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Discussion Starter · #1 ·
Hey everyone, here's my problem.... I bought an 86 zx600a a few months ago. With the exception of all fairing pieces removed, and the carbs removed, the bike was, for the most part, together and not running. The guy I bought it from said it was running, but needed the carbs to be rebuilt. So I had the carbs rebuilt by a professional bike mechanic, put them on the bike, and it fired right up. Had a good idle, and everything seemed fine. So after letting the bike warm up for a little bit, I gave it a little twist of the grip, and the bike reved up like it should until about 4000 rpm ish, then it bogged the motor down to the point that it would almost stall, then it would idle fine again. This was all done with the bike on the stand and air box off of the bike. The way that it would bog, wasn't like it was starving for fuel, it was more like something was partially blocking the exhaust. It seemed like it would rev up to the point where there was just too much pressure in the cylinder for the piston to push the exhaust out.

Also; the bike sat for a few years before I bought it.

Then I read about these bike being know for the valves sticking to the head. So my question is - is this a result of maybe something building a nest in the exhaust? Or Is this a valve problem? Or is it something total different?

I just thought I'd post this before getting elbow deep ripping the exhaust off or something, and then finding out this is just something I have overlooked/didn't think of. So - thoughts? Any help is apreciated.
 

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Ok you can hold your hand behind the exhaust to check flow. If you have a good strong pulse then your system is probably not the problem. If you still are not sure you can remove the mufflers and check. It sounds like your ignition advance may not be working. But it could be something else too. Do you have a timing light to check the advance with? Also a good service manual is vital if you are going to be doing the work yourself.
 

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Discussion Starter · #3 ·
Yeah, there's good pressure coming out of the exhaust, but my theory was that if say, just one header or something like that was plugged, it would still have 3 cylinders pushing exhaust through. I do have a service manual, but I don't have a timing light. I'm sure I could borrow one tho. This will probably be my next step. Sounds like an easy diagnosis.

Any other suggestions?
 

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I am not sure how a header would get plugged in a sealed system with the only opening being the mufflers. Another thing to check would be fuel level which is easy to check with a clear hose attached to the carb bowl drains. Attach one end to the drain and hold the other end above the carb bowl and loosen the drain screw. The fuel level will show when the gas flows into the tube. But it would have to be pretty low to cut off at 4 grand. Also check for a plugged vent. The gas cap vents the tank and you can open it and run it to see if that is the problem. Also if the carb vents are stopped up the fuel will fill too slow for high rpm running.
 

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Discussion Starter · #6 ·
I don't think it's a fuel delivery problem, but anything is possible. I'm not ruling anything out just yet. I'll do my best to get a video uploaded of what this bike is doing. I'll post the link up here once I get it recorded etc...
 

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I'd put the airbox back on first. Then check the valves.
Good points. Overly tight valves will not seat at high rpms and the revs will fall off. I think it would take more than one cylinder doing this to kill it at 4000. A badly worn camshaft will also do this but I wanted him to check the obvious things first before worrying him about something as drastic as that.

I thought about the airbox but figured if it idled ok then that was probably not the problem. However some of these bikes came with ram air so not having the box on could have that effect.
 

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Discussion Starter · #11 ·
UPDATE: Ok guys, just got in from the shed. since i had the bike running, and discovered this problem, the bike has pretty much sat because it was a huge discouragement for me (about 2 months). So yesterday is when i started working on the bike again. So today, I went over everything minor quick, and went to start it up. well as soon as i started cranking, i noticed a fuel leak. Turns out the carb on cylinder 3 is leaking. I tried to start the bike anyway, figuring it would maybe fire, not the case.

So I pulled the carbs off and then decided to glance into the intake port just out of curiosity, and noticed a bit of carbon. So i cranked the engine over watching inside the port, and I saw bubbles forming around the valve from the excessive fuel put out by carb 3.

So I started pulling everything off the bike to pull the valve cover off, and thats where I think I have found my problem. I set the engine up to check my clearances, and found that every single valve had ZERO clearance. I couldn't even get a piece of paper in between the valve stem and the adjustment screw. I dont have any feeler gauges handy, so looks like that's on the shopping list. Along with new plugs.

Also, I noticed that the guy that had the bike before me, had broken the bolt head off of the one bolt that holds the steel coolant line to the cylinder head. It looks like the guy tried to like weld a nut to the top of it to try and get it out, but it looks more like an EPIC fail. Any suggestions on getting that out?
 
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