It was the first Saturday for a while where we had decent weather (not that common at this time of the year here in the UK !) and I didn't have to spend time working, so I've been able to do a bit more on the Z650 itself rather than just the spare engines.
Getting the bike to run properly has been a frustrating experience. Definitely one step forward and two steps back !
When a bike of this age doesn't run, there are only a limited number of things it can be; fuel, air, mechanical or electrical.
I've mostly focused on the carburettors, because I knew they were a problem, but after several rebuilds I'm pretty sure they're now at the point where they should be functional even if they need a bit of fine tuning at a later stage. So that's fuel and air ticked off the list.
Mechanically, I'm reasonably certain the engine is sound. It turns over on the kickstarter, and I've changed the oil and filter (neither of which suggested any problems). It runs well for short periods of time, but then dies under load.
Just to be sure, I did a compression test. This is something you're supposed to do with a warmed-up engine, by turning it over with the electric start. Well, the electric starter on Z650's was always a bit of a weak point, and this one doesn't work at all.
But yes, it is possible to do it with a cold engine and just a kickstarter. It isn't as accurate as doing it with a warmed up engine, but still gives useful information - you're looking to see that all four cylinders have approximately the same reading, and that it is close to what the manual suggests it should be with a warmed-up engine.
To do it, you take the spark plugs out, and screw the compression tester into each one in turn. Then you turn the engine over as rapidly as you can with the kickstarter, until the reading on the compression tester stops increasing. It takes about 20 kicks per cylinder, after which I had to go for a lie down until I recovered !!
If all the readings are substantially too low, it suggests the piston rings or barrels are too worn. If one or two of the readings are close to zero, it suggests damaged valves or broken piston rings in just that cylinder or cylinders. If the readings are all over the place, it might be valve damage or broken piston rings, but more likely that the valve clearances need to be sorted out.
In this case, all the readings were close to each other and indicated good compression. Unfortunately I forgot to take photos of this task !
Having eliminated fuel, air and mechanical issues, that leaves electrical problems. Basically, we need it to spark healthily and at the right time on all four cylinders.
To get an initial impression, I unplugged each HT cap in turn, put a spare spark plug in and gave it a couple of turns on the kickstarter. What you should see is a healthy fat blue spark. What I actually saw was that the spark was orange and a bit weak, which suggested a problem.
Time to test the coils !
The bike is fitted with Dyna coils and electronic ignition. Both have been on for a good 10-15 years.
Testing the coils with a multimeter is a relatively simple job. The idea is to test the resistance in the primary and secondary circuits on each of the two coils.
These two photos show me testing the primary circuit on one of the coils, holding the multimeter probes against each of the two terminals for the wiring from the bike's electrical system. It is so hard trying to do this with one hand and take a photo with the other, which is why I had to do it as two photos !
With this particular model of coil (the Dyna1-1), the primary circuit should have a resistance of 2.9-3.2 ohms, so I set my multimeter to the low (20 ohm) setting. My multimeter is a cheap one, so I need to add the resistance of the leads; the 3.6 ohms it is showing is totally acceptable. If it was drastically out one way or the other it would indicate a problem.
Next is to test the secondary circuits. These are wrapped around (but not touching) the primary circuits inside the coils, and convert the 12 volts of the primary circuit to the thousands needed to get the spark to arc across the spark plug terminals.
To test these, I set my multimeter to the 20K ohm setting, pull out the HT leads and use the probes in the HT lead sockets. The reading we want is 13.5 to 14.5 kilo ohms. As you can see, this one (the left hand coil) has a good reading.
And here's the problem ! On this multimeter, a figure of one at the left hand end of the screen indicates infinite resistance. In other words, there is a break in the circuit and the coil is effectively scrap.
The good news is that Dyna sell the coils singly. The bad news is that it cost me £80 for just one. Hopefully it will arrive in a few days and I can see if I've solved the problem. It's always possible there's something else going on as well, of course - we'll see !
