-]

Funky fuel or an air leak. Scrap it and install a 4-stroke?

 

[Goat Herder]

Pressurize the engine case and look for leaks. Plug off the intake and out take type deal. Grab some soapy bubbles. Move the crank while you check. Chance are you have a leaking crank seal? ''My first gut hunch from over here''

In the small engine shop I worked in we had a beastly two stroke Steel brand chain saw that was cooking it's piston. Could only catch the air leak whilst it was running. Was a crank seal leaking and only then. Was a fun one to stump the average Joe. Moving the crank around while it was under pressure found the culprit.

Dang shame, Happy Time ticking time bombs giving real two stroke engines a bad rep somtimes. I had a H.T. motor that stumped me in this manner once as well. Was the crank seal at the clutch side. That thing had a weird attitude and would act up when it was warmed up lol.

There is Chinese knock offs of the Steel brand weed eaters, blowers and chainsaws out now too. lol

 

[crassius]

yeah, sounds like it's going lean - look for crap in the carb blocking a jet

don't forget to pry up on each end of the crank since a slightly sloppy bearing may cause seal leaks only when running

 

[2door]

Just as an experiment, try riding the bike with the choke partially closed. Get it up to the speed where the trouble starts then pinch off the air supply, just a little, to see if things get better or worse.

I concur with the others in that it sounds like a lean condition. Restricting the air will richen up the mix and possibly show you that excessive air is the culprit.

How are the vibrations? Could you be seeing that infamous 'fuel foaming' issue?

Tom

 

[maniac57]

I've tried adjusting the choke when the problem first surfaced. Choke made no difference at all except full choke kills it until it gets hot enough to start dieseling. Throttle position, fuel valve, air cleaner cover, choke, exhaust pipe (I tried the old pipe just in case), NOTHING affects it!
I resealed the manifold to carb and manifold to jug, cleaned the carb completely (again) base gasket is good, seals look good, intake not cracked, head gasket looks fine.
I'm beginning to think more and more there's a fuel contamination issue.
Unfortunately I'm so broke right now I literally cannot afford a dollar or two worth of fresh gas to test this theory.*shakes head*
I'm gonna rob my friends lawn mower for some tomorrow.
scotto, much as I love big engines, I can't afford a four stroke build right now. The only reason I have a bike at all is the affordability of the chinadoll kits.
Not being able to work makes things very difficult. The only money I have coming in is from odd jobs and bike repairs, which are very slow this time of year.
Why do you think I'm so cheap?

 

[a.graham52]

does it run ok under throttle and just diesels when you go shut it off? i ask because i had a carbed truck and one time i set the idle too high, it dieseled then. grasping at straws here.