Starting woes - could it be the dreaded sprag clutch?

I successfully completed my biggest ever one day ride earlier this month. I'll probably blog about it some time soon. It put me and Ruby under serious strain. I think it's only fair to say we were both struggling at times but, unlike me, wobbly motivation wasn't part of the equation for this monumental motorcycle.

Tim's Carribean Blue Sprophy 1200

As my trip went on, I noticed Ruby became ever more reluctant to start with her electric foot. This reluctance appeared only when she was hot. After a mug of tea, she fired up straight away. My first thought was the ignition pickups. That's because, when they fail, they do so only when the bike gets hot. It seems to bring out a problem in the field generated by the spinning magnet on the end of the crank. However, Ruby rumbled into life perfectly with a bump start. This would not happen if it were a problem with the ignition pickups. I got to visit Tim on my way back so benefitted from his considerable wisdom (thanks Tim). The idiot lights seemed to dim more than normal on the starter, suggesting a large current draw. That means an electrical short or similar. But we couldn't think what would do this in the starter circuit. The solenoid is mounted in rubber and earths through the starter motor, so that seemed unlikely. I later thought it might be a short in the electromagnetic windings that activate the solenoid switch. Mysterious business. 

Meanwhile, I got a ride on Tim's transformed Trophy - now a Sprophy, courtesy of a neatly colour-matched and fitted 900 Sprint fairing - and his 1050 Sprint. Both great fun, despite the rain that had started to sweep across the country. More thanks to Tim. 

To cut a long story short, the need to bump start when hot got more and more common over the course of the trip. It made refuelling an issue unless I was stopping for a half-hour or more. Ruby's MOT was due at the start of October. I had an extra day off work after my trip so I took her up to the brill bike shop in my home town (it has been there since the 1960s, now run by the son of the original owner). I deliberately didn't warm her up beforehand - started fine, breezed through the checks, and passed without advisory warnings. Great. Then I thought I'd get to the bottom of the problem. 

Back in my garage, engine now hot, I tried the starter and sure enough it wouldn't spin the engine enough to get her running. More like chug - chug - nothing. However, with helmet off and in silence, I could here a nasty metallic ringing sound after the chug - chug. I persisted with the result that I got fewer chugs and more ringing, until I just got a click from the solenoid. The video above shows this but, since I lost my actual camera, is poor mobile phone quality. You'll get the idea. The idea is "Blast and confound it, this is horrible". Notwithstanding the solenoid click, the ringing sounded like the dreaded sprag clutch just spinning ineffectually on its shaft.

Denso Starter Repair Kit

Could it be I'd have to sort out the sprags for the first time? Expensive. Time consuming. Worrying. I sure hoped not. Tim thought it was more likely a problem with the starter motor, because of the dimming lights. 

So I rang Trevor. He said he thought it might be the starter motor brushes, but it could be the sprag clutch. 

So I visited Trevor to acquire a Denso starter repair kit, comprising brushes, brush holder plate, O-rings and nuts. He was as wonderful as usual. Sharing my pain and furnishing me with a rebuild kit. Plus a little tour of his premises so I could ogle his latest project - a fantastically compact Egli-type racing frame for T300.  

Americans say 'tear down' rather than
'strip down'. It did feel like that as I got stuck in.

Back at home, the strip down began. I decided to get down to the starter as quickly as possible, to replace the brushes and then to test the motor without reconnecting all the gubbins that have to come off in the first place. That way, if the brushes weren't the answer, I'd be half-way through the dismantling necessary for the sprag job. 

Slot cut in button head screw

Dismantling went well until it came to removing the starter itself. It is retained by two M6x20mm hex-headed bolts. I'd swapped the originals for stainless steel button head machine screws. They use a 4mm Allen key and one of them rounded out. I may have caused this by not draining the coolant down, thereby having to wiggle the keys around the coolant return pipe from the rear of the cylinder block. Blast and tarnation. I then removed the alternator, to improve access and cut a slot in the head of the offending button-headed screw before tapping it around with a hammer and chisel. At least things were reasonably clean and my original careful rebuild work - and use of copper grease - was rewarded with a straightforward strip in all other respects.

Lots of black dust in the
starter body

Charred brush residue

Comparing replacement brush holder (left) with
blackened and worn out original (right)

With the starter out on my bench, I split it down and was struck first by the smell - hot or burnt electrical - and the volume of black dust inside the starter body. I allowed myself to feel a bit encouraged. Then I saw the brushes. They were worn down practically to the copper braids that deliver them with electricity. Clearly and happily, it was true that they needed replacing. I could also see the copper segments on the armature showed evidence of pitting, which I would expect from electrical arcing. Fortunately, this was not deep and was dressed out with a nylon pad. 

Cleaned and polished

Replaced in the brush housing

A good clean up and reassembly with Trevor's kit left me feeling optimistic. I used plenty of electrical contact cleaner, a thin screwdriver to rake out the gunge between copper segments and wiped down everything with paper. 

But I was still worried that the metallic ringing noise could only be the sprag clutch. What could I do? 

So I reinstalled the starter, connected the battery, and ....

.... brilliant. The starter spins with gusto, no sign of metallic ringing, and is clearly a very happy electrobunny again. I then stopped for the night and will discover tomorrow whether the dear bike will in fact fire up cold and hot. I think the answer will be 'YES!'