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K Series mis fire

Andrew Dent

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Been out this evening to make the most of empty roads, and on the way back the engine started to misfire. Had a look on the Stack at the voltage, and saw between 10.5 - 11.0v. Is this low enough to cause a misfire. My next concern is what is causing it. THe ignition LED I have was also coming on very slightly.


Either the diode pack on the alternator or the alternator itself needs looking at. Any tech suggestions to test these things out.


Are there any other alternative alternators for a SLR?


As fate would have it I was supposed to be at Oulton Park for the day tomorrow, but I'm off to Belgium for a couple of days work. May be a blessing in disguise.




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Never had this problem on a car, but on my BMW bike, a slightly glowing ignition light and a lack of decent charge (voltage wise) meant starter motor troubles. Poor memory means that I fail to remember what the fix actually was, except that on a BMW bike, it was expensive.....With the K series propensity for eating starter solenoids, if the alternator checks out on the multi-meter, charging wise (you should see >13.5 volts, IIRC. Checking amps would be problematic......), then the starter would be my mext port of call........
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Hello Andrew.



I'm not sure 10.5V is low enough to cause misfire, but I'm sure it's too low to ensure a proper battery charge. There is a simple way to do a primary test of the alternator: with a voltmeter directly on the battery, you should have about 11.8V engine stopped and about 13.5V engine running at 2000 rpm and more.


13.5 running and <11.5 stopped : change battery

<13 running : investigate alternator


It can be:

1- The belt : belt slips and alternator is too slow to ensure charge.

2- The diodes : One third (or two)of the alternative current is not rectified to direct current and is lost for the charge.

3- The regulator : it cuts the current in little "packets" which average gives the proper voltage at the battery.

4- The brushes : they lead the induction current to the alternator's rotor. It induces an alternative current in the stator, which is rectified in the diodes then "packeted" in the regulator and sent to the battery.


Easyer (and cheaper) : the belt. Just ensure proper tension. Be careful: it often also drives the water pump. If it seems too old, change it.

More frequent : brushes or regulator. On modern alternators, they are one and only spare part. You can always try and solder new brushes on your regulator. I did it with spare brushes from a vacuum cleaner...

Nobody wants to do it (except 2nd hand spare parts vendors...): diodes.


I know a possible link between the starter and the alternator:

There is always a big big wire between the battery(+) and the starter. The alternator "+BAT" wire must be connected to the battery(+). Sometimes the alternator is connected at the "starter end" of the big big wire, to spare a few grams of copper in the car manufacturing. If the bolt on the starter connection gets loose, you can have a battery charge problem. Just tighten it. Blatman says it's expensive on a BMW bike, so I guess it can be more complicated...


Hope it can help.



Cosworth 1600 BDR

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Had the same on a Fiat X 1/9 some years back. Ignition light glimmered when a load was put on but it could only be seen at night. Full beam headlights caused it, but not on dipped beam. Mine was the diode pack as I remember. Sold the car before I got around to altering/fixing it!!!
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