CB500 not running

[Borris]

Evening all,

I've been having problems with my cb500 twin 2003 for the last few weeks, culminating in it dying last week.

It started with the bike surging at around 4k revs, 5k revs and 6k revs, with it being ok between those values,say 4.3k - 4.8k revs being ok. Over 7k-12k revs/under 4k was fine too.

I also noticed my headlight wasn't as bright as I remember it being in reflections, looked ok when I got off and looked at it, but it's been a while since I've ridden in the dark.

When it finally stopped altogether, I was riding up a steep hill, it started to bog down, so I changed down a gear, but it kept slowing down, until it would only crawl along in first gear with no revs. Reving it would cause it to stall.

I could get it to start and crawl along for a bit without giving it any revs, then it just stalled fully. Trying to get it to start then and since just makes the engine turn over a few times then backfire.

I've tried starting it without the air filter in, it just turned over then backfired. Nothign else blocking the air box.

Fuel flow from the tap was ok. Topped up a few days ago,average 1 tank a week.

Carb diaphrams are starting to wear slightly, but no rips through. The sliders were free, didn't check the bowls, due to it being a pia. The butterfly valves turned ok, no obvious problems.

Checked the sparks, sparking ok against the frame. Checked the ohms of the primary+secondary through to leads,within tolerance.

Started to check the alternator ohms, but Haynes doesn't list the values start of the chapter, just which plug to undo next the the rectifier, but I think I was getting around 0.9 on the 2m setting when checking all three wires against each other. I also tried having one lead from the multimeter on the wires in the plug, and was getting a reading when touching the other to the upper engine part (with all the cooling fins, my guess at what they meant as ground).

Does this mean my alternator is gone, and would it be enough to be the sole cause of my bike problems? The battery, Motobatt MBTX9U, was still pretty strong, bought May 2013, not yet given me any trouble. Bike has been riden all yeart round for the last 4 years.

Going to have a go at checking the rectifier tomorrow.
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Suzuki EN 125-2
Honda CB 500

[stinkwheel]

If one of the three yellow wires coming from the alternator has continuity with earth (such as your engine fin or the metal of the ignition lock or the battery negative) then yes, it's shot.

Check again. Disconnect that plug. Measure resistance between each yellow wire and earth. There should be an open circuit (infinity or the highest reading your meter can do, same as when the probes are held apart). If it's getting a reading, there is a short in the alternator windings. New alternator stator or rewind time.

It's more common for the reg/rec to fail. You will not be able to static test the reg/rec with a standard multimeter but the light being dim certainly sounds like a charging failure of some sort. If the alternator checks out ok then do the following dynamic reg/rec test:

Charge the battery fully and see if the bike starts. It should and should run normally initially.

Use your multimeter to measure the voltage across the battery terminals.

It should be sitting at 12.5-13v when fully charged. This may drop a little when you turn on the ignition (no lights on) but should stay over 12. If it drops way down, your reg/rec or alternator may be dumping power to earth through a short.

Start the engine. Voltage drop momentarily, maybe as low as 9v with the starter motor. The engine should start and the voltage recover to over 12v, more likely 13 or 14v. When revved, the voltage should increase to the 14-15v range.

Anything under say 13.5v or over 14.5v at mid rev speed is a problem.

If the alternator is shot, the very first thing you should do after fitting a new one is a dynamic reg/rec test as described above to see if it failed as a result of a reg/rec issue.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

It's a Honda of the right sort of vintage for reg/rec failure to be likely. Reg/rec failure means you run out of electricity, which feels identical to running out of fuel.

When check the reg/rec output, it is important to check both at tickover, and at around 4000 rpm. A traditional shunt type reg/rec has 3 pairs of diodes that shunt power back and forth between them, generating heat, which over time kills the diodes. They don't all fail together - the pairs drop out one at a time. This means a reg/rec can work at tickover when one or two pairs can switch fast enough to work. Once the revs rise, the reg/rec can't switch fast enough, and the battery voltage falls.

If the reg/rec is dead, I highly recommend replacing it with MOSFET type. I just did this one my VFR 750. 60 quid for a used reg/rec from a recent Yamaha MT, a tenner for the Triumph wiring harness, and a bit of fiddling to fit my Honda plugs. I now have a working charging system.

[Borris]

Thanks
Will check tomorrow, just charged the battery up.
____________________
Suzuki EN 125-2
Honda CB 500