Kawasaki Z650 Exhaust Stud Bodge-Fix

I've had a problem with my Z650 recently; it seemed to only be running on 3 of the 4 cylinders.

Here's the story so far - sorry for the lack of photos in the initial bit, I was thinking about finding the problem at that stage rather than making a Hive post of it !

Identifying The Misbehaving Cylinder

The way to test this is relatively simple. Start the motor, run it just for about 10-15 seconds, and turn it off. Then flick a little water at each of the exhaust downpipes. If the water immediately disappears in a hissing flash of steam, it's all good. If it just sits there, it means something is wrong because the downpipe is too cool.

It is important to avoid actually touching the exhaust when you do this ! The pipes are easily hot enough to give you a very serious burn. Also, melted-on skin is really hard to remove from the chrome....

In this case, the pipe for cylinder number 2 was cold, which told me that was where the problem was.

Eliminating Possible Causes

My first thought was that the pilot circuit in number 2 carburettor was blocked yet again. So I took the carbs off, gave them a good clean and tried the engine again. No joy, still the same.

The next possibility was ignition issues. I pulled off each HT lead in turn and used a spare spark plug pushed into the cap and earthed against the head while kicking the engine over. In each case, the spark seemed healthy.

Then I remembered the test I should have done right at the beginning; running the motor for a few seconds, turning it off and checking the condition of each spark plug. Sure enough, number 2 was wet with fuel, the others were dry but a bit sooty from running with the choke on.

At this point, I was thinking the next thing I'd need to do would be to open the engine up, check the valve clearances and maybe start digging deeper into the valve gear.

Problem Found !

That was when I noticed something unexpected was wrong. The exhaust collar for cylinder number 2 was out of place. Closer inspection showed that the lower stud had snapped off halfway down, as shown on the photo below.

Fixing The Problem

The "official" solution to this is to remove the exhaust stud and replace it with a new one. But I knew this would be a major issue. The studs have been in the cylinder head for nearly 50 years, so galvanic corrosion means it's not a job I can do myself, I'd have to take the cylinder head off and take it to a specialist.

I didn't take a photo unfortunately, but the stud wasn't snapped off at the base, it was just the nut and the end of the stud that were missing. But it was too short to fit another nut and washer.

Then I realised that there might be an alternative solution. Lateral thinking is a wonderful ability when you work on old bikes !

I used a spare exhaust collar for this. It's the later design which is nowhere near as pretty as the early design, but is also far easier and cheaper to find on eBay ! Early design ones are rarer and far more expensive !

What I did was to file a notch in the collar large enough to take a nut and washer and deep enough to allow enough of the broken stud to poke through. It took about 3 hours of careful filing, but the photos below show the results.

Yes, I really did use the micrometer to check that the slot was an even depth throughout !

It may not look like it, but the two designs of collar are exactly the same height normally.

Below is the modified collar fitted back onto the bike. Close up photos always make it look grubbier than it really is !

While the number 2 downpipe was off, I took the opportunity to give it a good polish, as I could get at areas that are normally impossible to reach. The only problem is that now I want to do the same for the other three pipes !

Conclusion

So all in all it's a bit of a bodge solution, but it should do the job until it's time to take the cylinder head off for a complete refurbishment.

I haven't had time to test the bike yet and find out if this has solved the problem, but I hope it has. Having one pipe blowing that close to the head is very likely to have upset the exhaust valve seating due to lack of back pressure. If it's not raining tomorrow, hopefully I'll get the chance to give it a try.

All photos by me