TOPIC:

ewolpert wrote: My bike was running on Friday, now it is not. I had the bike running and was attempting to synch the carbs and I decided to give up for the day. It was running at a decently high rpm for a while, but I had done that previously. Now my bike wont start at all. I didnt change anything that would impact the bike not starting. Yesterday I took the battery in to make sure it was ok, and it was. I have changed the plugs, as the old ones were fouled from running rich. Yesterday I was able to get the bike to start up with the throttle open but it would quickly die. now I cannot get it to run at all.

After changing the plugs I decided, on a wave of frustration, check the compression. I dont know if my gauge broke, but it read 60psi on every cylinder (cold). .

I would try to lock that down first because if that is a true reading, the engine won't run at all.

This actually happened to me once after my rebuild. The gasket set was bad and the base gasket was disintegrating. After I turned it off and it cooled down, the base gasket let the cylinder shift enough that compression was leaking out past the head gasket. I actually bought a second compression gauge because I could not believe a newly bored and rebuilt engine was reading about 40 PSI.

I don't know what happened to yours but it won't run on 60 PSI.

No worries, sometimes we don't get the full story, so I took a punt. I re-read your first comments and appear to have misunderstood how this happened.

I doubt that overheating is the culprit, because of the even compression among the cylinders. Damage would likely not be so consistent to each and every cylinder.

Until leak down test results are known, would consider fuel supply and spark quality.

Are the spark plugs wet with fuel after an unsuccessful attempt to start the engine?

Perhaps water in the fuel, blockage in the tank, petcock or fuel line, or whatever might be preventing ample fresh fuel supply into the carb float bowls.
Would drain some fuel through the petcock, allowing the fuel to escape from the line just before it enters the carb.
Assure that fuel freely flows from each opened carb drain (which helps assure that each carb is receiving ample fuel supply, and may help flush the float valves).
Double-check fuel level in the tank.

Plug sparks should be continuous fat blue sparks.
Would measure battery voltage across the battery posts, and compare the voltage being recieved at the coil primary terminals (should be the same or only slightly less at the coils).
Would service the battery, charge it overnight, and have it load tested.
Would assure integrity of the connection where the negative battery cable attaches to ground (probably onto rear of the engine).

The goal is to assure strong plug sparks and correct air/fuel mixture into the cylinders.

Good Fortune!

"It was running at a decently high rpm for a while,"
Define "decently high rpm". Are you talking a high idle, or close to red line?'

you had no issues till you tried to synch the carbs? ........ you have the slides raised so high ( very high Idle).....low vacuum signal to the carb and choke circuits will not let the engine start/work.........

just my .02

MDZ, no not close to redline. closer to 4k and not sustained for more than 20 sec.

I attempted a "leakdown test." I use the quotes because I improvised. Rather than using a blowing tool as Patton suggested (although I did try that) I found I could use the spark plug adapter for the compression testing gauge which worked better. I did not hear any air leaking from either the intake or exhaust side, nor did I hear anything from the head gasket. I tried spraying a little water around to see if I could get bubbles to appear. Nada. I checked my compression gauge using my air compressor and it registered above 60 psi, so I can assume the gauge is not broken.

Now I need to come up with other possible explanations. I am pretty confident the fuel is not the problem. I have bench synched the carbs many times and the bike has always fired up. I am at a loss as to what could cause all cylinders to register virtually exactly the same compression reading when prior to today they were all closer to 100psi range with a deviation of about 15psi. In my opinion it has to be something that affects all of the cylinders simultaneously like the head gasket.

Perhaps a "Back to Basics"approach may work. To have the engine fire you must have fuel, good spark at the right time and compression.........looks like you are confident on the fuel aspect, I am not sure the WG coil mod is the best thing to do as it masks underlying electrical problems. You mention that you are getting a few weak arcs when you lay the plug on the head so I believe that may be your culprit, run through all your electrical connections making sure there is no corrosion, loose wires etc, check the points settings haven't moved at all............also make sure the kill switch is set to Run (sorry if this seems silly as it has happened before)

Your low vacuum readings and everything else suggests low compression. I'd like to see a pic of the plugs.
Steve

Might be time to try this...

Seriously, A blown head gasket is not going to affect all cylinders equally. It's not going to blow out the same amount on all 4 cylinders. Normally I'd say Cam timing might affect all 4 equally, but you didn't touch this area.
Some are going to cringe and tell you never to do this, but a quick shot of starter fluid may be useful in your diagnostics. You need compression, fuel and spark. I know you're confident you've got good fuel, but that's also the area you were working on when the bike quit. A quick spray of starter fluid will provide the fuel regardless of you a carbs conditon/settings. If you get a brief run with starter fluid, you'll know it's a fuel issue. If not, compression or spark.

Clogged petcock might be allowing ultra-slow filling of float bowls, which allows 20 seconds running before the float bowls are emptied.

Good Fortune!

MDZ, I'll give it a try. You are correct in that I was working on fuel when it went out. I suppose I should look into the spark thing again as I am not very confident about the condition of my coils.

Patton, I don't think it was the petcock as I was adjusting the float levels just yesterday and the carbs were filling fairly rapidly.

My compression started out fairly low on this bike (around 80-90psi). The bike has about 25k miles on it. Has it been anybody's experience that the rings and gaskets can be renewed without the pistons and boring out the motor? I don't want to spend $600 to get the motor running again. Or would I have to take it apart to know what I need? I would really hate to replace the rings and then find out I need to replace the pistons as well.

On a KZ650 25,000 miles low mileage. If the rings are shot (which I seriously doubt) it would be the result of serious abuse, neglected maintenance, or running with no air filtration.

If it was running ok before the rings didn't suddenly go kaput on their own. I believe something else must be the problem with it not starting. Did you try kickstarting it yet or bump starting it? Is the kill switch set to "RUN"? Ed