I didn't check the compression.  I put the carbs back on the bike with the screws adjusted to 3 turns and it ran...turned them back to 2.5 and it ran better.  I bypassed the frame petcock completely.  I rode her for a short bit today and eventually it seemed that it wasn't getting fuel. 

All in all it was a good day though...gotta get it tuned properly and start enjoying it.

Update:

Yesterday the bike was running like a brand new one with the mix screws set at 2.5 turns.  It idled so beautifully and the throttle was responsive.  Then the carbs got gummed up with rust from the tank.  Today, I took them off, took them apart and gave them a very good and thorough cleaning.  I then checked everything and put them back together.  Now, when the bike turns over the carbs are popping off again when it fires.

Because of the way it ran yesterday I am 100% certain that the problem lies somewhere within the carbs and not the timing or anything else like that but, for the life of me, I can't figure out what the problem is.  I am going to turn the mix screws to 2 and try...then 1.5.  Heck, I'm fast running out of ideas.

If you took your carbs apart again, did you re check the float heights? Also, what are your plugs looking like? They will tell you a lot...

Here are pics of the plugs.  Left and right respectively.  The carbon is dry.

Hmm, looks like you've been running rich to me, although there are plenty of other forum members out there who can be a lot more specific on how rich it looks and what to do about it... It is still hard to believe that mixture is the only thing popping your carbs off. I've heard of people backfiring hard enough to blow the tops of their carbs off, but never seen anyone blow the whole carb off the bike... You should get your mixture right, but also figure out why backfiring is causing the carbs to pop off... I'm still thinking you'll find a bad clamp or something else like that.

Well, I believe that the clamps are on their last legs but, just in the interim, I cut a couple of strips out of a soda can and put them under the clamps so that they'll tighten more.

I just turned the mix screws to 2 turns and popped the carbs off again.  ARGH!!! 

Ok, 1.5 turns and it runs beautifully.  Gotta clean out the tank and see if it will run on the tank.

Fuel mixture won't have any effect on blowing your carbs off.

I'm glad it's running, but I don't think you're out of the woods as to your carbs getting blown off.  I agree with The Mole.  I think we're on to the problem.  Your intake valves are open when they shouldn't be and the pistons are blowing your carbs off.  Are the heads new, or salvaged?  It sounds to me like the intake valves aren't seating properly.  There's nothing a carb can do to cause itself to be blown off the intake tubes.  The engine is doing it, and those plugs are FOULED, especially the left one.  The right plug looks fouled, and the left one looks awful.  Assuming the carbs were mixing right to begin with, intake valves not seating properly would contribute to carbon fouling, as you certainly have on those cruddy black plugs.  The fact that the color of your plugs don't match makes me suspect the heads you bought aren't up to snuff.  I wish you so much luck.  This is one of the weirdest problems I've ever heard of!  I've heard of this sort of thing happening to 2-strokes with broken reed valves, but a 4-stroke? never.  A carb is like a kazoo in reverse with gas being sprayed into it.  It's not going to pop itself off.  Could carbon be holding your intake valves open and now the carbon is reducing and the valves are closing enough to keep the carbs on the bike?  That's what I'm thinking right now.

Alright, carb gods, I have a new problem:
Yesterday I cleaned out the carbs and the bike ran like a champ.  I put over 5 miles on it around the neighborhood and it ran like a top.  Then I parked it to take my dog to the vet.  My brother and I were going to ride to my mother in law's house after the trip to the vet's. 

When I got back we fired up our bikes but mine was only running on the right cylinder.  We pulled the plugs and they looked ok.  Both were giving a spark but we could pull the plug wire off of the left plug and there was no change in the running.  It won't take throttle or anything.  The choke doesn't seem to make much of a difference either. 

I took the carbs apart this morning and gave them a once over...checked everything and put them back together and still...right cylinder only.  Any ideas?

I had a problem with a dead left cylinder.  It was a clogged pilot jet.  When you took your carbs apart this last time, could you see through the pilot jets?  Maybe your tank needs a good cleaning and/or inline filter?

Oh, and the float height measurement was  for me.  I had to go by the clear tube method.

Quote from: Paulcet on March 03, 2009, 10:28:40 AM
I had a problem with a dead left cylinder.  It was a clogged pilot jet.  When you took your carbs apart this last time, could you see through the pilot jets?  Maybe your tank needs a good cleaning and/or inline filter?

Oh, and the float height measurement was  for me.  I had to go by the clear tube method.

The pilot jets are clear, as are the mains.  Actually, I'm surprised that it even runs on one cylinder.  I've checked the float heights by the ruler method. 

Checked the diaphragm for pinholes?

Quote from: sledge on March 03, 2009, 12:32:22 PM
Checked the diaphragm for pinholes?

No, but how would a pinhole have gotten there with it sealed up in the top of the carb?
I checked the float heights with the tube method and both are really close to the same at the top of the float bowl.

Quote from: thousandlegs on March 03, 2009, 01:06:48 PM
No, but how would a pinhole have gotten there with it sealed up in the top of the carb?

Magic!

Ok, I just came in for a few.  Tonight, I'm going to break the carbs down again to check for anything out of the ordinary.  It's really not a big deal now...I've had them apart 1/2 dozen times now, lol. 

I am open to any additional info/help/suggestions.

My suggestion is check your valve clearances and figure out why you are developing so much pressure on the back side of the carbs to push them off.

Crank the engine over by hand, once the right piston(or left piston whichever one is not firing) is on the compression stroke simply put you hand over the carb boot and block it. As you continue to crank it over, if you feel air leaking out between your hand and the carb boot, well something is wrong.

What is the chance that you have an valve sticking open during the up stroke and that's blowing you're carb's off?

The carbs aren't coming off now.  I got new clamps and they're holding tight.  The issue now is that it's only running on the right cylinder.

I'm not a carb God, but I know it's a vacuum operated device and can't blow itself off as it has no ability to create positive pressure.  Whether it ran or not, I'm still thinking at least one of your exhaust valves isn't closing completely.  Now that the left cylinder isn't running at all, you may find your left exhaust valve is stuck open.  I hope it's not too open, or it'll hit the piston.  If that happened you'd know it!  I'm still thinking your new heads aren't up to snuff.

Quote from: bill14224 on March 03, 2009, 06:52:44 PM
I'm not a carb God, but I know it's a vacuum operated device and can't blow itself off as it has no ability to create positive pressure.  Whether it ran or not, I'm still thinking at least one of your exhaust valves isn't closing completely.  Now that the left cylinder isn't running at all, you may find your left exhaust valve is stuck open.  I hope it's not too open, or it'll hit the piston.  If that happened you'd know it!  I'm still thinking your new heads aren't up to snuff.

If he's getting pressure on the intake side, wouldn't that be an intake valve that sticking? I thought a sticky exhaust valve would lead to the exhaust being sucked back into the cylinder during the intake stroke, this seems more like the intake valve is open during the exhaust stroke, creating pressure on the intake side and popping the carbs off...

I also agree that new clamps probably just masked the problem. My understanding is that those clamps are there to prevent a vacuum leak, not to hold the carbs on the bike. I'm pretty sure you should be able to run the bike without the clamps in place. It would run super lean because of vacuum leak around the intake tubes, but the carbs wouldn't pop off...