Spark, But No Start (kind of)

The Bike: A 1980 KZ750 4 Cylinder

The Problem:

When I got the bike home, it wouldn't start. It would crank and crank and crank and try to start. But would't wouldn't quite go. I took all the spark plugs out, and they all have a decent spark. I also took the carbs off and cleaned them. (even though they were already pretty clean)

After I put the carbs back on, I could get it started after a few cranks and the throttle held wide open. I noticed that only the two right most exhaust pipes got hot (cylinders #3 & 4?...the two on the left were still cold. I tried swapping the spark plug wires on the left two, but they still didnt work.

Where do I start trouble shooting, considering all the plugs have spark?

If you have spark on all 4, it's most likely a carb issue. Make sure you coils are hooked up right too... one coil does 1 & 4, the other 2 & 3. If #3 and #4 are firing, I bet the pilot crcuits on carbs #1 & #2 are gunked up. Did you pull all the jets out (includeding the pilot jet under the plastic plug) and verify all the pilot pasages are clear? These carbs have quite a few small passages and if they are clogged, it won't run well. I'd also take your pilot mixture screws out and verify the passages to the throat of the carb are clean and the three "transition" holes are also clean.

If you look at a few of my more recent posts, you'll see where I had some carb issues with my '80 750 4 cylinder.

Also, have you made sure your valves are adjusted properly?

I cleaned everything when I took the carbs apart, including the jets under the plastic caps.

I have not done anything other than that yet. I will probably be tearing the engine apart at some point here to replace a few gaskets anyway, so I will check the valves when I do that probably.

I will verify the spark plug wire arrangement when I get home.

I don't know if you tried it, but the normal starting procedure for a cold engine is choke on full and no throttle. Ed

I have actually tried that...

One other thing... before you stick to much cash into this bike, I would do a compression check on all cylinders.
If I was you, I'd take the carbs off, and take out all the jets, air/fuel mixture screws, diaphragm slides and ensure that your passage from air box side to pilot jet to passageway that leads to the transition holes and pilot holes are clear. With the carbs off, do a compression check on the motor. Once done with this, put back together. make sure that the vaccuum ports on your carb manifold boots are also either plugged or hooked up to your petcock or air emission system. If all that passed the test, I'd try spraying some carb cleaner around your carb boot/carb area to check for vaccuum leaks. My #2 wouldn't fire at idle and found out I was getting air in btween the carb manifold boots and the head. Sealed it up and bike ran completely different.

I also just thought of this... make sure your air/fuel mixture pilot screws are about 2 turns out from lightly seated. Too rich or too lean could cause it not to fire. I'd also take a peek to make sure your butterflys are somewhat equal to one another, if not performa a bench synch. I then set my idle knob so that just the first of the 3 transition holes are visible. This seemed to be a good starting point for me to fine tune them.

Check out this thread... might find some useful stuff.
kzrider.com/index.php?option=com_kunena&...ew&catid=3&id=382252