Left 2 cylinders running hot

What could cause some cylinders to run hotter than others?

Could it just be the fuel/air mixture? I'm wondering if the carbs aren't balanced that it could cause this problem, but I'm not sure.

I don't think it could be a valve issue, but again, I'm not sure.

*Working on a 1984 GPz550

Speaking balance, did you perform a carb sync? Fully clean the carbs including the pilot circuits? How's the compression? Valve adjustment all in spec?

Since you say it idles fine in the other thread on this topic, I suspect the valves are fine.
www.kzrider.com/forum/2-engine/614070-gp...uggling-when-in-gear

It sounds like a main circuit problem in the carbs, or maybe a diaphragm has failed and so the slide does not raise up. You said it was running fine for 10 hours or so. Was that before the carb work or after the carb work? Did the problem show up suddenly or gradually?

As I said in the other thread, the sync can cause the temperature issue, but it won't cause the higher throttle opening problems. Sync imbalance disappears when you open the throttle.

Nessism wrote: Speaking balance, did you perform a carb sync? Fully clean the carbs including the pilot circuits? How's the compression? Valve adjustment all in spec?

I cleaned the carbs by removing all the jets, diaphragms, gaskets, floats etc and soaking them in pinesol/water mixture for a day. Then I rinsed them and cleaned them with carb cleaner and compressed air. I did not remove the carbs from the rack, however. I replaced the float bowl gaskets and reassembled the carbs. I then bench synced the carbs.

Compression is great - it is 145psi at each cylinder. I didn't take the cam cover off to check the valve clearance because in neutral, the engine doesn't misfire at idle or when holding the throttle open to 4000rpm.

loudhvx wrote: It sounds like a main circuit problem in the carbs, or maybe a diaphragm has failed and so the slide does not raise up. You said it was running fine for 10 hours or so. Was that before the carb work or after the carb work? Did the problem show up suddenly or gradually?

As I said in the other thread, the sync can cause the temperature issue, but it won't cause the higher throttle opening problems. Sync imbalance disappears when you open the throttle.

It happened after the carb work and it happened suddenly.
In first gear you can get through the bogging by giving it more throttle. In second gear, that doesn't work as well.

I can take the carbs off again and check the diaphragm and the jets and everything. I wouldn't expect anything to be dirty or clogged because I had the tank cleaned and coated by a mechanic friend of mine.

145 psi is very low for a Kz550 or Zx550. Every Kz550 and Zx550 I've tested is between 175 and 200 psi. The only time I've seen compression that low was with a procedural error or defective tester. Most testers I've seen lately are made wrong and give an artificially low reading. But also, not holding the throttle wide open can give a low reading.

I suspect your engine is fine, so no need to worry about it. It ran fine before the carb work so the current problem is most likely carb related.

The diaphragms should not have come into contact with any chemicals correct? They only get water and maybe mild dish detergent or soap to clean them.

Ya know, I don't recall if I had the throttle open when I tested compression. Just as a sanity check, I will test again tonight. I didn't realize 145 psi was low, I thought that was normal. I also thought that the delta between cylinders was more important. Thank you for that insight!!

The diaphragms have not come into contact with any chemicals. Like you mention, I have only rinsed them with water - never even any detergent.

I will remove the carbs though and give everything a thorough inspection.

I'm still a little confused why the engine feels hotter on one side compared to the other.

Valve lash is perfect, float heights are perfect, no air leaks, etc... I double checked the compression on a warm engine and it read 150-155 psi on all cylinders.

These are my spark plugs:

This is from cylinder 1



This is from cylinder 4



They both seem to be in ok to lean condition, but that they don't look exceptionally different makes me more confused why one side of the engine is hotter (cylinder 1 & 2 side) than the other.

175 psi would be roughly 12/1 compression ratio. That sounds high to me for a KZ engine.

If a cylinder is running hot I would suspect a lean condition. If it happened suddenly I would go looking for dirt in the suspect carbs.

175 psi is on the low end of what I see on Kz550's. Every 550 I've run compression on over the last 25 years or so has ranged from about 175 to well over 180 approaching 200. That is with using probably more than 4 different compression testers across a dozen bikes or so. The only time I've seen it much lower was when someone does the test incorrectly or uses one of the screw-in type gauges with the check valve in the wrong place. That is on 10:1 gpz engines and 9.5:1 Kz engines alike.

The factory spec is 145 psi to 220 psi (page 172, white base manual)

That's strange....pressure and volume vary inversely. If you cut the volume in half (compression ratio of 2/1) you double the pressure. Cutting the volume by a factor of 9.5 results in a pressure increase of times 9.5.

A compression ratio of 9.5/1 results in a theoretical maximum compression of 9.5 x 14.7 = 140 psi, assuming the cylinder at BDC is fulled with air at atmospheric pressure. 14.7 psi is normal atmospheric pressure at sea level.

My last check on my wife's KZ550 was 125/110/115/120 I thought that was OK.

That does not take into account the fact that the valves do not close at BDC, which makes the pressure lower, even more so with more radical cams.