Won't start... out of ideas

[Yvan]

I've reassembled my engine after having to do major surgery.

Lots of big fat blue spark.

Good compression when turning over the engine. No leaks or hisses.

Carb has fuel, full bowl, not flooding (unless I hold down float bypass button), solid seal -- no vacuum leak.

I've tried every combination of full choke, half choke, no choke, no throttle, some throttle, WOT

No sputters or hint of combustion whatsoever.

What else is there?
The rings are fine or I wouldn't get good compression.
There aren't any valves.
Clutch isn't an issue, because I'm trying to start it at the crankshaft (yes, tried the old fashioned way too).
I've tried another carb -- both have worked fine in the past.

 

[butre]

is your magnet on right? magnet on backwards usually won't start. your crankcase compression is good too, right?

 

[Deleted member 12676]

I second butre's suggestion of checking the crank seals.
that's also very true about the magneto.
once you get the engine too wet it just won't start. You may even have a pool of fuel in the crank.
take the plug out and push start it. but the ignition has to be dead to keep from damaging the high voltage coil. disconnect one of the wires going to the CDI first. that will dry out the engine a bit.
then put the plug back in and hold the throttle wide open and push start it and have a friend spray in starter fluid into the filter while the engine is turning over. best to do this on a downhill. once it starts sputtering then return the throttle to a minimal open position and see if it keeps trying to run. then see if its better or worse with the choke on. (make sure you are certain which way the lever moves to choke it)
Let us know what happens

 

[Yvan]

Thanks for the tips. I don't think it is the magnet, because the engine has run fine in the past, and I haven't touched the ignition system.

I removed the head over the weekend and everything looked OK, but I guess there is no way of telling if there is fuel in the crank without splitting the case, so I'll try drying it out like you suggest and then trying starter fluid.

 

[Yvan]

Still no luck

• I spun it quite a bit with the plug out and the carb off to dry it out.
• Then I let it sit up-side down for a few hours.
• I checked the magneto. It is a 1 o'clock at TDC.
• I regapped the plug. Gobs of spark.
• I removed the head/cylinder and shot lots of compressed air in the bottom of the crank.
• I tried starting fluid.

Not even a sputter.

It's like gas stopped being flammable. Oh, I tried fresh gas too.

I noticed that the spark plug boot leaks a little. If I hold it, I get little shocks that feel like sliding over a carpeted floor and touching metal. I'll fix that, but it doesn't explain anything.

I have to say, I don't believe peoples' descriptions of spark plugs that fire outside of the cylinder but not in it. I mean, physics doesn't change in the dark; but something is weird.

 

[Deleted member 12676]

with pressure it takes more voltage to spark the gap which is why it can spark outside of the engine but not in it.
Disconnect the stator coil wires from the CDI and measure the AC voltage when pushing the bike. It should be at least 30 volts. If not then replace it.

 

[Yvan]

Hmmm. I'm getting about 26VAC under load; 75VAC no load.

 

[Yvan]

How does one measure/evaluate crankcase compression?

 

[Yvan]

What gap seems to work well with the stock plugs?

 

[crassius]

one can judge the integrity of the case by examination of seals and all the seams of the case - anything crooked or seeping oil may be causing a loss of compression

gap at .028in is usually good

I'm beginning to suspect that "having to do major surgery" may be the cause - do you have a repair guy in the area?